Vulnerability-Lookup

OESA-2026-3320 (CVE-2026-50538)

Vulnerability from osv_openeuler – Published: 2026-08-13 13:57 – Updated: 2026-08-13 13:57 – Source website

Summary

libvncserver security update

Details

libvncserver is a set of programs using the RFB (Remote Frame Buffer) protocol. They are designed to "export" a frame buffer via net: you set up a server and can connect to it via VNC viewers. If the server supports WebSockets (which LibVNCServer does), you can also connect using an in-browser VNC viewer like noVNC. It is already in wide use for administration, but it is not that easy to program a server yourself.

Security Fix(es):

An attacker-controlled heap out-of-bounds write vulnerability exists in the Tight decoder of libvncclient within libvncserver. An attacker can trigger a heap out-of-bounds write via crafted packets, potentially leading to denial of service or remote code execution.(CVE-2026-50538)

Severity

8.8 (High)


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

References

URL

Type

	https://www.openeuler.org/zh/security/security-bu…	ADVISORY
	https://nvd.nist.gov/vuln/detail/CVE-2026-50538	ADVISORY

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "aarch64": [
          "libvncserver-0.9.13-6.oe2203sp4.aarch64.rpm",
          "libvncserver-debuginfo-0.9.13-6.oe2203sp4.aarch64.rpm",
          "libvncserver-debugsource-0.9.13-6.oe2203sp4.aarch64.rpm",
          "libvncserver-devel-0.9.13-6.oe2203sp4.aarch64.rpm"
        ],
        "src": [
          "libvncserver-0.9.13-6.oe2203sp4.src.rpm"
        ],
        "x86_64": [
          "libvncserver-0.9.13-6.oe2203sp4.x86_64.rpm",
          "libvncserver-debuginfo-0.9.13-6.oe2203sp4.x86_64.rpm",
          "libvncserver-debugsource-0.9.13-6.oe2203sp4.x86_64.rpm",
          "libvncserver-devel-0.9.13-6.oe2203sp4.x86_64.rpm"
        ]
      },
      "package": {
        "ecosystem": "openEuler:22.03-LTS-SP4",
        "name": "libvncserver",
        "purl": "pkg:rpm/openEuler/libvncserver\u0026distro=openEuler-22.03-LTS-SP4"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.9.13-6.oe2203sp4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "severity": "High"
  },
  "details": "libvncserver is a set of programs using the RFB (Remote Frame Buffer) protocol. They are designed to \u0026amp;quot;export\u0026amp;quot; a frame buffer via net: you set up a server and can connect to it via VNC viewers. If the server supports WebSockets (which LibVNCServer does), you can also connect using an in-browser VNC viewer like noVNC. It is already in wide use for administration, but it is not that easy to program a server yourself.\r\n\r\nSecurity Fix(es):\n\nAn attacker-controlled heap out-of-bounds write vulnerability exists in the Tight decoder of libvncclient within libvncserver. An attacker can trigger a heap out-of-bounds write via crafted packets, potentially leading to denial of service or remote code execution.(CVE-2026-50538)",
  "id": "OESA-2026-3320",
  "modified": "2026-08-13T13:57:21Z",
  "published": "2026-08-13T13:57:21Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.openeuler.org/zh/security/security-bulletins/detail/?id=openEuler-SA-2026-3320"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-50538"
    }
  ],
  "schema_version": "1.7.2",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "libvncserver security update",
  "upstream": [
    "CVE-2026-50538"
  ]
}

CVE-2026-50538 (GCVE-0-2026-50538)

Vulnerability from cvelistv5 – Published: 2026-08-21 21:03 – Updated: 2026-08-24 19:39

Title

libvncclient Tight decoder has an attacker-controlled heap out-of-bounds write

Summary

LibVNCClient is a library for easy implementation of a VNC client. In versions 0.9.12 through 0.9.15, a malicious (or man-in-the-middle) VNC server can force a connecting `libvncclient` to write attacker-controlled data past the end of its framebuffer. This is an out-of-bounds heap write with attacker-controlled length, contents, and offset. It needs no authentication (the attacker is the server), works in a default build with default settings, and fires from a single `FramebufferUpdate` the moment the victim connects. It crashes any client unconditionally (denial of service); we also demonstrated it overwriting an application callback pointer and redirecting execution to attacker-chosen code (code execution) under the default configuration. Commit 540332be3e0acc566fa64da6f1b4680c72c724dd patches the issue.

Severity

8.8 (High)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-24 19:39 UTC

CWE

CWE-122 - Heap-based Buffer Overflow
CWE-787 - Out-of-bounds Write

Assigner

GitHub_M mitre root

References

2 references

URL	Tags
https://github.com/LibVNC/libvncserver/security/a…	x_refsource_CONFIRM
https://github.com/LibVNC/libvncserver/commit/540…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
LibVNC	libvncserver	Affected: >= 0.9.12, <= 0.9.15

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-50538",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "total"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-08-24T19:39:31.246844Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-08-24T19:39:57.811Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "libvncserver",
          "vendor": "LibVNC",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 0.9.12, \u003c= 0.9.15"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "LibVNCClient is a library for easy implementation of a VNC client. In versions 0.9.12 through 0.9.15, a malicious (or man-in-the-middle) VNC server can force a connecting `libvncclient` to write attacker-controlled data past the end of its framebuffer. This is an out-of-bounds heap write with attacker-controlled length, contents, and offset. It needs no authentication (the attacker\nis the server), works in a default build with default settings, and fires from a single `FramebufferUpdate` the moment the victim connects. It crashes any client unconditionally (denial of service); we also demonstrated it overwriting an application callback pointer and redirecting execution to attacker-chosen code (code execution) under the default configuration. Commit 540332be3e0acc566fa64da6f1b4680c72c724dd patches the issue."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "HIGH",
            "baseScore": 8.8,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "HIGH",
            "integrityImpact": "HIGH",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-122",
              "description": "CWE-122: Heap-based Buffer Overflow",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-787",
              "description": "CWE-787: Out-of-bounds Write",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-21T21:03:58.147Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8"
        },
        {
          "name": "https://github.com/LibVNC/libvncserver/commit/540332be3e0acc566fa64da6f1b4680c72c724dd",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/LibVNC/libvncserver/commit/540332be3e0acc566fa64da6f1b4680c72c724dd"
        }
      ],
      "source": {
        "advisory": "GHSA-v9pm-47h4-jcq8",
        "discovery": "UNKNOWN"
      },
      "title": "libvncclient Tight decoder has an attacker-controlled heap out-of-bounds write"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-50538",
    "datePublished": "2026-08-21T21:03:58.147Z",
    "dateReserved": "2026-06-04T20:37:18.653Z",
    "dateUpdated": "2026-08-24T19:39:57.811Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

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.

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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

OESA-2026-3320 (CVE-2026-50538)

CVE-2026-50538 (GCVE-0-2026-50538)

Tags

Sightings

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