Vulnerability · Published Nov 5, 2026 · Assigned by Checkmarx
CVE-2026-118517Symfony: HtmlSanitizer allowLinkHosts() / allowMediaHosts() Bypass via URLs Without a Well-Formed Authority (Incomplete Fix for CVE-2026-45066)
Medium 4.3
In plain English
A medium-severity security flaw in symfony from sensiolabs. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 18% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | symfony | no fix listed | DEBIAN-CVE-2026-118517 |
| Debian:13 | symfony | no fix listed | DEBIAN-CVE-2026-118517 |
| Debian:14 | symfony | 7.4.23+dfsg-1 | DEBIAN-CVE-2026-118517 |
| Ubuntu:26.04:LTS | symfony | no fix listed | UBUNTU-CVE-2026-118517 |
| Ubuntu:26.10 | symfony | no fix listed | UBUNTU-CVE-2026-118517 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| sensiolabs | symfony | 6.1.0 – before 6.4.507.0.0 – before 7.4.238.0.0 – before 8.1.11 |
Weakness: CWE-184, CWE-436 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N