graphql-php is affected by a Denial of Service via quadratic complexity in OverlappingFieldsCanBeMerged validation

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 (ashwakn04@gmail.com)