CVE-2026-31908 -- CVSS 9.1 Vulnerability Briefing

CVE-2026-31908 | CVSS 9.1 (Critical) | Exploit: No known exploit

What Is It

CVE-2026-31908 is a header injection vulnerability in Apache APISIX, affecting the forward-auth plugin and allowing attackers to inject malicious HTTP headers through specific plugin configurations.

Technical Detail

The flaw exists in the forward-auth plugin of Apache APISIX, where insufficient sanitization of header values enables an attacker to inject arbitrary HTTP headers when certain plugin configurations are in place. An attacker who can influence request headers or interact with the forward-auth flow can craft input that passes malicious header content to upstream services or authentication endpoints. Depending on how downstream services consume those headers, this could result in authentication bypass, privilege escalation, or manipulation of access control decisions enforced by the upstream authentication service.

Exploitation Status

No known exploit code has been observed or published as of April 21, 2026. This vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. There is no confirmed in-the-wild exploitation at this time, though the critical CVSS score of 9.1 reflects the potential severity if exploitation is attempted.

Who Is Targeting This

No specific threat actor attribution at this time. No campaigns or targeted sectors have been associated with this vulnerability in available intelligence.

What To Do

Organizations running Apache APISIX versions from 2.x onward should apply the vendor-supplied patch as soon as it becomes available and treat this as a high-priority remediation given the critical severity rating. As an interim measure, administrators should audit forward-auth plugin configurations and restrict or disable the plugin on any gateway instances where it is not strictly required. Monitoring for anomalous or unexpected header values in requests passing through APISIX, particularly those routed to authentication backends, can serve as a detection signal. Follow the Apache APISIX security advisory for confirmed affected version ranges and patch availability, as the full version scope was not completely specified in the initial disclosure.