Symfony HtmlSanitizer: srcset Images Skip Media Scheme and Host Checks

Published as "HtmlSanitizer Does Not URL-Sanitize srcset and imagesrcset: Media Scheme and Host Allow-List Bypass"

low Symfony Symfony components · Last reviewed Nov 10, 2026

CVE: CVE-2026-121949
GHSA: None
Packagist: PKSA-wwnc-rwy3-v11m
Reported: Nov 4, 2026

Lets sanitized HTML load images from hosts outside allowMediaHosts() through srcset or imagesrcset attributes.

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 passed srcset and imagesrcset attribute values through untouched. Even when you limit images to certain hosts or schemes, sanitized HTML could make a visitor's browser load images from anywhere. It doesn't let an attacker run JavaScript.

The src attribute on the same <img> was checked correctly. No URL check was registered for these two attributes. See the Symfony advisory and the fix.

Who is affected

Apps that sanitize untrusted HTML with symfony/html-sanitizer when both of these are true:

The config allows srcset or imagesrcset, through allowSafeElements(), allowStaticElements() or allowing them by name.
The app relies on allowMediaHosts(), allowMediaSchemes() or forceHttpsUrls() to limit media URLs.

<img>, <source> and <link rel="preload"> elements are all affected. If you don't restrict media hosts or schemes, there's no rule to get around. Symfony rates the issue low (CVSS 2.3).

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. After the update, each URL in a srcset list is checked against your media rules, and the attribute is dropped when none pass. This fix depends on the one for CVE-2026-118517, which ships in the same releases.

Reported independently by Shriyansh Gupta, Ilia Alshanetsky and 李智遥.

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: srcset Images Skip Media Scheme and Host Checks (CVE-2026-121949)
Details: https://phpnews.net/security/symfony/pksa-t3wq-xfwt-t1h9

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/pksa-t3wq-xfwt-t1h9 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