CVE-2026-40476: graphql-go DOS Vulnerability

CVE-2026-40476 Overview

CVE-2026-40476 is an Algorithmic Complexity Attack vulnerability affecting graphql-go, a Go implementation of GraphQL. In versions 15.31.4 and below, the OverlappingFieldsCanBeMerged validation rule performs O(n²) pairwise comparisons of fields sharing the same response name. An attacker can exploit this by sending a query with thousands of repeated identical fields, causing excessive CPU usage during the validation phase before query execution begins. This vulnerability is particularly dangerous because it bypasses existing QueryDepth and QueryComplexity mitigation rules.

Critical Impact

Unauthenticated remote attackers can cause denial of service conditions by submitting specially crafted GraphQL queries, leading to excessive CPU consumption and potential service unavailability.

Affected Products

• graphql-go versions 15.31.4 and below
• Go implementations utilizing the vulnerable OverlappingFieldsCanBeMerged validation rule

Discovery Timeline

• 2026-04-17 - CVE CVE-2026-40476 published to NVD
• 2026-04-20 - Last updated in NVD database

Technical Details for CVE-2026-40476

Vulnerability Analysis

This vulnerability stems from an inefficient algorithmic implementation in the GraphQL validation layer. The OverlappingFieldsCanBeMerged rule is designed to ensure that fields with the same response name can be merged correctly during query resolution. However, the implementation uses a pairwise comparison approach with O(n²) time complexity, making it vulnerable to abuse.

When processing a GraphQL query, the validation engine compares each field against every other field with the same response name to check for conflicts. While this works efficiently for typical queries with a small number of fields, an attacker can weaponize this behavior by constructing queries containing thousands of identical field selections. The quadratic growth in comparison operations leads to severe CPU exhaustion.

The attack is particularly effective because it targets the validation phase, which occurs before any rate limiting or complexity analysis based on query execution. Standard GraphQL security measures like QueryDepth limits and QueryComplexity calculations do not prevent this attack since they evaluate the query structure rather than the validation algorithm's performance characteristics.

Root Cause

The root cause is CWE-407: Inefficient Algorithmic Complexity. The OverlappingFieldsCanBeMerged validation rule implements field comparison using a nested iteration pattern that scales quadratically with the number of fields sharing the same response name. This design fails to account for maliciously crafted inputs containing an abnormally high number of repeated fields, creating an exploitable resource consumption vulnerability.

Attack Vector

This vulnerability is exploitable over the network without authentication. An attacker can craft a malicious GraphQL query containing thousands of fields with identical response names and submit it to any exposed GraphQL endpoint running a vulnerable version of graphql-go.

The attack executes during the query validation phase, meaning standard GraphQL query complexity limits offer no protection. The crafted query may appear structurally valid while triggering excessive computational overhead during field overlap validation.

A typical attack involves submitting a query with many repeated field selections (e.g., thousands of identical field names), which forces the validation engine into an O(n²) comparison loop. For example, a query with 10,000 repeated fields would trigger approximately 100 million pairwise comparisons, consuming significant CPU resources and potentially causing service degradation or denial of service for other users.

Detection Methods for CVE-2026-40476

Indicators of Compromise

• Unusual spikes in CPU utilization on GraphQL server processes without corresponding increases in legitimate traffic
• GraphQL requests with abnormally large query payloads containing repetitive field structures
• Increased request latency or timeout errors on GraphQL endpoints
• Web application firewall or API gateway logs showing large GraphQL queries with thousands of identical field names

Detection Strategies

• Implement request payload size limits at the network or application layer to prevent excessively large GraphQL queries
• Monitor for GraphQL queries containing an unusual number of field selections with identical response names
• Deploy anomaly detection on GraphQL endpoint CPU consumption patterns
• Configure logging to capture and analyze GraphQL query structures for suspicious repetitive patterns

Monitoring Recommendations

• Set up alerting for sustained high CPU usage on GraphQL service instances
• Monitor GraphQL request sizes and flag queries exceeding normal payload thresholds
• Track validation phase duration metrics to detect queries triggering excessive processing time
• Review web server access logs for repeated requests from single sources targeting GraphQL endpoints

How to Mitigate CVE-2026-40476

Immediate Actions Required

• Upgrade graphql-go to version 15.31.5 or later immediately
• Implement request payload size limits to prevent excessively large GraphQL queries
• Consider adding rate limiting on GraphQL endpoints to reduce the impact of potential attacks
• Review and monitor GraphQL endpoints for unusual query patterns or performance degradation

Patch Information

The vulnerability has been addressed in graphql-go version 15.31.5. Users should upgrade to this version or later to remediate the vulnerability. The fix optimizes the OverlappingFieldsCanBeMerged validation algorithm to prevent the O(n²) behavior exploited in this attack.

For more information, refer to the GitHub Release v15.31.5 and the GitHub Security Advisory GHSA-68jq-c3rv-pcrr.

Workarounds

• Implement request size limits at the reverse proxy or API gateway level to reject abnormally large GraphQL payloads
• Add custom validation middleware to limit the maximum number of fields per query before processing
• Deploy web application firewall rules to detect and block queries with excessive field repetition
• Consider temporarily disabling or rate-limiting affected GraphQL endpoints until patching is complete