Skip to main content

Boteraser | Website and Server Security Solutions

🛡️ CVE-2026-45304 — symfony

🟠 CVSS 7.5 — High ✅ No Known Exploit CWE-776 NVD
7.5
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

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](https://github.com/snakeyaml/snakeyaml/blob/master/src/main/java/org/yaml/snakeyaml/LoaderOptions.java). 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](https://github.com/symfony/symfony/commit/e77391b2e4f18821198f010d573674c8ed4a970a) for branch 5.4.

Credits

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

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality none, integrity none, availability high.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: None.
  • Integrity impact: None.
  • Availability impact: High — total loss, or loss the attacker controls.

Weakness class

CVE-2026-45304 is classified as CWE-776: XML Entity Expansion. Nested entity definitions expand enormously when parsed, consuming memory and CPU.

Affected software

CVE-2026-45304 is recorded against 3 packages.

  • symfony (from 8.0.0 up to 8.0.12)
  • symfony/symfony (from 8.0.0 up to 8.0.12)
  • symfony/yaml (from 8.0.0 up to 8.0.12)

Timeline and source

Published on 14 July 2026 and last revised on 15 July 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

github.com
github.com
github.com
github.com
github.com
github.com

Other advisories for this package

symfony has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-776: XML Entity Expansion) in other software:

CVE-2026-45304 on other distributions

Each distribution ships its own build and its own fixed version. Pick the one you run:

Details

Severity HIGH
CVSS Score 7.5
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CWE CWE-776
Public Exploit ✅ No
Source NVD
Published 2026-07-14
Updated 2026-08-20
Modified 2026-07-15

Affected Packages

Software From version Fixed in
symfony 8.0.0 8.0.12
symfony/symfony 8.0.0 8.0.12
symfony/yaml 8.0.0 8.0.12

Similar Threats

Site Security Check

Is symfony part of your stack?

CVE-2026-45304 is rated CVSS 7.5 High. BotEraser scans your installation against known CVE records and tells you whether this vulnerability applies to the versions you actually run.

Scan My Site Free →

No credit card required  ·  Results in minutes

ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the vulnerabilities listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.

Browse related advisories

All advisoriesCVECVE 2026
✓