Symfony ExpressionLanguage: Deeply Nested Expressions Crash PHP

Published as "CVE-2026-109449: Unbounded Nesting in the Expression Parser Lets Untrusted Expressions Crash the Process"

medium Symfony Symfony components · Last reviewed Oct 31, 2026

CVE: CVE-2026-109449
GHSA: None
Packagist: PKSA-vq1j-sc3k-jzzk
Reported: Oct 14, 2026

Lets an attacker crash the PHP process with one request in apps that parse or validate expressions written by users.

Affected and fixed versions

In 2 packages: symfony/expression-language 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 – 8.1.8	8.1.9	Supported
7.4.0 – 7.4.20	7.4.21	Supported
6.4.0 – 6.4.47	6.4.48	Supported
5.4.0 – 5.4.53	5.4.54	Supported

Show 16 older branches, no longer supported

8.0.0 – 8.0.x	8.1.9
7.0.0 – 7.0.x	7.4.21
7.1.0 – 7.1.x	7.4.21
7.2.0 – 7.2.x	7.4.21
7.3.0 – 7.3.x	7.4.21
6.0.0 – 6.0.x	6.4.48
6.1.0 – 6.1.x	6.4.48
6.2.0 – 6.2.x	6.4.48
6.3.0 – 6.3.x	6.4.48
2.4.0 – 2.x	5.4.54
3.0.0 – 3.x	5.4.54
4.0.0 – 4.x	5.4.54
5.0.0 – 5.0.x	5.4.54
5.1.0 – 5.1.x	5.4.54
5.2.0 – 5.2.x	5.4.54
5.3.0 – 5.3.x	5.4.54

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/expression-language:^8.1.9
composer require symfony/expression-language:^7.4.21
composer require symfony/expression-language:^6.4.48
composer require symfony/expression-language:^5.4.54

What this means

An attacker who can send an expression to your app can crash the PHP process with a single, deeply nested expression. The ExpressionLanguage parser had no limit on how deep an expression could nest.

Who is affected

Apps that parse expressions from untrusted users with symfony/expression-language (also part of symfony/symfony). That includes apps that check user input with the #[Assert\ExpressionSyntax] constraint, because it parses the value it validates. Apps that only parse expressions their own developers wrote aren't exposed.

What to do

Update to 5.4.54, 6.4.48, 7.4.21 or 8.1.9. If you're on 8.0, update to 8.1.9.

The first fix, in 6.4.45, 7.4.18 and 8.1.6, missed some ways of nesting, so those releases are still affected. See the Symfony advisory and the fix. The parser now rejects anything nested more than 256 levels deep as an invalid expression.

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 ExpressionLanguage: Deeply Nested Expressions Crash PHP (CVE-2026-109449)
Details: https://phpnews.net/security/symfony/cve-2026-109449

Affected versions, as Composer constraints:
- symfony/expression-language, symfony/symfony: >=2.4.0,<3.0.0|>=3.0.0,<4.0.0|>=4.0.0,<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,<5.4.54|>=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,<6.4.48|>=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,<7.4.21|>=8.0.0,<8.1.0|>=8.1.0,<8.1.9

Fixed releases on branches that still get security fixes:
- 8.1.0 – 8.1.8: update to 8.1.9
- 7.4.0 – 7.4.20: update to 7.4.21
- 6.4.0 – 6.4.47: update to 6.4.48
- 5.4.0 – 5.4.53: update to 5.4.54

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-2026-109449 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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