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Low, CVSS 2.7

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

CVE-2026-45304GHSA-4qpc-3hr4-r2p4

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (symfony/symfony 8.0.12, 7.4.12, 6.4.40, 5.4.52; symfony/yaml 8.0.12, 7.4.12, 6.4.40, 5.4.52), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • Packagist symfony/symfony

    Affected
    < 5.4.52>= 6.0.0, < 6.4.40>= 7.0.0, < 7.4.12>= 8.0.0, < 8.0.12
    Fixed in
    8.0.12, 7.4.12, 6.4.40, 5.4.52

  • Packagist symfony/yaml

    Affected
    < 5.4.52>= 6.0.0, < 6.4.40>= 7.0.0, < 7.4.12>= 8.0.0, < 8.0.12
    Fixed in
    8.0.12, 7.4.12, 6.4.40, 5.4.52

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
2.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U · score source
EPSS
  • CVE-2026-45304: 0.68% estimated exploitation activity in the next 30 days; percentile 52.62 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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.…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-4qpc-3hr4-r2p4

Agent brief

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# Patch Next advisory brief: CVE-2026-45304

Advisory: Symfony's YAML Parser Vulnerable to Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs")
IDs: CVE-2026-45304, GHSA-4qpc-3hr4-r2p4
Record: https://patchnext.com/advisories/cve-2026-45304 (revision 1, source checked 26 Oct 2026, 14:48 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: symfony/symfony 8.0.12, 7.4.12, 6.4.40, 5.4.52; symfony/yaml 8.0.12, 7.4.12, 6.4.40, 5.4.52. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist symfony/symfony: affected < 5.4.52; >= 6.0.0, < 6.4.40; >= 7.0.0, < 7.4.12; >= 8.0.0, < 8.0.12; fixed in 8.0.12, 7.4.12, 6.4.40, 5.4.52
- Packagist symfony/yaml: affected < 5.4.52; >= 6.0.0, < 6.4.40; >= 7.0.0, < 7.4.12; >= 8.0.0, < 8.0.12; fixed in 8.0.12, 7.4.12, 6.4.40, 5.4.52

## Suggested inspection (read-only)
- Packagist symfony/symfony: check the installed version with your package manager.
- Packagist symfony/yaml: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on symfony/symfony, symfony/yaml.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4qpc-3hr4-r2p4 (GitHub-reviewed advisory GHSA-4qpc-3hr4-r2p4 lists affected ranges: Packagist symfony/yaml >=0 =6.0.0 =7.0.0 =8.0.0 =0 =6.0.0 =7.0.0 =8.0.0 <8.0.12)

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