Vulnerability-Lookup

CVE-2026-55839 (GCVE-0-2026-55839)

Vulnerability from cvelistv5 – Published: 2026-08-18 15:27 – Updated: 2026-08-18 17:46

Title

Kestra: Stored XSS via custom Markdown [[link]] attribute injection

Summary

Kestra is an open-source, event-driven orchestration platform. Prior to 1.3.24, Kestra's custom Markdown parser in ui/src/utils/markdown_plugins/link.ts allows a user with permission to create or update a Flow description to inject JavaScript event-handler attributes through the custom [[link]] syntax, causing stored cross-site scripting when another user opens the description or information panel in the Flow list. This issue is fixed in version 1.3.24.

Severity

8.7 (High)


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

SSVC

Exploitation: none Automatable: no Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-18 17:28 UTC

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

GitHub_M mitre root Scorecard

References

4 references

URL	Tags
https://github.com/kestra-io/kestra/security/advi…	x_refsource_CONFIRM
https://github.com/kestra-io/kestra/pull/16835	x_refsource_MISC
https://github.com/kestra-io/kestra/commit/6c8e6d…	x_refsource_MISC
https://github.com/kestra-io/kestra/releases/tag/…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
kestra-io	kestra	Affected: < 1.3.24	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-55839

Vulnerability from fkie_nvd - Published: 2026-08-18 16:17 - Updated: 2026-09-16 13:42

Severity

8.7 (High) - CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/kestra-io/kestra/commit/6c8e6d099ed172cbb6b003b7fb30b7bb1f8f710e
	security-advisories@github.com	https://github.com/kestra-io/kestra/pull/16835
	security-advisories@github.com	https://github.com/kestra-io/kestra/releases/tag/v1.3.24
	security-advisories@github.com	https://github.com/kestra-io/kestra/security/advisories/GHSA-34pm-923j-7wf8

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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  "affected": [
    {
      "affectedData": [
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          "product": "kestra",
          "vendor": "kestra-io",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 1.3.24"
            }
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    }
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GHSA-34PM-923J-7WF8

Vulnerability from github – Published: 2026-08-18 16:31 – Updated: 2026-08-18 16:31

Summary

Kestra vulnerable to stored XSS via custom Markdown [[link]] attribute injection

Details

Summary

Kestra’s Markdown renderer supports a custom [[link ...]] syntax that is converted into a custom HTML element. The custom Markdown parser allows attacker-controlled attributes to be rendered into the generated element without proper allowlisting or sanitization.

As a result, a user who can create or edit Markdown-rendered content, such as a Flow description, can inject JavaScript event-handler attributes. When another user views or interacts with the affected Markdown-rendered UI element, the JavaScript executes in that user’s browser.

This is not a normal raw-HTML Markdown payload such as <img onerror=...>. The payload uses Kestra’s custom [[link]] Markdown syntax, and the dangerous HTML attributes are introduced by Kestra’s own Markdown plugin.

Affected version

Tested on Kestra 1.3.22.

Affected components

Source review identified the issue in the custom Markdown link renderer:

ui/src/utils/markdown_plugins/link.ts
ui/src/utils/markdown.ts
ui/src/components/layout/Markdown.vue

The vulnerable code path renders parsed attributes directly into an HTML string:

const attrs = token.attrs
  ? token.attrs.map(([name, value]) => `${name}="${value}"`).join("")
  : "";

return `<router-md ${attrs}>`;

The implementation does not safely restrict attribute names or values before inserting the generated HTML into the DOM.

Proof of concept

Create or edit a Flow with the following YAML:

id: markdown_xss_flow_desc
namespace: company.team
description: |
  [[link x="y" style="position:fixed;inset:0;z-index:9999;background:rgba(255,0,0,0.05)" onmouseover="alert(document.domain)"]]

tasks:
  - id: hello
    type: io.kestra.plugin.core.log.Log
    message: hello

Save the Flow.

Then navigate to the Flow list:

http://localhost:8080/ui/main/flows

Click the description/info icon next to the malicious Flow.

Actual result

The browser executes JavaScript from the Flow description. In the PoC, an alert is triggered showing the current domain.

Expected result

Markdown-rendered Flow descriptions should not be able to inject arbitrary JavaScript event handlers or attacker-controlled HTML attributes into the page.

The custom [[link]] syntax should only generate safe attributes required for the intended router-link behavior.

Security impact

An attacker with permission to create or update a Flow in a namespace can store a malicious Markdown payload in the Flow description. When another Kestra user views the Flow list and opens the Flow description/info panel, attacker-controlled JavaScript executes in the victim’s browser.

Depending on the victim’s privileges, this may allow the attacker to perform actions as the victim within the Kestra UI, access data visible to the victim, or pivot into more privileged namespace or administrative functionality.

This crosses a security boundary in multi-user Kestra deployments where Flow authors are less privileged than administrators or other operators who review workflows.

Suggested remediation

The custom Markdown link plugin should not render arbitrary attributes into raw HTML.

Recommended fixes:

Allowlist only the exact attributes required by the custom router link component.
Reject all attributes beginning with on, such as onclick, onmouseover, and onfocus.
Reject or sanitize dangerous attributes such as style.
HTML-escape all attribute values before rendering.
Ensure attributes are joined safely with spaces.
Sanitize the final rendered Markdown output before passing it to v-html.
Add regression tests for custom [[link]] payloads containing event-handler attributes.

Minimal payload

[[link x="y" style="position:fixed;inset:0;z-index:9999;background:rgba(255,0,0,0.05)" onmouseover="alert(document.domain)"]]

Evidence collected

The payload was stored in a Flow description.
The Flow list displayed the malicious Flow.
Opening the Flow description/info panel triggered JavaScript execution in the browser.
The payload did not require execution of the Flow.
The payload used Kestra’s custom [[link]] syntax rather than raw HTML.

