Executive brief
Russh is a library used by developers to build SSH clients and servers in the Rust programming language. A flaw in the server-side authentication logic allows security settings from one login attempt to incorrectly persist and affect a subsequent login attempt for a different user on the same connection. While this does not directly grant access, it causes a mismatch in how the server handles different users, potentially leading to inconsistent security enforcement or unexpected authentication behavior.
Technical details
A vulnerability exists in the russh server authentication path where internal 'userauth' state is preserved across SSH_MSG_USERAUTH_REQUEST messages even when the request principal (username or service) changes. According to RFC 4252, servers must flush accumulated authentication state if these fields change. In affected versions, russh-owned state—including remaining allowed methods, partial-success flags, and in-progress method state—remains associated with the connection. An unauthenticated remote attacker can exploit this by sending a sequence of authentication requests for different users, causing the state from the first user to influence the authentication requirements or responses for the second. This issue is patched in version 0.61.0.
Affected products
- Eugeny russh >= 0.34.0-beta.1, < 0.61.0
Timeline
- 2026-05-20: advisory: GitHub Security Advisory published
- 2026-06-10: disclosed: CVE published to NVD