Symfony HtmlSanitizer: Hostless URLs Slip Past Link and Media Host Lists

Published as "CVE-2026-118517: HtmlSanitizer allowLinkHosts() / allowMediaHosts() Bypass via URLs Without a Well-Formed Authority: Incomplete Fix for CVE-2026-45066"

low Symfony Symfony components · Last reviewed Nov 7, 2026

CVE: CVE-2026-118517
GHSA: None
Packagist: PKSA-n87g-t5cn-m1yq
Reported: Nov 4, 2026

Lets links or media point to hosts outside allowLinkHosts() or allowMediaHosts() when allowRelativeLinks or allowRelativeMedias is on.

Affected and fixed versions

In 2 packages: symfony/html-sanitizer and symfony/symfony. Find the row for the version you run, and update to the release next to it.

If you run	Update to	Support
8.1.0 – 8.1.10	8.1.11	Supported
7.4.0 – 7.4.22	7.4.23	Supported
6.4.0 – 6.4.49	6.4.50	Supported

Show 8 older branches, no longer supported

8.0.0 – 8.0.x	8.1.11
7.0.0 – 7.0.x	7.4.23
7.1.0 – 7.1.x	7.4.23
7.2.0 – 7.2.x	7.4.23
7.3.0 – 7.3.x	7.4.23
6.1.0 – 6.1.x	6.4.50
6.2.0 – 6.2.x	6.4.50
6.3.0 – 6.3.x	6.4.50

Upgrade

Run the line for the branch you are on. 1 other package listed above is affected too; update it the same way.

composer require symfony/html-sanitizer:^8.1.11
composer require symfony/html-sanitizer:^7.4.23
composer require symfony/html-sanitizer:^6.4.50

What this means

Symfony's HtmlSanitizer can keep links and media URLs that point to hosts you didn't allow. An attacker who can submit HTML to your app could get a link to their own site past your host allow-list.

This is an incomplete fix for CVE-2026-45066. The sanitizer and browsers read some URL spellings differently: the sanitizer sees no host and keeps the URL, while the browser finds a host and goes to it. See the Symfony advisory and the fix.

Who is affected

Your app is exposed if it sanitizes untrusted HTML with symfony/html-sanitizer and its HtmlSanitizerConfig combines either of these:

allowLinkHosts() with allowRelativeLinks(true)
allowMediaHosts() with allowRelativeMedias(true)

The default configuration is not affected. With relative links and relative media turned off, which is the default, every form of the bypass is blocked. Symfony rates the issue low for that reason.

What to do

Update symfony/html-sanitizer (or symfony/symfony) to 6.4.50, 7.4.23 or 8.1.11, depending on your branch. Symfony's advisory notes that turning off allowRelativeLinks() and allowRelativeMedias() removes every form of the bypass, if you can live without relative URLs until you update.

One behaviour change: a bare https://, or any URL with an empty host part, is now dropped where it used to be kept under allowRelativeLinks(true).

Reported by Ilia Alshanetsky.

Check your app with an agent

Paste this into a coding agent that can read your project, such as Claude Code, Codex or Amp. It checks your composer.lock against the versions above and tells you what to update, without changing anything.

Check whether the PHP project you have access to is affected by this security advisory, and tell me what to update. Do a read-only check: don't edit files, don't run composer update or composer require, and don't try to reproduce the flaw.

Advisory: Symfony HtmlSanitizer: Hostless URLs Slip Past Link and Media Host Lists (CVE-2026-118517)
Details: https://phpnews.net/security/symfony/cve-2026-118517

Affected versions, as Composer constraints:
- symfony/html-sanitizer, symfony/symfony: >=6.1.0,<6.2.0|>=6.2.0,<6.3.0|>=6.3.0,<6.4.0|>=6.4.0,<6.4.50|>=7.0.0,<7.1.0|>=7.1.0,<7.2.0|>=7.2.0,<7.3.0|>=7.3.0,<7.4.0|>=7.4.0,<7.4.23|>=8.0.0,<8.1.0|>=8.1.0,<8.1.11

Fixed releases on branches that still get security fixes:
- 8.1.0 – 8.1.10: update to 8.1.11
- 7.4.0 – 7.4.22: update to 7.4.23
- 6.4.0 – 6.4.49: update to 6.4.50

1. Find every composer.lock in the project outside vendor/ (a monorepo can have one per app) and read the installed version of each package above. Packages installed as dependencies of other packages count too.
2. Compare each installed version with its constraint: "|" separates alternative ranges, and "," means both bounds apply.
3. For each affected install, read "What this means" at https://phpnews.net/security/symfony/cve-2026-118517 and check whether the project meets the conditions it describes (a feature in use, a config value, a driver). Say which conditions you could check and which you couldn't.
4. Report a table: lock file | package | installed version | affected (yes, no or unknown) | conditions met (yes, no or unknown) | update to | evidence. For each affected package, give the command that would fix it, without running it: composer update <package> --with-dependencies when it is only installed as a dependency of another package, or composer require with the fixed version when composer.json requires it directly. If the installed version is on a branch with no fixed release listed, say it needs a move to a newer branch.

Sources

← All Symfony advisories