CraftCMS vulnerable to reflective XSS via incomplete return URL sanitization

Exploitation

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

CVSS 4.0

6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N · score source

EPSS

• CVE-2026-31859: 0.19% estimated exploitation activity in the next 30 days; percentile 8.96 (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 fix for CVE-2025-35939 in craftcms/cms introduced a striptags() call in src/web/User.php to sanitize return URLs before they are stored in the session. However, striptags() only removes HTML tags (angle brackets) -- it does not inspect or filter URL schemes. Payloads like javascript:alert(document.cookie) contain no HTML tags and pass through striptags() completely unmodified, enabling reflected XSS when the return URL is rendered in an href attribute. Details The patched code in is: striptags() removes HTML tags (e.g., <script, <img) from a string, but it is not a URL sanitizer. When the sanitized return URL is subsequently rendered in an href attribute context (e.g., <a href="{{ returnUrl }}"), the following dangerous payloads survive striptags() completely unmodified: 1. javascript: protocol URLs -- javascript:alert(document.cookie) contains no HTML tags, so striptags() returns it verbatim. When placed in an href, clicking the link executes the JavaScript. 2. data:…

Published by the source

11 Mar 2026, 00:26 UTC

Last modified by the source

25 May 2026, 06:15 UTC

Source last checked

26 Oct 2026, 14:40 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-fvwq-45qv-xvhv

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# Patch Next advisory brief: CVE-2026-31859

Advisory: CraftCMS vulnerable to reflective XSS via incomplete return URL sanitization
IDs: CVE-2026-31859, GHSA-fvwq-45qv-xvhv
Record: https://patchnext.com/advisories/cve-2026-31859 (revision 1, source checked 26 Oct 2026, 14:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: craftcms/cms 5.9.7, 4.17.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist craftcms/cms: affected >= 4.15.3, <= 4.17.2; >= 5.7.5, <= 5.9.6; fixed in 4.17.3, 5.9.7

## Suggested inspection (read-only)
- Packagist craftcms/cms: 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 craftcms/cms.
- 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://github.com/advisories/GHSA-fvwq-45qv-xvhv (GitHub-reviewed advisory GHSA-fvwq-45qv-xvhv lists affected ranges: Packagist craftcms/cms >=4.15.3 =5.7.5 <=5.9.6)

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