CVE-2026-31908: Apache APISIX Header Injection Vulnerability

CVE-2026-31908 Overview

CVE-2026-31908 is a header injection vulnerability in Apache APISIX, an open-source API gateway. The flaw resides in the forward-auth plugin, where specific configurations allow attackers to inject malicious HTTP headers into upstream requests. The vulnerability affects Apache APISIX versions 2.12.0 through 3.15.0 and is fixed in version 3.16.0. Attackers can exploit the issue over the network without authentication or user interaction, making it suitable for remote exploitation against exposed API gateways. The vulnerability is categorized under [CWE-75: Failure to Sanitize Special Elements into a Different Plane].

Critical Impact

Unauthenticated attackers can inject arbitrary headers through the forward-auth plugin, enabling authentication bypass, request smuggling, and impersonation against upstream services.

Affected Products

• Apache APISIX 2.12.0 through 3.15.0
• Deployments using the forward-auth plugin with susceptible configurations
• API gateway instances exposed to untrusted network traffic

Discovery Timeline

• 2026-04-14 - CVE-2026-31908 published to NVD
• 2026-04-14 - Apache project published advisory to the dev mailing list
• 2026-04-17 - Last updated in NVD database

Technical Details for CVE-2026-31908

Vulnerability Analysis

Apache APISIX provides a forward-auth plugin that delegates authentication decisions to an external service. The plugin forwards client requests to an authentication endpoint and propagates response headers back to the upstream service. CVE-2026-31908 stems from insufficient sanitization of headers returned by the authentication service or supplied through request input. Attackers can manipulate the plugin configuration paths to inject arbitrary headers into the request flow toward the upstream. Because upstream services typically trust headers set by the gateway, this injection undermines downstream authorization and identity context.

Root Cause

The root cause is improper neutralization of special elements when constructing HTTP headers within the forward-auth plugin. The plugin fails to validate or filter header names and values before passing them to the upstream request, violating header plane separation as described in CWE-75. Configurations that broaden which headers are forwarded amplify the issue by accepting attacker-controlled values.

Attack Vector

An unauthenticated remote attacker sends crafted HTTP requests to an APISIX route that uses the forward-auth plugin. By controlling input that influences forwarded headers, the attacker injects headers such as X-User, Authorization, or arbitrary custom headers consumed by upstream applications. Upstream services then act on those headers, allowing identity spoofing or privilege escalation depending on backend trust assumptions. The attack requires no privileges and no user interaction, and exploitation traffic is indistinguishable from legitimate API calls without targeted detection rules.

No public proof-of-concept code is currently available. Refer to the Apache Mailing List Discussion and the Openwall OSS Security Update for technical details.

Detection Methods for CVE-2026-31908

Indicators of Compromise

• Unexpected or duplicated HTTP headers in upstream access logs originating from APISIX nodes
• Requests to routes configured with the forward-auth plugin containing CRLF sequences or unusual header names in client-supplied fields
• Upstream authentication bypass events where identity headers (e.g., X-User, X-Forwarded-User) carry attacker-controlled values

Detection Strategies

• Inventory all APISIX routes that enable the forward-auth plugin and audit their header allow-lists against version 3.16.0 defaults
• Compare headers received by upstream services to a baseline of headers expected from the gateway, flagging deviations
• Inspect APISIX access logs for anomalous header patterns associated with the auth flow

Monitoring Recommendations

• Forward APISIX and upstream service logs to a centralized analytics platform for correlation across the request path
• Alert on authentication or authorization decisions made using headers that should be set exclusively by the gateway
• Monitor configuration changes to the forward-auth plugin and review them through change control

How to Mitigate CVE-2026-31908

Immediate Actions Required

• Upgrade Apache APISIX to version 3.16.0 or later on all gateway nodes
• Audit every route using the forward-auth plugin and restrict the request_headers, upstream_headers, and client_headers allow-lists to the minimum required
• Rotate any credentials or session tokens that may have been exposed through manipulated header flows

Patch Information

Apache has released Apache APISIX 3.16.0, which fixes CVE-2026-31908. The advisory is published in the Apache Mailing List Discussion and the Openwall OSS Security Update. Operators running versions 2.12.0 through 3.15.0 should upgrade without delay.

Workarounds

• Disable the forward-auth plugin on routes that do not require external authentication delegation
• Replace permissive header forwarding configurations with explicit allow-lists of header names known to be safe
• Place a web application firewall in front of APISIX to strip or normalize client-supplied headers consumed by the plugin

# Configuration example: restrict forward-auth header propagation
# Apply on each route using the plugin until upgrade to 3.16.0 is complete
plugins:
  forward-auth:
    uri: "https://auth.internal.example.com/verify"
    request_headers:
      - "Authorization"
    upstream_headers:
      - "X-User-ID"
    client_headers: []
    keep_request_body: false