CVE-2026-118517
Symfony: HtmlSanitizer allowLinkHosts() / allowMediaHosts() Bypass via URLs Without a Well-Formed Authority (Incomplete Fix for CVE-2026-45066)
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.
INFO
Published Date :
Nov. 5, 2026, 5:17 p.m.
Last Modified :
Nov. 5, 2026, 5:17 p.m.
Remotely Exploit :
Yes !
Source :
596c5446-0ce5-4ba2-aa66-48b3b757a647
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | ||||
| CVSS 4.0 | LOW | 596c5446-0ce5-4ba2-aa66-48b3b757a647 | ||||
| CVSS 4.0 | LOW | 596c5446-0ce5-4ba2-aa66-48b3b757a647 |
Solution
- Update Symfony to versions 6.4.50, 7.4.23, or 8.1.11.
- Review and apply vendor-recommended patches for HtmlSanitizer.
- Validate URL sanitization logic with specific test cases.
- Ensure host allow-lists are correctly configured.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-118517.
| URL | Resource |
|---|---|
| https://github.com/symfony/symfony/commit/f2f47d2eef6c2552c6c5c24953caa94f7901cb43 | |
| https://github.com/symfony/symfony/security/advisories/GHSA-c76g-9c4c-gxrw |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-118517 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-118517
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-118517 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-118517 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
New CVE Received by 596c5446-0ce5-4ba2-aa66-48b3b757a647
Nov. 05, 2026
Action Type Old Value New Value Added 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. Added CVSS V4.0 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 Added CWE CWE-184 Added CWE CWE-436 Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/118xxx/CVE-2026-118517.json">CVE-2026-118517</a> Added Reference https://github.com/symfony/symfony/commit/f2f47d2eef6c2552c6c5c24953caa94f7901cb43 Added Reference https://github.com/symfony/symfony/security/advisories/GHSA-c76g-9c4c-gxrw -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Nov. 05, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N Added SSVC {'id': 'CVE-2026-118517', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-11-05T16:49:56.410466Z'}