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TI-2026-85519 Aliases CVE-2026-118517

Host allow-list bypass in Symfony HtmlSanitizer via malformed URLs

Priority MEDIUM 45CWE-184Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
Passive
Attack complexity
Low
Attack requirements
Present
Impact type
—
Confidentiality · integrity · availability
None · Low · None
Fix status
Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA.

Summary

Symfony from 6.1.0 before 6.4.50, from 7.0.0 before 7.4.23 and from 8.0.0 before 8.1.11 has a flaw in the HtmlSanitizer component. UrlSanitizer parses URLs differently from browsers, so host checks skip URLs without a well-formed authority that browsers resolve to another host. An attacker can get untrusted hosts into sanitized output when a host allow-list is used with allowRelativeLinks(true) or allowRelativeMedias(true). The default configuration is not affected.

Written with Claude Haiku 5.5 from the sources below · 2026-11-06

Description from the CNA

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00262 18th percentileScored 2026-11-07
Fix Fixed No fixed version in the sources yet

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

No affected products listed by the sources yet.

Remediation

Upgrade Symfony to 6.4.50 for the 6.x line, 7.4.23 for the 7.x line or 8.1.11 for the 8.x line.

Written with Claude Haiku 5.5 from the sources below · 2026-11-06

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

  • Release ·

    sensiolabs symfony 6.4.50, 7.4.23, 8.1.11

    2 records Highest priority Priority MEDIUM 1 MEDIUM, 1 LOW

    Other records, highest priority first

    • TI-2026-85550 Symfony HtmlSanitizer skips URL checks on srcset and imagesrcsetPriority LOW 12
    Show all 2 records →

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product
  • Red Hat

Timeline

  1. 2026-11-05 Threatist record created from CVE-2026-118517
  2. 2026-11-05 CVSS score added · CISA ADP
  3. 2026-11-05 CISA SSVC decision points changed
  4. 2026-11-05 Priority changed: LOW → MEDIUM
  5. 2026-11-06 Affected versions changed · Red Hat
  6. 2026-11-06 CVSS score added · Red Hat

References

Patch 1

Advisory 1

  • TI-2019-13616 An issue was discovered in Symfony 3.4.0 through 3.4.34, 4.2.0 through 4.2.11, and 4.3.0… CVSS 3.1CRITICAL 9.8
  • TI-2018-10604 An issue was discovered in Http Foundation in Symfony 2.7.0 through 2.7.48, 2.8.0 through… CVSS 3.1MEDIUM 6.5
  • TI-2019-08131 In Symfony before 2.7.51, 2.8.x before 2.8.50, 3.x before 3.4.26, 4.x before 4.1.12, and… CVSS 3.1CRITICAL 9.8
  • TI-2016-02159 Symfony before 2.8.6 and 3.x before 3.0.6 allows remote attackers to bypass… CVSS 3.0CRITICAL 9.8
  • TI-2019-08459 An issue was discovered in Symfony before 4.2.12 and 4.3.x before 4.3.8. CVSS 3.1CRITICAL 9.8
  • TI-2018-08136 An issue was discovered in the Ldap component in Symfony 2.8.x before 2.8.37, 3.3.x… CVSS 3.0CRITICAL 9.8
See all 107 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences