CVE-2026-45304: YAML Parser Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs")

Published May 20, 2026
·
Updated

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::PARSEEXCEPTIONONALIAS 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.

Other sources

CVE-2026-45304: YAML Parser Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs")

— Packagist

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, Symfony\Component\Yaml\Parser resolved YAML collection aliases recursively, allowing a small untrusted YAML input to expand into a multi-gigabyte structure and exhaust memory. This issue is fixed in versions 5.4.52, 6.4.40, 7.4.12, and 8.0.12.

— NVD

Affected Software

49 affected componentsFixes available
composer/symfony/yaml>=8.0.0<8.0.12
8.0.12
composer/symfony/yaml>=7.0.0<7.4.12
7.4.12
composer/symfony/yaml>=6.0.0<6.4.40
6.4.40
composer/symfony/symfony>=8.0.0<8.0.12
8.0.12
composer/symfony/symfony>=7.0.0<7.4.12
7.4.12
composer/symfony/symfony>=6.0.0<6.4.40
6.4.40
composer/symfony/symfony<5.4.52
5.4.52
composer/symfony/yaml<5.4.52
5.4.52
SensioLabs Symfony<5.4.52
SensioLabs Symfony>=6.0.0<6.4.40
SensioLabs Symfony>=7.0.0<7.4.12
SensioLabs Symfony>=8.0.0<8.0.12
composer/symfony/symfony>=2.0.0, <3.0.0
composer/symfony/symfony>=3.0.0, <4.0.0
composer/symfony/symfony>=4.0.0, <5.0.0
composer/symfony/symfony>=5.0.0, <5.1.0
composer/symfony/symfony>=5.1.0, <5.2.0
composer/symfony/symfony>=5.2.0, <5.3.0
composer/symfony/symfony>=5.3.0, <5.4.0
composer/symfony/symfony>=5.4.0, <5.4.52
5.4.52
composer/symfony/symfony>=6.0.0, <6.1.0
composer/symfony/symfony>=6.1.0, <6.2.0
composer/symfony/symfony>=6.2.0, <6.3.0
composer/symfony/symfony>=6.3.0, <6.4.0
composer/symfony/symfony>=6.4.0, <6.4.40
6.4.40
composer/symfony/symfony>=7.0.0, <7.1.0
composer/symfony/symfony>=7.1.0, <7.2.0
composer/symfony/symfony>=7.3.0, <7.4.0
composer/symfony/symfony>=7.4.0, <7.4.12
7.4.12
composer/symfony/symfony>=8.0.0, <8.0.12
8.0.12
composer/symfony/yaml>=2.0.0, <3.0.0
composer/symfony/yaml>=3.0.0, <4.0.0
composer/symfony/yaml>=4.0.0, <5.0.0
composer/symfony/yaml>=5.0.0, <5.1.0
composer/symfony/yaml>=5.1.0, <5.2.0
composer/symfony/yaml>=5.2.0, <5.3.0
composer/symfony/yaml>=5.3.0, <5.4.0
composer/symfony/yaml>=5.4.0, <5.4.52
5.4.52
composer/symfony/yaml>=6.0.0, <6.1.0
composer/symfony/yaml>=6.1.0, <6.2.0
composer/symfony/yaml>=6.2.0, <6.3.0
composer/symfony/yaml>=6.3.0, <6.4.0
composer/symfony/yaml>=6.4.0, <6.4.40
6.4.40
composer/symfony/yaml>=7.0.0, <7.1.0
composer/symfony/yaml>=7.1.0, <7.2.0
composer/symfony/yaml>=7.3.0, <7.4.0
composer/symfony/yaml>=7.4.0, <7.4.12
7.4.12
composer/symfony/yaml>=8.0.0, <8.0.12
8.0.12
debian/symfony
5.4.53+dfsg-0+deb12u16.4.41+dfsg-0+deb13u16.4.49+dfsg-0+deb13u17.4.22+dfsg-17.4.23+dfsg-1

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade composer/symfony/yaml to a version that resolves this vulnerability.

    Fixed in 8.0.12
  2. Upgrade

    Upgrade composer/symfony/yaml to a version that resolves this vulnerability.

    Fixed in 7.4.12
  3. Upgrade

    Upgrade composer/symfony/yaml to a version that resolves this vulnerability.

    Fixed in 6.4.40
  4. Upgrade

    Upgrade composer/symfony/symfony to a version that resolves this vulnerability.

    Fixed in 8.0.12
  5. Upgrade

    Upgrade composer/symfony/symfony to a version that resolves this vulnerability.

    Fixed in 7.4.12
  6. Upgrade

    Upgrade composer/symfony/symfony to a version that resolves this vulnerability.

    Fixed in 6.4.40
  7. Upgrade

    Upgrade composer/symfony/symfony to a version that resolves this vulnerability.

    Fixed in 5.4.52
  8. Upgrade

    Upgrade composer/symfony/yaml to a version that resolves this vulnerability.

    Fixed in 5.4.52
  9. Upgrade

    Upgrade debian/symfony to a version that resolves this vulnerability.

    Fixed in 5.4.53+dfsg-0+deb12u1Fixed in 6.4.41+dfsg-0+deb13u1Fixed in 6.4.49+dfsg-0+deb13u1Fixed in 7.4.22+dfsg-1Fixed in 7.4.23+dfsg-1
  10. Upgrade

    Upgrade Symfony to a version that resolves this vulnerability.

    Fixed in 5.4.52
  11. Upgrade

    Upgrade Symfony to a version that resolves this vulnerability.

    Fixed in 6.4.40
  12. Upgrade

    Upgrade Symfony to a version that resolves this vulnerability.

    Fixed in 7.4.12
  13. Upgrade

    Upgrade Symfony to a version that resolves this vulnerability.

    Fixed in 8.0.12
  14. Configuration

    Enable Yaml::PARSE_EXCEPTION_ON_ALIAS when parsing fully untrusted YAML input to reject all aliases.

    Symfony YAML parser Yaml::PARSE_EXCEPTION_ON_ALIAS = enabled
  15. Configuration

    Configure the maximum number of collection-alias resolutions to 128 via Parser::__construct(), Yaml::parse(), or Yaml::parseFile().

    Symfony YAML parser maxAliasesForCollections = 128

Event History

May 20, 2026
Advisory Published
via Packagist·08:00 AM
Data Sourced
via Packagist·08:00 AM
DescriptionSeverityAffected Software
May 27, 2026
Advisory Published
via GitHub·09:33 PM
Data Sourced
via GitHub·09:33 PM
DescriptionWeaknessAffected Software
Jul 14, 2026
CVE Published
via MITRE·06:48 PM
Data Sourced
via MITRE·06:48 PM
DescriptionWeakness
Data Sourced
via NVD·07:17 PM
RemedyDescriptionSeverityWeaknessAffected Software

Parent advisories

This vulnerability appears in the following advisories.

Frequently Asked Questions

1

What is the severity of CVE-2026-45304?

CVE-2026-45304 has a risk level of 40.

2

What are the affected software components for CVE-2026-45304?

The affected software components include composer/symfony/yaml and composer/symfony/symfony.

3

How do I resolve the vulnerability CVE-2026-45304?

To resolve CVE-2026-45304, update the affected Symfony components to the latest secure versions.

4

What are the potential risks associated with CVE-2026-45304?

CVE-2026-45304 can lead to exponential expansion during YAML parsing, potentially causing denial of service.

5

When was CVE-2026-45304 published?

CVE-2026-45304 was published on May 27, 2026.

Company

Resources

Contact

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