CVE-2026-29000: Professional Cybersecurity Analysis

Executive Summary

CVE-2026-29000 represents a critical authentication bypass vulnerability in the pac4j-jwt library with a maximum CVSS score of 10.0. This vulnerability allows remote attackers to forge authentication tokens and impersonate any user, including administrators, by exploiting weaknesses in JWE (JSON Web Encryption) processing within the JwtAuthenticator component.

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1. Vulnerability Assessment and Severity Evaluation

Severity Classification

• CVSS Score: 10.0 (Critical)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Attack Vector: Network

Technical Assessment

This vulnerability represents a complete authentication bypass, which justifies the maximum severity rating:

• Authentication Circumvention: Attackers can authenticate as any user without valid credentials
• Privilege Escalation: Direct path to administrative access
• Remote Exploitation: No local access required
• Low Barrier to Entry: Only requires possession of the server's public RSA key (often publicly available)

Risk Factors

1. Public Key Availability: RSA public keys are frequently exposed through:

   • JWKS (JSON Web Key Set) endpoints
   • Application configuration files
   • Public repositories
   • SSL/TLS certificates

2. Widespread Impact: pac4j is a widely-used security framework for Java applications, particularly in:

   • Enterprise authentication systems
   • API gateways
   • Microservices architectures
   • Single Sign-On (SSO) implementations

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2. Attack Vectors and Exploitation Methods

Vulnerability Mechanism

The flaw exists in how JwtAuthenticator processes encrypted JWTs (JWE tokens). The vulnerability chain operates as follows:

1. JWE Wrapping: Attacker creates a PlainJWT (unsigned JWT) with arbitrary claims
2. Encryption with Public Key: The PlainJWT is wrapped in JWE encryption using the server's RSA public key
3. Signature Verification Bypass: The JwtAuthenticator decrypts the JWE but fails to verify the inner JWT's signature
4. Authentication Success: The server accepts the forged token as legitimate

Attack Sequence

1. Reconnaissance Phase:
   - Obtain server's RSA public key (from JWKS endpoint, certificates, or configuration)
   - Identify target user or administrator account

2. Token Forgery:
   - Create PlainJWT with claims:
     {
       "sub": "admin@company.com",
       "roles": ["ADMIN", "SUPERUSER"],
       "exp": [future_timestamp]
     }
   
3. JWE Wrapping:
   - Encrypt PlainJWT using RSA-OAEP with server's public key
   - Create valid JWE structure (header.encrypted_key.iv.ciphertext.tag)

4. Exploitation:
   - Submit forged JWE token to protected endpoints
   - Bypass authentication and gain unauthorized access

Exploitation Complexity

Low - Attackers need:

• Server's RSA public key (often publicly accessible)
• Standard JWT libraries (available in all major languages)
• Basic understanding of JWE structure

Proof of Concept Scenario

# Conceptual exploitation flow (simplified)

# 1. Obtain public key
public_key = fetch_jwks_endpoint("https://target.com/.well-known/jwks.json")

# 2. Create malicious claims
claims = {
    "sub": "administrator",
    "roles": ["ADMIN"],
    "exp": future_timestamp()
}

# 3. Create PlainJWT (no signature)
plain_jwt = create_plain_jwt(claims)

# 4. Wrap in JWE using public key
forged_token = jwe_encrypt(plain_jwt, public_key, alg="RSA-OAEP")

# 5. Authenticate with forged token
response = requests.get(
    "https://target.com/admin/dashboard",
    headers={"Authorization": f"Bearer {forged_token}"}
)

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3. Affected Systems and Software Versions

Vulnerable Versions

Version Branch	Vulnerable Versions	First Patched Version
4.x series	< 4.5.9	4.5.9
5.x series	< 5.7.9	5.7.9
6.x series	< 6.3.3	6.3.3

Affected Components

• Primary: org.pac4j:pac4j-jwt module
• Specific Class: JwtAuthenticator when processing JWE tokens
• Configuration: Systems using encrypted JWT authentication

Ecosystem Impact

Applications and frameworks that may be affected:

1. Direct Dependencies:

   • Java web applications using pac4j-jwt
   • Spring Boot applications with pac4j integration
   • Play Framework applications
   • Vert.x applications

2. Indirect Dependencies:

   • CAS (Central Authentication Service) implementations
   • Knox Gateway installations
   • Custom SSO solutions
   • API management platforms

3. Deployment Environments:

   • Enterprise authentication servers
   • Cloud-native microservices
   • API gateways
   • Identity and Access Management (IAM) systems

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4. Recommended Mitigation Strategies

Immediate Actions (Priority 1)

1. Emergency Patching

   <!-- Update Maven dependency immediately -->
   <dependency>
       <groupId>org.pac4j</groupId>
       <artifactId>pac4j-jwt</artifactId>
       <version>6.3.3</version> <!-- or 5.7.9, 4.5.9 -->
   </dependency>
   

2. Token Validation Audit

   • Review authentication logs for suspicious token patterns
   • Look for JWE tokens with unusual characteristics
   • Identify potential unauthorized access attempts

3. Temporary Workarounds (if immediate patching is impossible):

   • Disable JWE token support temporarily
   • Implement additional authentication layers
   • Enforce IP whitelisting for administrative access
   • Enable multi-factor authentication (MFA) for all privileged accounts

Short-term Remediation (Priority 2)

1. Comprehensive Update Strategy

   # Identify all affected dependencies
   mvn dependency:tree | grep pac4j-jwt
   
   # Update all instances
   mvn versions:use-latest-versions -Dincludes=org.pac4j:pac4j-jwt
   

2. Configuration Hardening

   • Implement strict JWT validation policies
   • Require signed JWTs (JWS) in addition to encryption
   • Configure explicit algorithm whitelisting
   • Disable algorithm negotiation

3. Monitoring Enhancement

   • Deploy detection rules for JWE token anomalies
   • Monitor for authentication patterns indicating exploitation
   • Alert on administrative access from new sources

Long-term Security Measures (Priority 3)

1. Architecture Review

   • Evaluate necessity of JWE encryption vs. TLS-only protection
   • Consider migration to more robust authentication mechanisms
   • Implement defense-in-depth strategies

2. Security Controls

   // Enforce strict JWT validation
   JwtAuthenticator authenticator = new JwtAuthenticator();
   authenticator.setSignatureConfiguration(new RSASignatureConfiguration());
   authenticator.setEncryptionConfiguration(new RSAEncryptionConfiguration());
   
   // Require both signature AND encryption
   authenticator.setRequireSignature(true);
   authenticator.setRequireEncryption(true);
   

3. Key Management

   • Rotate RSA key pairs immediately
   • Implement proper key lifecycle management
   • Restrict public key exposure where possible
   • Use Hardware Security Modules (HSMs) for key storage

4. Dependency Management

   • Implement automated vulnerability scanning
   • Subscribe to security advisories for pac4j
   • Establish regular dependency update cycles
   • Use Software Composition Analysis (S