CVE-2026-48108: 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/sshread.rs - russh/src/server/mod.rs

The same readsshid() 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: SSHMAXBANNERLEN - /home/mjc/projects/openssh-portable/ssh.h: SSHMAXPREBANNERLINES - /home/mjc/projects/openssh-portable/kex.c: client-side handling of server pre-banner lines - /home/mjc/projects/openssh-portable/sshapi.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.

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

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

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

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

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

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

assert!( result.iserr(), "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 remotesshid; the final client identification string is what feeds the key-exchange transcript. remotesshid 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.