Bug 2567785 (CVE-2026-118517)

Summary: CVE-2026-118517 symfony/html-sanitizer: symfony/html-sanitizer: Host allowlist bypass via URL parsing inconsistency
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in the Symfony HtmlSanitizer component. Due to differences in how URLs are parsed compared to web browsers, the component fails to properly detect hosts in certain malformed URLs when host allowlists are enforced. A remote attacker can exploit this vulnerability by submitting specially crafted URLs to bypass host validation, resulting in untrusted links or media being retained in sanitized output.
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Description OSIDB Bzimport 2026-11-05 22:25:53 UTC 
The Symfony HtmlSanitizer component UrlSanitizer parses URLs differently from browsers, allowing the allowLinkHosts() and allowMediaHosts() checks to be bypassed for URLs without a well-formed authority, resulting in untrusted hosts being referenced in sanitized output in Symfony versions 6.1.x prior to 6.4.50, 7.x prior to 7.4.23, and 8.x prior to 8.1.11.



UrlSanitizer parses URLs with league/uri, which follows RFC 3986, while browsers follow the WHATWG URL standard. For URLs without a well-formed authority, such as http:evil.example, ///evil.example, or https://://evil.example/ the sanitizer finds no host, whereas browsers resolve them to evil.example. The guard added for CVE-2026-45066 missed the zero-slash form, and the host 0 was also skipped because PHP treats it as falsy.



This is exploitable when a host allow-list is combined with allowRelativeLinks(true) or allowRelativeMedias(true). The default configuration is not affected.