CVE-2026-39413: LightRAG JWT Auth Bypass Vulnerability

CVE-2026-39413 Overview

CVE-2026-39413 is an Authentication Bypass vulnerability affecting LightRAG, a framework that provides simple and fast retrieval-augmented generation capabilities. Prior to version 1.4.14, the LightRAG API is vulnerable to a JWT algorithm confusion attack where an attacker can forge authentication tokens by specifying 'alg': 'none' in the JWT header. Since the jwt.decode() call does not explicitly deny the 'none' algorithm, a crafted token without a signature will be accepted as valid, leading to unauthorized access to protected API endpoints.

Critical Impact

Attackers can bypass authentication entirely by crafting JWT tokens with the 'none' algorithm, potentially gaining unauthorized access to LightRAG API functionality and any sensitive data processed through the retrieval-augmented generation system.

Affected Products

• LightRAG versions prior to 1.4.14
• LightRAG API implementations using default JWT configuration

Discovery Timeline

• 2026-04-08 - CVE-2026-39413 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-39413

Vulnerability Analysis

This vulnerability falls under CWE-347 (Improper Verification of Cryptographic Signature). The flaw exists in how the LightRAG API handles JWT token validation. JSON Web Tokens support multiple signing algorithms including HMAC (HS256, HS384, HS512), RSA (RS256, RS384, RS512), and notably, the 'none' algorithm which indicates no signature verification is required.

When the API's jwt.decode() function processes incoming tokens, it fails to explicitly block the 'none' algorithm from being accepted. This allows an attacker to modify or craft JWT tokens with "alg": "none" in the header and remove the signature portion entirely. The server will accept these unsigned tokens as valid, completely bypassing the authentication mechanism.

The attack requires network access and some knowledge of the target system's expected JWT claims structure. While the complexity is higher due to the need to craft valid token payloads, successful exploitation grants access to high-confidentiality data processed through the RAG system.

Root Cause

The root cause is improper configuration of the JWT decoding library. The jwt.decode() function in the vulnerable versions does not specify an explicit list of allowed algorithms or explicitly deny the 'none' algorithm. This permissive default behavior allows algorithm confusion attacks where the attacker controls which algorithm the server uses for signature verification.

Proper JWT implementations should always specify an explicit allowlist of acceptable algorithms using the algorithms parameter in the decode function, which would prevent the 'none' algorithm from being accepted.

Attack Vector

The attack is network-based and targets the authentication layer of the LightRAG API. An attacker can exploit this vulnerability by intercepting or observing a valid JWT structure, then crafting a new token with the following characteristics:

1. Modify the JWT header to set "alg": "none"
2. Craft or modify the payload with desired claims (user ID, permissions, etc.)
3. Remove the signature portion of the token entirely
4. Submit the forged token to protected API endpoints

The vulnerability manifests in the JWT verification process where unsigned tokens are incorrectly accepted as valid. For technical implementation details and remediation guidance, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-39413

Indicators of Compromise

• JWT tokens with "alg": "none" in the decoded header appearing in API request logs
• Authentication events with tokens lacking a valid signature component (missing or empty third segment after the second period)
• Unusual API access patterns from unexpected sources without corresponding valid credential issuance
• Elevated privilege actions performed by users without matching authentication records

Detection Strategies

• Implement API gateway or WAF rules to inspect and block JWT tokens with the 'none' algorithm in the header
• Monitor authentication logs for tokens with missing or malformed signature segments
• Deploy runtime application self-protection (RASP) to detect JWT manipulation attempts
• Review server-side logs for jwt.decode() calls processing tokens without valid signatures

Monitoring Recommendations

• Enable verbose logging on JWT processing functions to capture algorithm values used during token validation
• Set up alerts for authentication anomalies including successful access without corresponding token issuance events
• Monitor for patterns of authentication bypass attempts across multiple endpoints
• Implement security information and event management (SIEM) rules to correlate suspicious JWT patterns with API access logs

How to Mitigate CVE-2026-39413

Immediate Actions Required

• Upgrade LightRAG to version 1.4.14 or later immediately
• Audit existing JWT implementations to ensure explicit algorithm allowlists are configured
• Review API access logs for evidence of exploitation using forged tokens
• Rotate any JWT signing secrets as a precautionary measure if exploitation is suspected
• Implement additional authentication controls while patching is in progress

Patch Information

The vulnerability has been addressed in LightRAG version 1.4.14. Users should upgrade to this version or later to receive the security fix. The patch ensures that the jwt.decode() function explicitly specifies allowed algorithms and rejects tokens using the 'none' algorithm.

For detailed patch information and upgrade instructions, refer to the GitHub Security Advisory.

Workarounds

• If immediate upgrade is not possible, implement an API gateway or reverse proxy rule to reject requests containing JWTs with "alg": "none" in the header
• Add custom middleware to validate JWT algorithm before processing tokens in the application layer
• Temporarily restrict API access to trusted IP addresses or require additional authentication factors
• Deploy a web application firewall (WAF) rule to inspect and block malformed JWT tokens

# Example: Upgrade LightRAG to patched version
pip install --upgrade lightrag>=1.4.14

# Verify installed version
pip show lightrag | grep Version