What is CVE-2026-45305?

CVE-2026-45305 is a low-severity inefficient regular expression (ReDoS) 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. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.

Which packages are affected by CVE-2026-45305?

symfony/yaml (composer) versions < 5.4.52; symfony/symfony (composer) versions < 5.4.52.

Is there a fix for CVE-2026-45305?

Yes. CVE-2026-45305 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.

Is CVE-2026-45305 exploitable, and should I be worried?

Whether CVE-2026-45305 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.

What actually determines whether CVE-2026-45305 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.

CVE-2026-45305 in symfony/yaml

Q: Which packages are affected by CVE-2026-45305?

symfony/yaml (composer) versions < 5.4.52; symfony/symfony (composer) versions < 5.4.52.

Q: Is there a fix for CVE-2026-45305?

Yes. CVE-2026-45305 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.

Q: Is CVE-2026-45305 exploitable, and should I be worried?

Whether CVE-2026-45305 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.

Q: What actually determines whether CVE-2026-45305 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.

Q: How do I fix CVE-2026-45305?

Upgrade: 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.

Summary

Symfony's YAML Parser has a ReDoS via Catastrophic Backtracking in Parser::cleanup() Regex

Description

Symfony\Component\Yaml\Parser::cleanup() strips the optional %YAML directive header, leading comments, and document start/end markers before parsing. The original regexes contained overlapping quantifiers, most notably '#^%YAML[: ][\d.]+.*\n#u', whose [\d.]+ and .* overlap on the dot, that exhibit catastrophic backtracking on crafted input. A single oversized %YAML directive header (or comment / document-marker line) makes the parser hang for an arbitrarily long time, denying service.

Resolution

The four regexes in Parser::cleanup() (YAML directive header, leading comments, document-start marker, document-end marker) have been rewritten with possessive quantifiers and unambiguous character classes so backtracking cannot occur.

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

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

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.

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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

What is CVE-2026-45305? CVE-2026-45305 is a low-severity inefficient regular expression (ReDoS) 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. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
Which packages are affected by CVE-2026-45305?
- symfony/yaml (composer) (versions < 5.4.52)
- symfony/symfony (composer) (versions < 5.4.52)
Is there a fix for CVE-2026-45305? Yes. CVE-2026-45305 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.
Is CVE-2026-45305 exploitable, and should I be worried? Whether CVE-2026-45305 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
What actually determines whether CVE-2026-45305 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.
How do I fix CVE-2026-45305?
- 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-2026-45304Lowsymfony/yaml: Symfony's YAML Parser Vulnerable to Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs")CVE-2026-45133Lowsymfony/yaml: Symfony hardened the parser when handling untrusted input CVE-2013-1348Highsymfony/symfony: Symphony Vulnerable to PHP Code Injection via YAML Parsing CVE-2013-1397Highsymfony/symfony: Symfony Arbitrary PHP code Execution