CVE-2026-31908: Apache APISIX Header Injection Vulnerability

CVE-2026-31908 Overview

A critical header injection vulnerability exists in Apache APISIX, a popular open-source API gateway. The vulnerability allows attackers to exploit certain configurations in the forward-auth plugin to inject malicious HTTP headers. This flaw can be leveraged by unauthenticated remote attackers to manipulate authentication decisions, bypass security controls, or potentially escalate privileges within the API gateway environment.

Critical Impact

Unauthenticated attackers can inject arbitrary headers through the forward-auth plugin, potentially bypassing authentication mechanisms and compromising backend service security.

Affected Products

• Apache APISIX versions 2.12.0 through 3.15.0
• Environments utilizing the forward-auth plugin with vulnerable configurations
• API gateway deployments relying on header-based authentication forwarding

Discovery Timeline

• April 14, 2026 - CVE-2026-31908 published to NVD
• April 16, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31908

Vulnerability Analysis

This vulnerability is classified as CWE-75 (Failure to Sanitize Special Elements into a Different Plane), commonly known as header injection or HTTP response splitting. The flaw resides in the forward-auth plugin of Apache APISIX, which is designed to delegate authentication decisions to external services.

When certain plugin configurations are applied, the forward-auth plugin fails to properly sanitize or validate header values before forwarding them to backend authentication services. This allows attackers to craft malicious requests containing injected header content that gets processed by upstream services as legitimate headers.

The attack is accessible via network without requiring authentication or user interaction, making it particularly dangerous in internet-facing deployments. Successful exploitation can compromise both confidentiality and integrity of the affected system, potentially allowing attackers to impersonate users, bypass access controls, or manipulate backend service behavior.

Root Cause

The root cause stems from insufficient input validation in the forward-auth plugin's header handling logic. When the plugin processes incoming requests to forward authentication headers to an external service, it does not adequately sanitize special characters such as carriage return (\r) and line feed (\n) sequences. This allows attackers to inject additional headers by including these control characters in request parameters that are subsequently reflected in forwarded headers.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending specially crafted HTTP requests to an Apache APISIX instance with a vulnerable forward-auth plugin configuration. The malicious payload is included in request headers or parameters that the plugin forwards to the authentication service.

By injecting control characters, the attacker can add arbitrary headers to the forwarded request. These injected headers may include authentication tokens, user identity claims, or other security-relevant values that the backend authentication service trusts, potentially resulting in authentication bypass or privilege escalation.

For detailed technical information, refer to the Apache Mailing List Discussion and Openwall OSS-Security Update.

Detection Methods for CVE-2026-31908

Indicators of Compromise

• HTTP requests containing carriage return (%0d) or line feed (%0a) encoded characters in header values
• Unusual authentication successes from unexpected source IPs or with malformed request patterns
• Log entries showing headers with unexpected newline characters or multiple header values
• Backend authentication services receiving headers not present in original client requests

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block CRLF injection patterns in HTTP headers
• Deploy runtime application security monitoring to identify anomalous header manipulation attempts
• Enable detailed logging on the forward-auth plugin to capture full header contents for forensic analysis
• Use SentinelOne Singularity to monitor API gateway processes for suspicious behavior patterns

Monitoring Recommendations

• Monitor Apache APISIX access logs for requests with encoded special characters (%0d, %0a, %0d%0a)
• Alert on authentication service requests containing headers not whitelisted in plugin configuration
• Track authentication bypass attempts and correlate with unusual header patterns
• Implement network traffic analysis to identify header injection attack patterns targeting APISIX endpoints

How to Mitigate CVE-2026-31908

Immediate Actions Required

• Upgrade Apache APISIX to version 3.16.0 or later immediately
• Audit all forward-auth plugin configurations for potentially vulnerable header forwarding settings
• Implement input validation at the WAF or load balancer level to filter CRLF injection attempts
• Review authentication logs for signs of prior exploitation

Patch Information

Users are strongly recommended to upgrade to Apache APISIX version 3.16.0, which contains the security fix for this vulnerability. The patch addresses the improper header sanitization in the forward-auth plugin by implementing proper input validation and encoding of header values before forwarding them to authentication services.

For official patch details and upgrade instructions, consult the Apache Mailing List Discussion.

Workarounds

• Disable the forward-auth plugin if not actively required until patching is complete
• Implement strict header whitelisting at the reverse proxy or WAF layer to block requests with special characters
• Configure authentication services to reject requests containing suspicious header patterns
• Use network segmentation to limit access to APISIX admin API and authentication endpoints

# Configuration example - Disable forward-auth plugin temporarily
# In your APISIX configuration file (config.yaml)
plugins:
  - name: forward-auth
    enable: false
    
# Alternatively, apply strict header validation via NGINX (if used as upstream)
# Add to nginx.conf server block
if ($http_authorization ~* "[\r\n]") {
    return 400;
}