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Fencer Intel
medium 4.3

CVE-2026-118517 json

Symfony: HtmlSanitizer allowLinkHosts() / allowMediaHosts() Bypass via URLs Without a Well-Formed Authority (Incomplete Fix for CVE-2026-45066)

published 4d ago modified 3d ago also EUVD-2026-117579

cvss
4.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 18%
public exploits
0
none known

Summary

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.

Patch / fix

  • symfony Debian 13 6.4.50+dfsg-0+deb13u1
  • symfony Debian 14 7.4.23+dfsg-1

debian advisories DEBIAN-CVE-2026-118517

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Vendor advisories

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red hat product security vex CVE-2026-118517 updated cvss 4.3

symfony/html-sanitizer: symfony/html-sanitizer: Host allowlist bypass via URL parsing inconsistency

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
4d ago

Exploit prediction

0.26% chance of exploitation in the next 30 days, higher than 18% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 4.0 2.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
CISA ADP 3.1 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
CNA 4.0 2.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CNA 3.1 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

Affected products

  • sensiolabs symfony

Classification

Weakness
  • CWE-184 Incomplete List of Disallowed Inputs
  • CWE-436 Interpretation Conflict

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.