league/commonmark: Denial of service via deeply nested XML output

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Impact XmlRenderer pretty-prints XML by emitting depth-proportional indentation whitespace for every opening and closing tag. For a tree of depth n, the indentation alone sums to O(n²) bytes of output (and corresponding memory), reachable through MarkdownToXmlConverter — e.g. strrepeat(' ', $depth) . "x\n", a single line of nested blockquotes — or through a direct XmlRenderer::renderDocument() call on an attacker-influenced AST. This affects applications that convert untrusted Markdown to XML, which is an opt-in output path. The parser's maxnestinglevel bounds the depth of parser-created trees, but its default is high enough to reach damaging sizes, can be raised by the host application, and does not constrain custom or programmatically built ASTs handed straight to the renderer. The result is a memory / output-size amplification rather than a hard crash, which is why this issue is rated Medium rather than High. No confidentiality or integrity impact. XML rendering was introduced in…

Published by the source

6 Aug 2026, 20:42 UTC

Last modified by the source

6 Aug 2026, 20:45 UTC

Source last checked

26 Oct 2026, 14:50 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mj63-m3rc-8ppr

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-mj63-m3rc-8ppr

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# Patch Next advisory brief: GHSA-mj63-m3rc-8ppr

Advisory: league/commonmark: Denial of service via deeply nested XML output
IDs: GHSA-mj63-m3rc-8ppr
Record: https://patchnext.com/advisories/ghsa-mj63-m3rc-8ppr (revision 2, source checked 26 Oct 2026, 14:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: league/commonmark 2.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist league/commonmark: affected >= 2.0.0, < 2.9.0; fixed in 2.9.0

## Suggested inspection (read-only)
- Packagist league/commonmark: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on league/commonmark.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-mj63-m3rc-8ppr (OSV record GHSA-mj63-m3rc-8ppr lists affected ranges: Packagist league/commonmark >=2.0.0 <2.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-mj63-m3rc-8ppr (GitHub-reviewed advisory GHSA-mj63-m3rc-8ppr lists affected ranges: Packagist league/commonmark >=2.0.0 <2.9.0)

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