CVE-2026-50013 Overview
Hoverfly, an open source API simulation tool, contains a race condition vulnerability in versions prior to 1.12.8. When Hoverfly runs in Diff mode, the AddDiff() function writes to the shared responsesDiff map without any synchronization primitive such as a mutex. Concurrent proxy requests trigger Go's built-in race detector, producing a fatal error: concurrent map read and map write that terminates the entire Hoverfly process. Attackers can trigger this condition by sending multiple simultaneous requests to the proxy. The flaw is tracked under CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization). Version 1.12.8 patches the issue.
Critical Impact
Unauthenticated remote attackers can crash Hoverfly instances running in Diff mode by issuing concurrent HTTP requests, causing service-wide denial of service.
Affected Products
- Hoverfly versions prior to 1.12.8
- SpectoLabs Hoverfly instances running in Diff mode
- API simulation environments proxying concurrent traffic through Hoverfly
Discovery Timeline
- 2026-09-11 - CVE-2026-50013 published to NVD
- 2026-09-14 - Last updated in NVD database
Technical Details for CVE-2026-50013
Vulnerability Analysis
Hoverfly's Diff mode captures differences between simulated and real API responses for later analysis. The AddDiff() function stores these differences in a shared map named responsesDiff. The implementation omits any synchronization primitive to guard concurrent access to that map.
Go's runtime enforces safety for concurrent map operations. When two goroutines write to the same map without a mutex, the runtime raises a fatal error rather than allowing memory corruption. This behavior turns an unsynchronized write into an immediate process termination.
Because Hoverfly operates as a proxy, concurrent requests are the default operational pattern rather than an edge case. Any attacker able to reach the proxy can reliably trigger the race by sending parallel requests, resulting in complete loss of availability. See the GitHub Security Advisory GHSA-qrh4-p6v4-mrfg for the vendor writeup.
Root Cause
The root cause is a missing mutex around writes to the responsesDiff map inside AddDiff(). The developer treated the map as goroutine-safe, but Go maps require external synchronization such as sync.Mutex or sync.Map for concurrent writers.
Attack Vector
Exploitation requires network access to a Hoverfly proxy configured in Diff mode. No authentication or user interaction is needed. Sending two or more overlapping HTTP requests through the proxy is sufficient to trigger the fatal runtime error and crash the process.
No verified public exploit code is available. The trigger consists of concurrent HTTP requests to any proxied endpoint. Details of the fix are documented in GitHub Pull Request #1227.
Detection Methods for CVE-2026-50013
Indicators of Compromise
- Hoverfly process logs containing the string fatal error: concurrent map read and map write.
- Unexpected Hoverfly process termination followed by service restart events in the container or systemd journal.
- Spikes of concurrent inbound requests to the Hoverfly proxy port immediately preceding a crash.
Detection Strategies
- Monitor Hoverfly stderr output for Go runtime fatal errors referencing map access.
- Correlate process exit events with inbound request bursts to identify triggering traffic.
- Deploy synthetic health checks that alert when the Hoverfly admin API becomes unreachable.
Monitoring Recommendations
- Ingest Hoverfly container and host logs into a centralized logging platform for keyword-based alerting on fatal error.
- Track process restart counts and mean time between failures for Hoverfly workloads.
- Alert on abnormal concurrency patterns targeting the proxy listener from unexpected source IPs.
How to Mitigate CVE-2026-50013
Immediate Actions Required
- Upgrade Hoverfly to version 1.12.8 or later, which introduces synchronization around the responsesDiff map.
- Disable Diff mode on Hoverfly instances that cannot be upgraded immediately.
- Restrict network exposure of the Hoverfly proxy to trusted internal networks only.
Patch Information
The fix is included in Hoverfly Release v1.12.8. The code change is available in Pull Request #1227, which adds mutex protection around concurrent writes to the shared diff map.
Workarounds
- Run Hoverfly in Simulate, Capture, or Modify mode instead of Diff mode until patching is complete.
- Place Hoverfly behind an authenticated reverse proxy that rate limits concurrent connections.
- Use a process supervisor such as systemd or Kubernetes with automatic restart to reduce downtime while upgrades are staged.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
