Summary

The channel window adjust message of the SSH protocol is used to track the free space in the receive buffer of the other side of a channel. The current implementation takes the value from the message and adds it to an internal state value. This can result in a integer overflow. If the Rust code is compiled with overflow checks, it will panic. A malicious client can crash a server.

Details

According https://datatracker.ietf.org/doc/html/rfc4254#section-5.2, The value must not overflow.
The incorrect handling is done in server/encrypted.rs and client/encrypted.rs in the handling of CHANNEL_WINDOW_ADJUST.

let amount = map_err!(u32::decode(&mut r))?;
...
channel.recipient_window_size += amount;

It could be replaced with something like

  if let Some(ref mut channel) = enc.channels.get_mut(&channel_num) {
                        // rfc 4254: The window MUST NOT be increased above 2^32 - 1 bytes.
                        new_size = channel.recipient_window_size.saturating_add(amount);
                        channel.recipient_window_size = new_size;
                    }
...

PoC

A customized client code would be required to send a message with a big value like u32_max. Not done yet.

Impact

This problem seems only critical to a server. One user can crash the server, which might take down the service. A malicious server could also crash a single client, but this seems not very critical.