Advisory details:
github_osv / GHSA-4qpc-3hr4-r2p4
| Advisory ID | github_osv/GHSA-4qpc-3hr4-r2p4 |
| Aliases |
CVE-2026-45304
|
| 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](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. |
| Status | Published |
| Exploitability | 0.5 |
| Weighted Severity | 2.4 |
| Risk | 1.2 |
| Affected and Fixed Packages | Package Details |
| Introduced and Fixed Package Commit Patches | Package Commit Patches Details |
| System | Score | Found at |
|---|---|---|
| cvssv4 | 2.7 | https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-4qpc-3hr4-r2p4/GHSA-4qpc-3hr4-r2p4.json |
| generic_textual | LOW |
No EPSS data available for this advisory.
| Patch URL | |
|---|---|
| There are no known patches. | |
| Attack Vector (AV) | Attack Complexity (AC) | Attack Requirements (AT) | Privileges Required (PR) | User Interaction (UI) | Vulnerable System Impact Confidentiality (VC) | Vulnerable System Impact Integrity (VI) | Vulnerable System Impact Availability (VA) | Subsequent System Impact Confidentiality (SC) | Subsequent System Impact Integrity (SI) | Subsequent System Impact Availability (SA) |
|---|---|---|---|---|---|---|---|---|---|---|
network adjacent local physical |
low high |
none present |
none low high |
none passive active |
high low none |
high low none |
high low none |
high low none |
high low none |
high low none |
Vector: SSVCv2/E:N/A:Y/T:P/P:M/B:A/M:M/D:T/2026-07-14T19:13:27Z/
Decision: Track
Source URL: https://github.com/cisagov/vulnrichment/blob/develop/2026/45xxx/CVE-2026-45304.json
Source Advisory: vulnrichment/CVE-2026-45304
[{'Exploitation': 'none'},
{'Automatable': 'yes'},
{'Technical Impact': 'partial'},
{'Mission Prevalence': 'minimal'},
{'Public Well-being Impact': 'material'},
{'Mission & Well-being': 'medium'}]