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GHSA-qc95-4862-92fh
No affected components available
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.
Measures severity based on intrinsic characteristics of the vulnerability, independent of environment.
No exploitation activity has been observed at this time. Continue routine monitoring.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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