CVE-2026-45304

CVE-2026-45304 is a low-severity security vulnerability in symfony/yaml (composer), affecting versions < 5.4.52. It is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12.

Does this CVE actually affect you?

Kodem shows which CVEs are reachable and running in your applications, so you fix what's exploitable, not just what's listed.

Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

Runtime intelligence, not another scanner.

Summary

Symfony's YAML Parser Vulnerable to Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs")

Description

Symfony\Component\Yaml\Parser resolves YAML aliases (*anchor) during parsing. Aliases that reference collections (arrays, stdClass, TaggedValue-wrapped collections) can themselves point to other collections containing aliases, creating exponential expansion at resolution time. A small input can blow up into a multi-gigabyte structure and exhaust memory: the classic "Billion Laughs" denial-of-service against any parser exposed to untrusted YAML.

Resolution

The Parser now counts collection alias resolutions in a shared ParserState object, with a default limit of 128, following the SnakeYAML model. Scalar aliases remain unrestricted since they cannot drive exponential growth. The limit is configurable via a new $maxAliasesForCollections argument on Parser::__construct(), Yaml::parse() and Yaml::parseFile(). A new Yaml::PARSE_EXCEPTION_ON_ALIAS flag also rejects all aliases outright when parsing fully untrusted input.

The patch for this issue is available here for branch 5.4.

Credits

Symfony would like to thank Pietro Tirenna (Shielder) for reporting the issue and Nicolas Grekas for fixing it.

Impact

Affected versions

symfony/yaml (< 5.4.52) symfony/symfony (< 5.4.52) symfony/symfony (>= 6.0.0, < 6.4.40) symfony/symfony (>= 7.0.0, < 7.4.12) symfony/symfony (>= 8.0.0, < 8.0.12) symfony/yaml (>= 6.0.0, < 6.4.40) symfony/yaml (>= 7.0.0, < 7.4.12) symfony/yaml (>= 8.0.0, < 8.0.12)

Security releases

symfony/yaml → 5.4.52 (composer) symfony/symfony → 5.4.52 (composer) symfony/symfony → 6.4.40 (composer) symfony/symfony → 7.4.12 (composer) symfony/symfony → 8.0.12 (composer) symfony/yaml → 6.4.40 (composer) symfony/yaml → 7.4.12 (composer) symfony/yaml → 8.0.12 (composer)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

Already deployed Kodem?

See it in your environmentNew to Kodem? Get a demo →

Remediation advice

Upgrade the following packages to resolve this vulnerability:

symfony/yaml to 5.4.52 or later; symfony/symfony to 5.4.52 or later; symfony/symfony to 6.4.40 or later; symfony/symfony to 7.4.12 or later; symfony/symfony to 8.0.12 or later; symfony/yaml to 6.4.40 or later; symfony/yaml to 7.4.12 or later; symfony/yaml to 8.0.12 or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

  1. What is CVE-2026-45304? CVE-2026-45304 is a low-severity security vulnerability in symfony/yaml (composer), affecting versions < 5.4.52. It is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12.
  2. Which packages are affected by CVE-2026-45304?
    • symfony/yaml (composer) (versions < 5.4.52)
    • symfony/symfony (composer) (versions < 5.4.52)
  3. Is there a fix for CVE-2026-45304? Yes. CVE-2026-45304 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.
  4. Is CVE-2026-45304 exploitable, and should I be worried? Whether CVE-2026-45304 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  5. What actually determines whether CVE-2026-45304 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  6. How do I fix CVE-2026-45304?
    • Upgrade symfony/yaml to 5.4.52 or later
    • Upgrade symfony/symfony to 5.4.52 or later
    • Upgrade symfony/symfony to 6.4.40 or later
    • Upgrade symfony/symfony to 7.4.12 or later
    • Upgrade symfony/symfony to 8.0.12 or later
    • Upgrade symfony/yaml to 6.4.40 or later
    • Upgrade symfony/yaml to 7.4.12 or later
    • Upgrade symfony/yaml to 8.0.12 or later

Other vulnerabilities in symfony/yaml

CVE-IDSeverityPackage summary
CVE-2026-45305Lowsymfony/yaml: Symfony's YAML Parser has a ReDoS via Catastrophic Backtracking in Parser::cleanup() RegexCVE-2026-45133Lowsymfony/yaml: Symfony hardened the parser when handling untrusted inputCVE-2013-1348Highsymfony/symfony: Symphony Vulnerable to PHP Code Injection via YAML ParsingCVE-2013-1397Highsymfony/symfony: Symfony Arbitrary PHP code Execution

Stop the waste.
Protect your environment with Kodem.

Get a personalized demo
Get a personalized demo