Authentication with JWT allows use of “none”-algorithm

Vulnerability

CVE-2021-22160 affects Apache Pulsar instances that use JWT-based token authentication. The JWT library did not validate the signature when the token's algorithm header was set to 'none'. This is a known weakness in JWT implementations where the library trusts the algorithm indicated in the token itself. Versions prior to 2.7.2 are affected. [1][2]

Exploitation

An attacker with network access to a Pulsar broker can craft a JWT token with the algorithm set to 'none', a blank signature, and any desired claims (e.g., user or admin role). No prior authentication or valid credentials are required. The broker's authentication check will accept the forged token as valid, granting the attacker the privileges associated with the forged identity. [1]

Impact

Successful exploitation allows an attacker to connect to Pulsar as any user, including administrative accounts. This can lead to full compromise of the Pulsar instance: unauthorized access to topics, reading or writing messages, and modifying cluster configuration. The attack completely bypasses the intended authentication mechanism. [1]

Mitigation

The fix was released in Apache Pulsar version 2.7.2 on 2021-05-26. Users should upgrade to this version or later. [2] The fix enforces that the JWT signature algorithm must be validated against the expected key. No workaround is available for older versions; upgrading is the only recommended mitigation.