Symfony HttpKernel: Inline Fragments Let Forwarded Headers Change URLs

Published as "Inline Fragments Let Untrusted Forwarded Headers Set the Host, Scheme and Port of the URLs Generated for the Main Request"

low Symfony Symfony components · Last reviewed Nov 10, 2026

CVE: CVE-2026-123339
GHSA: None
Packagist: PKSA-3gxv-7xhq-mgmz
Reported: Nov 4, 2026

Lets client headers set the host of absolute URLs after an inline fragment renders, in apps reached from 127.0.0.1 without it as a trusted proxy.

Affected and fixed versions

In 2 packages: symfony/http-kernel 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
5.4.0 – 5.4.55	5.4.56	Supported

Show 16 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
6.0.0 – 6.0.x	6.4.50
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
4.0.14 – 4.x	5.4.56
5.0.0 – 5.0.x	5.4.56
5.1.0 – 5.1.x	5.4.56
5.2.0 – 5.2.x	5.4.56
5.3.0 – 5.3.x	5.4.56
3.4.14 – 3.x	4.0.0
2.8.44 – 2.x	3.0.0

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/http-kernel:^8.1.11
composer require symfony/http-kernel:^7.4.23
composer require symfony/http-kernel:^6.4.50
composer require symfony/http-kernel:^5.4.56

What this means

After a page renders an inline fragment, a client can choose the host, scheme and port of every absolute URL your app generates for the rest of that request. Those URLs can end up in pages a shared cache stores, or in emails your app sends, pointing users at the attacker's site.

While an inline fragment ran, Symfony briefly trusted 127.0.0.1 as a proxy. The router rebuilt its settings from the main request during that window, using forwarded headers the client sent.

Who is affected

Apps on symfony/http-kernel (also part of symfony/symfony) that render inline fragments, with render() in Twig or ESI tags without a gateway cache. The app must also be reached over HTTP from 127.0.0.1 without that address in framework.trusted_proxies. The Symfony advisory gives examples: a TLS terminator, reverse proxy, HTTP cache or sidecar on the same host.

Apps served through php-fpm, mod_php, or a proxy on another host aren't affected. With framework.trusted_hosts set, the host can't be changed, but the scheme and port still can.

What to do

Update to 5.4.56, 6.4.50, 7.4.23 or 8.1.11. If you're on 8.0, update to 8.1.11. The fix restores the router's settings after each fragment. Symfony also advises that a proxy on 127.0.0.1 set the forwarded headers itself rather than passing the client's along, and that you set framework.trusted_hosts.

Reported by Nicolas Grekas.

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 HttpKernel: Inline Fragments Let Forwarded Headers Change URLs (CVE-2026-123339)
Details: https://phpnews.net/security/symfony/pksa-68d7-b14m-3bjc

Affected versions, as Composer constraints:
- symfony/http-kernel, symfony/symfony: >=2.8.44,<3.0.0|>=3.4.14,<4.0.0|>=4.0.14,<5.0.0|>=5.0.0,<5.1.0|>=5.1.0,<5.2.0|>=5.2.0,<5.3.0|>=5.3.0,<5.4.0|>=5.4.0,<5.4.56|>=6.0.0,<6.1.0|>=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
- 5.4.0 – 5.4.55: update to 5.4.56

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-68d7-b14m-3bjc 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

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