CVE-2026-40476

The OverlappingFieldsCanBeMerged validation rule exhibits quadratic time complexity when processing queries with many repeated fields sharing the same response name. An attacker can send a crafted query like { hello hello hello ... } with thousands of repeated fields, causing excessive CPU usage during validation before execution begins.

This is not mitigated by existing QueryDepth or QueryComplexity rules.

Observed impact (tested on v15.31.4):

• 1000 fields: ~0.6s
• 2000 fields: ~2.4s
• 3000 fields: ~5.3s
• 5000 fields: request timeout (>20s)

Root cause: collectConflictsWithin() performs O(n²) pairwise comparisons of all fields with the same response name. For identical repeated fields, every comparison returns "no conflict" but the quadratic iteration count causes resource exhaustion.

Fix: Deduplicate structurally identical fields before pairwise comparison, reducing the complexity from O(n²) to O(u²) where u is the number of unique field signatures (typically 1 for this attack pattern).

Credit: Ashwak N ([email protected])

CVE-2026-40476 has a CVSS score of 7.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (15.31.5); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-40476? CVE-2026-40476 is a medium-severity security vulnerability in webonyx/graphql-php (composer), affecting versions <= 15.31.4. It is fixed in 15.31.5.
2. How severe is CVE-2026-40476? CVE-2026-40476 has a CVSS score of 7.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of webonyx/graphql-php are affected by CVE-2026-40476? webonyx/graphql-php (composer) versions <= 15.31.4 is affected.
4. Is there a fix for CVE-2026-40476? Yes. CVE-2026-40476 is fixed in 15.31.5. Upgrade to this version or later.
5. Is CVE-2026-40476 exploitable, and should I be worried? Whether CVE-2026-40476 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40476 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40476? Upgrade webonyx/graphql-php to 15.31.5 or later.