symfony/yaml security advisory

CVE-2026-45133: YAML Parser Stack Exhaustion via Unbounded Recursion in Nested Blocks, Sequences, and Mappings

CVE-2026-45133 is a low-severity denial of service vulnerability in the PHP package symfony/yaml (CVSS 7.5), published 14 July 2026. It is fixed in 5.4.52, 6.4.40, 7.4.12 and 8.0.12, one per release line; earlier versions on each line are affected. It can be exploited without an account.

low 7.5 published 2026-07-14 · updated 2026-07-15

Affected versions: >= 2.0.0, < 5.4.52
>= 6.0.0, < 6.4.40
>= 7.0.0, < 7.4.12
>= 8.0.0, < 8.0.12
Fixed in: 5.4.52, 6.4.40, 7.4.12, 8.0.12
Newest release: 8.1.10
Severity: Low · CVSS 7.5
Type: Denial of service
CWE-674, CWE-776, CWE-1333
Exploited in the wild: Not on the CISA KEV list
checked 2026-11-04

Also affects

The same advisory covers another package in our list. Fix whichever one your project installs.

symfony/symfony >= 2.0.0, < 5.4.52; >= 6.0.0, < 6.4.40; >= 7.0.0, < 7.4.12; >= 8.0.0, < 8.0.12 fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12

Is your symfony/yaml version affected?

Run composer show symfony/yaml in your project, then enter the version. We check it against this and every other known symfony/yaml advisory.

Who can exploit CVE-2026-45133

Access: No account. Anyone who can send a request to an app that uses the affected code.

CVSS vector (scored by NVD): CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Your app is only exposed if it uses the affected part of symfony/yaml. The advisory below says which part that is. If you are not sure, upgrade anyway: it costs less than finding out.

How to fix CVE-2026-45133 in symfony/yaml

Find the installed version: composer show symfony/yaml. If you never required it yourself, composer why symfony/yaml shows which package pulls it in.
Update it: composer update symfony/yaml --with-all-dependencies. You need the fixed version for your release line (5.4.52, 6.4.40, 7.4.12 and 8.0.12).
If Composer will not go that high, your composer.json constraint is holding it back. Widen it (for example "symfony/yaml": "^8.0.12" if you can take that version), or upgrade the package that requires it.
Run composer audit. It reads the same advisory data and should no longer list CVE-2026-45133.
Commit composer.lock and deploy it. On the server, run composer install --no-dev from the lock file, not composer update.

If you cannot upgrade yet

General hardening for denial of service issues while the upgrade waits. None of it replaces the fix.

Rate-limit the affected endpoint and cap request body, header and upload sizes at the web server.
If your app does not use the affected feature, note that in your audit, but still plan the upgrade.

The CVE-2026-45133 advisory

Symfony is a PHP framework for web and console applications and a set of reusable PHP components. Prior to 5.4.52, 6.4.40, 7.4.12, and 8.0.12, when the parser is exposed to attacker-controlled input, deeply nested mappings or sequences cause both the block-level (Parser::parseBlock()) and inline (Inline::parseSequence() / Inline::parseMapping()) parsers to recurse without a depth limit. A crafted document exhausts the PHP stack and crashes the worker. This issue is fixed in versions 5.4.52, 6.4.40, 7.4.12, and 8.0.12.

Description from the CVE record, published by GitHub (the CVE Numbering Authority), via the NVD entry for CVE-2026-45133. Advisory title: “CVE-2026-45133: YAML Parser Stack Exhaustion via Unbounded Recursion in Nested Blocks, Sequences, and Mappings”.

References

GitHub security advisory GHSA-c2p3-7m5p-cv8x github.com
Symfony security advisory symfony.com
Release v5.4.52 github.com
Release v6.4.40 github.com
Release v7.4.12 github.com
Release v8.0.12 github.com

Other symfony/yaml advisories

high CVE-2026-109413: Unbounded Recursion in the YAML Inline Lexer Lets a Single Document Crash the Process 2026-08-30
low CVE-2026-45304: YAML Parser Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs") 2026-05-20
low CVE-2026-45305: YAML Parser ReDoS via Catastrophic Backtracking in Parser::cleanup() Regex 2026-05-20
high CVE-2013-1397: Ability to enable/disable object support in YAML parsing and dumping 2013-01-15
high CVE-2013-1348: Ability to enable/disable PHP parsing in Yaml::parse() 2013-01-15

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Where this data comes from

Affected and fixed versions: the Packagist security advisories API (GitHub Advisory Database and FriendsOfPHP), the same data composer audit uses. Synced 2026-11-03.
Description, score, weakness and references: NIST National Vulnerability Database (NVD), checked 2026-11-04. This product uses data from the NVD API but is not endorsed or certified by the NVD.
Exploited-in-the-wild status: the CISA Known Exploited Vulnerabilities catalog, checked 2026-11-04.
Compiled by Daniel Lasek at Laskyr from the sources above. The summary, “who can exploit it”, “how to fix it” and hardening sections are our plain-English reading of the record and are generated from it. The quoted description is the record's own text. Spot an error? Tell us.