EUVD-2026-326611

fkie_cve-2026-48108

Russh is a Rust SSH client & server library. From version 0.34.0-beta.1 to before version 0.61.0, 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. This issue has been patched in version 0.61.0.

GHSA-76r6-x97p-67vr — Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input

Affected: crates.io: russh

### 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…

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EUVD-2026-326611

fkie_cve-2026-48108

GHSA-76r6-x97p-67vr — Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input