GHSA-qc95-4862-92fh
GHSA-qc95-4862-92fh is a CWE-184 vulnerability in symfony/html-sanitizer. O3 Security confirms whether GHSA-qc95-4862-92fh is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Symfony has an HtmlSanitizer allowLinkHosts() / allowMediaHosts() Bypass via URL-Parser Differentials and <area> Misclassification
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-qc95-4862-92fh.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
symfony/html-sanitizer🐘symfony/html-sanitizer🐘symfony/html-sanitizer🐘symfony/symfony🐘symfony/symfony🐘symfony/symfonyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Description
symfony/html-sanitizer lets applications sanitise untrusted HTML. The configuration methods allowLinkHosts([...]) and allowLinkSchemes([...]) are intended to restrict <a href> targets to an allowlist of hosts/schemes; allowMediaHosts() / allowMediaSchemes() do the same for <img src> etc.
Three distinct bypasses allow a content author to smuggle off-allowlist URLs past these checks. First, UrlSanitizer::parse() parses the input following RFC-3986, while browsers follow the WHATWG URL Standard which normalises \ to / before parsing the authority of "special" schemes; so an input like https://evil\@trusted.com/ parses with host trusted.com server-side but navigates to https://evil/ in the browser. Second, WHATWG collapses any run of / after the scheme into //, while RFC-3986 does not; so https:/evil.com/ and https:///evil.com/ parse as host-less (skipping the host allowlist) but resolve to evil.com in the browser. Third, UrlAttributeSanitizer checks 'a' === $element to route to the link policy and falls through to the media policy otherwise, but <area> is a navigable hyperlink equivalent to <a>; so <area href> was sanitised against the media policy (which typically allows data: and may have no host allowlist), bypassing allowLinkHosts() / allowLinkSchemes() entirely.
Resolution
UrlSanitizer::sanitize() now rejects URLs that contain a backslash or that use a special scheme (http, https, ftp, ws, wss) followed by a single slash or three slashes before parsing, eliminating the parser-differential bypasses. UrlAttributeSanitizer now applies the link policy to both <a> and <area> elements.
The patch for this issue is available here for branch 5.4.
Credits
Symfony would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | symfony/html-sanitizer | ≥ 6.1.0&&< 6.4.40 | 6.4.40 |
| 🐘Packagist | symfony/html-sanitizer | ≥ 7.0.0&&< 7.4.12 | 7.4.12 |
| 🐘Packagist | symfony/html-sanitizer | ≥ 8.0.0&&< 8.0.12 | 8.0.12 |
| 🐘Packagist | symfony/symfony | ≥ 6.1.0&&< 6.4.40 | 6.4.40 |
| 🐘Packagist | symfony/symfony | ≥ 7.0.0&&< 7.4.12 | 7.4.12 |
| 🐘Packagist | symfony/symfony | ≥ 8.0.0&&< 8.0.12 | 8.0.12 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for symfony/html-sanitizer. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update symfony/html-sanitizer to 6.4.40 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-qc95-4862-92fh is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-qc95-4862-92fh is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-qc95-4862-92fh. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-qc95-4862-92fh in your dependencies?
O3 detects GHSA-qc95-4862-92fh across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.