Severity

8.7 (High)


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

Show details on source website

JSON

To clipboard

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      "package": {
        "ecosystem": "Maven",
        "name": "io.kestra:kestra"
      },
      "ranges": [
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            },
            {
              "fixed": "1.3.24"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-55839"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-18T16:31:38Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "## Summary\n\nKestra\u2019s Markdown renderer supports a custom `[[link ...]]` syntax that is converted into a custom HTML element. The custom Markdown parser allows attacker-controlled attributes to be rendered into the generated element without proper allowlisting or sanitization.\n\nAs a result, a user who can create or edit Markdown-rendered content, such as a Flow description, can inject JavaScript event-handler attributes. When another user views or interacts with the affected Markdown-rendered UI element, the JavaScript executes in that user\u2019s browser.\n\nThis is not a normal raw-HTML Markdown payload such as `\u003cimg onerror=...\u003e`. The payload uses Kestra\u2019s custom `[[link]]` Markdown syntax, and the dangerous HTML attributes are introduced by Kestra\u2019s own Markdown plugin.\n\n## Affected version\n\nTested on Kestra `1.3.22`.\n\n## Affected components\n\nSource review identified the issue in the custom Markdown link renderer:\n\n* `ui/src/utils/markdown_plugins/link.ts`\n* `ui/src/utils/markdown.ts`\n* `ui/src/components/layout/Markdown.vue`\n\nThe vulnerable code path renders parsed attributes directly into an HTML string:\n\n```ts\nconst attrs = token.attrs\n  ? token.attrs.map(([name, value]) =\u003e `${name}=\"${value}\"`).join(\"\")\n  : \"\";\n\nreturn `\u003crouter-md ${attrs}\u003e`;\n```\n\nThe implementation does not safely restrict attribute names or values before inserting the generated HTML into the DOM.\n\n## Proof of concept\n\nCreate or edit a Flow with the following YAML:\n\n```yaml\nid: markdown_xss_flow_desc\nnamespace: company.team\ndescription: |\n  [[link x=\"y\" style=\"position:fixed;inset:0;z-index:9999;background:rgba(255,0,0,0.05)\" onmouseover=\"alert(document.domain)\"]]\n\ntasks:\n  - id: hello\n    type: io.kestra.plugin.core.log.Log\n    message: hello\n```\n\nSave the Flow.\n\nThen navigate to the Flow list:\n\n```text\nhttp://localhost:8080/ui/main/flows\n```\n\nClick the description/info icon next to the malicious Flow.\n\n## Actual result\n\nThe browser executes JavaScript from the Flow description. In the PoC, an alert is triggered showing the current domain.\n\n## Expected result\n\nMarkdown-rendered Flow descriptions should not be able to inject arbitrary JavaScript event handlers or attacker-controlled HTML attributes into the page.\n\nThe custom `[[link]]` syntax should only generate safe attributes required for the intended router-link behavior.\n\n## Security impact\n\nAn attacker with permission to create or update a Flow in a namespace can store a malicious Markdown payload in the Flow description. When another Kestra user views the Flow list and opens the Flow description/info panel, attacker-controlled JavaScript executes in the victim\u2019s browser.\n\nDepending on the victim\u2019s privileges, this may allow the attacker to perform actions as the victim within the Kestra UI, access data visible to the victim, or pivot into more privileged namespace or administrative functionality.\n\nThis crosses a security boundary in multi-user Kestra deployments where Flow authors are less privileged than administrators or other operators who review workflows.\n\n## Suggested remediation\n\nThe custom Markdown link plugin should not render arbitrary attributes into raw HTML.\n\nRecommended fixes:\n\n1. Allowlist only the exact attributes required by the custom router link component.\n2. Reject all attributes beginning with `on`, such as `onclick`, `onmouseover`, and `onfocus`.\n3. Reject or sanitize dangerous attributes such as `style`.\n4. HTML-escape all attribute values before rendering.\n5. Ensure attributes are joined safely with spaces.\n6. Sanitize the final rendered Markdown output before passing it to `v-html`.\n7. Add regression tests for custom `[[link]]` payloads containing event-handler attributes.\n\n## Minimal payload\n\n```md\n[[link x=\"y\" style=\"position:fixed;inset:0;z-index:9999;background:rgba(255,0,0,0.05)\" onmouseover=\"alert(document.domain)\"]]\n```\n\n## Evidence collected\n\n* The payload was stored in a Flow description.\n* The Flow list displayed the malicious Flow.\n* Opening the Flow description/info panel triggered JavaScript execution in the browser.\n* The payload did not require execution of the Flow.\n* The payload used Kestra\u2019s custom `[[link]]` syntax rather than raw HTML.",
  "id": "GHSA-34pm-923j-7wf8",
  "modified": "2026-08-18T16:31:38Z",
  "published": "2026-08-18T16:31:38Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/kestra-io/kestra/security/advisories/GHSA-34pm-923j-7wf8"
    },
    {
      "type": "WEB",
      "url": "https://github.com/kestra-io/kestra/commit/6c8e6d099ed172cbb6b003b7fb30b7bb1f8f710e"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/kestra-io/kestra"
    },
    {
      "type": "WEB",
      "url": "https://github.com/kestra-io/kestra/releases/tag/v1.3.24"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Kestra vulnerable to stored XSS via custom Markdown [[link]] attribute injection"
}

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CVE-2026-55839 (GCVE-0-2026-55839)

FKIE_CVE-2026-55839

GHSA-34PM-923J-7WF8

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