Bleach clean() / Cleaner() fails to sanitize dangerous URI schemes in allowed formaction attributes

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

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary Bleach clean() / Cleaner() fails to sanitize dangerous URI schemes in allowed formaction attributes. Bleach applies URI protocol sanitization only to attributes listed in attrvalisuri. While URI-bearing attributes such as action, href, src, and poster are included in that set, formaction is not. As a result, if a downstream application explicitly allows formaction on submit-capable controls in untrusted HTML, Bleach preserves dangerous values such as javascript:alert(1) instead of stripping them. This can lead to submit-triggered JavaScript execution in applications that rely on Bleach to sanitize untrusted HTML and allow the relevant tag/attribute combination. --- Details The issue appears to be a URI-sanitization coverage gap in Bleach’s sanitizer logic. Relevant code paths: bleach/sanitizer.py — BleachSanitizerFilter.allowtoken (around line 553) bleach/vendor/html5lib/filters/sanitizer.py — attrvalisuri (around line 525) In BleachSanitizerFilter.allowtoken, URI protocol…

Published by the source

16 Jun 2026, 14:07 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-gj48-438w-jh9v

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-gj48-438w-jh9v

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# Patch Next advisory brief: GHSA-gj48-438w-jh9v

Advisory: Bleach clean() / Cleaner() fails to sanitize dangerous URI schemes in allowed formaction attributes
IDs: GHSA-gj48-438w-jh9v
Record: https://patchnext.com/advisories/ghsa-gj48-438w-jh9v (revision 2, source checked 26 Oct 2026, 13:45 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: bleach 6.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI bleach: affected < 6.4.0; fixed in 6.4.0

## Suggested inspection (read-only)
- `python -m pip show bleach`: prints the installed version. Run with the environment that serves production active.

## 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 bleach.
- 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-gj48-438w-jh9v (OSV record GHSA-gj48-438w-jh9v lists affected ranges: PyPI bleach >=0 <6.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-gj48-438w-jh9v (GitHub-reviewed advisory GHSA-gj48-438w-jh9v lists affected ranges: PyPI bleach >=0 <6.4.0)

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