Navidrome allows an authentication bypass in Subsonic API with non-existent username

Summary

In certain Subsonic API endpoints, authentication can be bypassed by using a non-existent username combined with an empty (salted) password hash. This allows read-only access to the server’s resources, though attempts at write operations fail with a “permission denied” error.

Details

A flaw in the authentication check process allows an attacker to specify any arbitrary username that does not exist on the system, along with a salted hash of an empty password. Under these conditions, Navidrome treats the request as authenticated, granting access to various Subsonic endpoints without requiring valid credentials.

Proof of Concept (PoC)

1. Generate a random salt:

javascript // e.g., salt = "x1vbudn1m6d" Math.random().toString(36).substring(2, 15)

2. Calculate the MD5 hash of an empty password plus the salt:

shell # Using the example salt above echo -n "x1vbudn1m6d" | md5sum 81f0c0fb5d202ab0d012e6eaeb722d79 -

3. Send a request specifying a fake user, with the hash and salt values:

GET https://[host]/rest/getPlaylists?u=FakeUser&t=81f0c0fb5d202ab0d012e6eaeb722d79&s=x1vbudn1m6d&v=1.16.1&c=castafiore&f=json

Impact

An attacker can use any non-existent username to bypass the authentication system and gain access to various read-only data in Navidrome, such as user playlists. However, any attempt to modify data fails due to insufficient permissions, limiting the impact to unauthorized viewing of information.