CVE-2026-6874: ericc-ch copilot-api RCE Vulnerability

CVE-2026-6874 Overview

A vulnerability has been identified in ericc-ch copilot-api versions up to 0.7.0. This security flaw affects the /token endpoint within the Header Handler component, where manipulation of the Host argument can lead to reliance on reverse DNS resolution for authentication or access control decisions. The vulnerability is remotely exploitable and has been publicly disclosed.

Critical Impact

Remote attackers can exploit this vulnerability to potentially bypass security controls by manipulating DNS resolution, which may lead to unauthorized access or security policy circumvention.

Affected Products

• ericc-ch copilot-api versions up to 0.7.0

Discovery Timeline

• 2026-04-23 - CVE CVE-2026-6874 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-6874

Vulnerability Analysis

This vulnerability falls under CWE-350 (Reliance on Reverse DNS Resolution for a Security-Critical Action). The copilot-api application performs reverse DNS lookups to validate or authorize requests, which is an inherently insecure practice. Reverse DNS records are controlled by the owner of the IP address space and can be configured to return arbitrary hostnames, making them unreliable for security decisions.

The affected component, the Header Handler processing requests to /token, appears to use the Host header value in conjunction with reverse DNS lookups. This design flaw allows attackers to potentially spoof their identity or bypass access restrictions by controlling DNS resolution results.

Root Cause

The root cause of this vulnerability is the application's reliance on reverse DNS resolution for security-critical operations. When the Header Handler processes requests to the /token endpoint, it trusts the hostname returned by reverse DNS queries without proper validation. This architectural flaw violates the principle that security decisions should not rely on easily spoofable DNS information.

Attack Vector

The attack can be performed remotely over the network. An attacker can exploit this vulnerability by:

1. Crafting HTTP requests to the /token endpoint with manipulated Host header values
2. Setting up DNS infrastructure to return attacker-controlled reverse DNS records
3. Exploiting the application's trust in the reverse DNS resolution results to bypass authentication or authorization checks

The exploit has been publicly disclosed. For technical details, see the GitHub CVE Issue Report and VulDB Vulnerability #359039.

Detection Methods for CVE-2026-6874

Indicators of Compromise

• Unusual or suspicious Host header values in requests to the /token endpoint
• Unexpected reverse DNS query patterns originating from the application server
• Authentication logs showing access from mismatched IP addresses and hostnames
• Anomalous traffic patterns targeting the /token endpoint

Detection Strategies

• Monitor HTTP access logs for requests to /token with abnormal Host header values
• Implement DNS query logging to detect unusual reverse DNS lookup patterns
• Deploy web application firewall (WAF) rules to inspect and validate Host headers
• Configure network-level monitoring to identify potential DNS spoofing attempts

Monitoring Recommendations

• Enable detailed logging for the Header Handler component and /token endpoint
• Set up alerts for authentication anomalies or unexpected access patterns
• Monitor DNS infrastructure for unauthorized or suspicious record changes
• Review application logs for evidence of Host header manipulation attempts

How to Mitigate CVE-2026-6874

Immediate Actions Required

• Upgrade ericc-ch copilot-api beyond version 0.7.0 when a patched version becomes available
• Implement additional authentication mechanisms that do not rely on DNS resolution
• Add strict validation for Host header values against an allowlist
• Consider deploying a reverse proxy to sanitize and validate incoming headers

Patch Information

The vendor was contacted about this vulnerability but did not respond. No official patch has been confirmed at this time. Organizations should monitor the project repository for updates and consider implementing compensating controls until an official fix is released. For more information, refer to the VulDB Submission #795212.

Workarounds

• Implement IP-based allowlisting as a secondary validation layer
• Configure web server or reverse proxy to validate and sanitize the Host header before forwarding requests
• Disable or restrict access to the /token endpoint from untrusted networks
• Deploy network segmentation to limit exposure of the copilot-api service

# Example nginx configuration to validate Host headers
server {
    # Only allow specific trusted host values
    if ($host !~* ^(trusted-host\.example\.com|api\.internal\.local)$) {
        return 444;
    }
    
    location /token {
        # Additional access restrictions
        allow 10.0.0.0/8;
        deny all;
        
        proxy_pass http://copilot-api-backend;
    }
}