CryptPad has a Sanitizer Bypass in Diffmarked.js that Allows Arbitrary HTML Injection and Potential XSS

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-2026-26028: 0.24% estimated exploitation activity in the next 30 days; percentile 15.48 (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 CryptPad’s HTML sanitizer in Diffmarked.js can be bypassed due to incomplete filtering of restricted tags. Because the sanitizer only validates the src attribute of <iframe <video, and <audio elements, and does not restrict other attributes, an attacker can inject arbitrary HTML through srcdoc. This completely defeats CryptPad’s intended bounce sandboxing and allows link injection or other interactive content inside user-controlled documents. Details The sanitizer defines forbidden and restricted tags but treats <iframe as “restricted” instead of “forbidden”: https://github.com/cryptpad/cryptpad/blob/0dd3c1f53d56dffb06651b86ead6b9b387920173/www/common/diffMarked.js#L403-L407 The actual enforcement only checks the src attribute, nothing else: https://github.com/cryptpad/cryptpad/blob/0dd3c1f53d56dffb06651b86ead6b9b387920173/www/common/diffMarked.js#L445-L449 Because only src is validated, adding a benign blob: src but malicious srcdoc results in unrestricted rendering. PoC An…

Published by the source

26 May 2026, 19:05 UTC

Last modified by the source

9 Jun 2026, 05:15 UTC

Source last checked

26 Oct 2026, 12:52 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-g2g4-47gv-p72v

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-g2g4-47gv-p72v

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# Patch Next advisory brief: CVE-2026-26028

Advisory: CryptPad has a Sanitizer Bypass in Diffmarked.js that Allows Arbitrary HTML Injection and Potential XSS
IDs: CVE-2026-26028, GHSA-g2g4-47gv-p72v
Record: https://patchnext.com/advisories/cve-2026-26028 (revision 2, source checked 26 Oct 2026, 12:52 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 response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm cryptpad: affected <= 5.9.0

## Suggested inspection (read-only)
- `npm ls cryptpad --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on cryptpad.
- 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-g2g4-47gv-p72v (OSV record GHSA-g2g4-47gv-p72v lists affected ranges: npm cryptpad >=0 <=5.9.0)
- Verified: https://github.com/advisories/GHSA-g2g4-47gv-p72v (GitHub-reviewed advisory GHSA-g2g4-47gv-p72v lists affected ranges: npm cryptpad >=0 <=5.9.0)

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