Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input

Summary

russh did not enforce the SSH identification-string rules as deliberately as OpenSSH. In particular, the server-side identification reader used the same permissive path as the client, allowing pre-banner lines from clients, and the reader did not enforce a bounded number of pre-banner lines.

For a library server built on russh, this could allow a remote peer to hold connection setup resources in the cleartext pre-authentication phase with malformed identification input that should have been rejected early.

Details

RFC 4253 section 4.2 defines the SSH protocol version exchange. The identification string is a single line terminated by CR LF, must fit within 255 characters including CR LF, and clients should not send pre-banner lines before their SSH identification string.

Before the fix, russh's identification reader lived in:

• russh/src/ssh_read.rs
• russh/src/server/mod.rs

The same read_ssh_id() behavior was used for both client and server contexts. That allowed server-side parsing to accept preliminary banner lines from clients, even though RFC 4253 only describes server-side pre-identification text. The reader also discarded preliminary lines without a line-count cap, so a peer could repeatedly send short non-SSH lines and keep the connection in identification parsing until an application-level timeout or external resource limit intervened.

This also creates a remotely observable parser-state oracle inside a single connection. A client can send candidate identification lines one after another: lines not recognized as SSH identification are discarded as pre-banner text, while an accepted identification string terminates banner parsing and advances the connection into key exchange. A strict server would reject the first invalid client pre-banner line and force a reconnect for each probe. This can disclose server-side parser acceptance behavior and make fingerprinting cheaper, though it does not disclose application secrets, credentials, keys, or authenticated user data.

The patch splits the behavior between generic/server-banner-tolerant reading and stricter client-identification reading. It also adds explicit limits for line length and pre-banner line count.

Relevant branch commit:

• 3de4a68 Harden SSH identification parsing

RFC / OpenSSH Comparison

RFC 4253 section 4.2 says each side sends an identification string of the form SSH-protoversion-softwareversion SP comments CR LF. It allows a server to send other lines before its identification string, but says a client must be able to process such lines. It does not grant the same pre-banner allowance to clients.

OpenSSH portable enforces explicit identification limits:

• /home/mjc/projects/openssh-portable/ssh.h: SSH_MAX_BANNER_LEN
• /home/mjc/projects/openssh-portable/ssh.h: SSH_MAX_PRE_BANNER_LINES
• /home/mjc/projects/openssh-portable/kex.c: client-side handling of server pre-banner lines
• /home/mjc/projects/openssh-portable/ssh_api.c: rejects pre-banner lines when acting as a server

I checked /home/mjc/projects/openssh-portable at 45b30e0a5. OpenSSH uses an implementation banner line limit of 8192 bytes and a pre-banner line cap of 1024, which is more permissive than RFC 4253's 255-character identification-string limit. The relevant alignment is the parser shape: OpenSSH permits bounded pre-banner lines when reading a server banner as a client, but rejects pre-banner lines when acting as a server and reading a client identification string.

The russh fix follows that shape: accept bounded pre-banner lines only where the protocol allows them, and reject malformed or excessive identification input early.

PoC

Inline highest-CVSS PoC: unauthenticated remote client pre-banner input to the server identification parser. This demonstrates AV:N/AC:L/PR:N/UI:N.

#[tokio::test]
async fn poc_server_accepts_client_pre_banner_before_ssh_id() {
    use russh::server;
    use tokio::io::{AsyncReadExt, AsyncWriteExt};

    let config = std::sync::Arc::new(server::Config::default());
    let (mut client, server_stream) = tokio::io::duplex(4096);

    let server = tokio::spawn(async move {
        server::run_stream(config, server_stream, NoAuthHandler).await
    });

    let mut server_id = Vec::new();
    client.read_until(b'\n', &mut server_id).await.unwrap();

    client
        .write_all(b"attacker-controlled pre-banner\r\nSSH-2.0-poc\r\n")
        .await
        .unwrap();

    let result = tokio::time::timeout(std::time::Duration::from_millis(250), server).await;

    assert!(
        result.is_err(),
        "vulnerable code keeps processing after accepting a client pre-banner before SSH identification"
    );
}

On vulnerable code, the server-side reader accepts the client pre-banner line and continues instead of rejecting the malformed identification input promptly. The fixed parser rejects client pre-banner lines on the server path.

Impact

Suggested CVSS v3.1:

• CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Score: 5.3

Reasoning:

• AV:N: reachable over the SSH transport connection
• AC:L: no special race or unusual setup is required
• PR:N, UI:N: occurs before authentication and needs no user interaction
• C:N, I:N: no confidentiality or integrity impact demonstrated
• A:L: malformed identification input can consume connection setup resources until rejected by timeout or external limits

Additional impact investigation did not identify a stronger confidentiality, integrity, downgrade, or code-execution primitive. The accepted client pre-banner line is discarded before key exchange and does not become part of remote_sshid; the final client identification string is what feeds the key-exchange transcript. remote_sshid is otherwise exposed to library handlers and debug formatting, but the discarded pre-banner text does not influence authentication state, strict-kex negotiation, KEX algorithm selection, or later packet framing.

One parser-boundary nuance on vulnerable code is that behavior can depend on read chunking: if a client pre-banner line and the real SSH identification line are delivered in the same read, the old parser can discard the buffered identification line and then wait or disconnect; if delivered separately, the old server path can accept the pre-banner and continue. This supports malformed pre-authentication availability impact, but not a demonstrated confidentiality or integrity impact.

Fix / Patch Direction

Use a stricter server-side client-identification reader, enforce the RFC identification-line length, and cap preliminary banner lines. The server path should reject client pre-banner lines instead of treating them like allowed server pre-identification text.