league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes

Exploitation

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

CVSS

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

EPSS

• CVE-2026-71478: 0.36% estimated exploitation activity in the next 30 days; percentile 29.26 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The AttributesExtension's href/src unsafe-link filter (AttributesHelper::filterAttributes()) can be bypassed by embedding control bytes in a javascript: URL that browsers discard before parsing the scheme. Two variants: - Tab/newline inside the scheme — a literal ASCII TAB (0x09), CR (0x0D), or LF (0x0A), e.g. java<TABscript:alert(1). Per the WHATWG URL Standard's "basic URL parser" step 3, browsers "remove all ASCII tab or newline from input". - Leading C0 controls — e.g. <0x01javascript:alert(1). Per step 1 of the same algorithm, browsers remove any leading or trailing C0 control or space. (A leading space alone does not bypass, because parseAttributes() already trim()s the value; other C0 bytes are not trimmed.) The filter is a literal anchored-prefix regex (RegexHelper::isLinkPotentiallyUnsafe() / REGEXUNSAFEPROTOCOL) that matches neither obfuscated form, so in both cases the browser still executes javascript:alert(1). This is confirmed reproducible even with…

Published by the source

6 Aug 2026, 20:30 UTC

Last modified by the source

21 Aug 2026, 05:28 UTC

Source last checked

26 Oct 2026, 14:50 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-29pj-957v-52mc

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-29pj-957v-52mc

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# Patch Next advisory brief: CVE-2026-71478

Advisory: league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes
IDs: CVE-2026-71478, GHSA-29pj-957v-52mc
Record: https://patchnext.com/advisories/cve-2026-71478 (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 6.1)
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 >= 1.5.0, <= 2.8.3; 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-29pj-957v-52mc (OSV record GHSA-29pj-957v-52mc lists affected ranges: Packagist league/commonmark >=1.5.0 <2.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-29pj-957v-52mc (GitHub-reviewed advisory GHSA-29pj-957v-52mc lists affected ranges: Packagist league/commonmark >=1.5.0 <=2.8.3)

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