AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION

Summary

A malicious SSH server can wedge an AsyncSSH client, and an authenticated
client can wedge an AsyncSSH server, by sending a channel maximum packet size of 0 in SSH_MSG_CHANNEL_OPEN_CONFIRMATION (server→client) or
SSH_MSG_CHANNEL_OPEN (client→server). AsyncSSH stores the peer-supplied value
verbatim with no lower-bound check; the first time channel data is written,
SSHChannel._flush_send_buf enters a synchronous infinite loop that cannot
be interrupted by asyncio.wait_for or any timeout. The loop body has no
await, so it blocks the entire asyncio event loop — for a server, one
malicious authenticated channel freezes all current and future connections.

RFC 4254 §5.1 leaves receiver behavior for a peer-reported "maximum packet
size = 0" undefined, so the value must be rejected rather than stored.

Root cause

asyncssh/channel.py:

# process_open (server side)        -- line 465
self._send_pktsize = send_pktsize    # peer value, no >= 1 check

# process_open_confirmation (client) -- line 528
self._send_pktsize = send_pktsize    # peer value, no >= 1 check

# _flush_send_buf                    -- lines 305-320
while self._send_buf and self._send_window:
    pktsize = min(self._send_window, self._send_pktsize)  # 0 when peer sends 0
    buf, datatype = self._send_buf[0]
    if len(buf) > pktsize:          # True for any buffered data
        data = buf[:pktsize]        # empty (b'')
        del buf[:pktsize]           # no-op
    ...
    self._send_window -= len(data)  # -= 0, unchanged

With _send_pktsize == 0, pktsize is 0, so buf[:0] is empty,
del buf[:0] is a no-op, and _send_window is never decremented — the
while condition is permanently true, and with no await in the body the
event loop is blocked.

Impact

• Client vector (primary): a malicious SSH server replies to the client's
  channel open with maximum packet size = 0; the client wedges on its first
  channel write. The attacker is the server, so it needs no valid credentials.
• Server vector: an authenticated client opens a channel with
  maximum packet size = 0; any server-side channel write wedges the AsyncSSH
  server's event loop, freezing every current and future connection. A
  single low-privilege account can take the whole server down.

Both vectors are a single SSH message, deterministic, and cause total
availability loss for the affected process.

Affected versions

<= 2.23.1 (latest release, 2026-06-06); also present on master
(channel.py:465/528 unguarded). Verified end-to-end on 2.23.1.

Verification

The maintainer's proposed fix (reject send_pktsize == 0 in connection.py
_process_channel_open / _process_channel_open_confirmation) was applied to
2.23.1 and re-tested end-to-end over TCP:

• Unpatched: malicious server (paramiko forcing max_packet_size=0 in
  OPEN_CONFIRMATION) + real asyncssh client → client event loop wedges.
• Patched: the guard fires inside _process_channel_open_confirmation, the
  malicious value is rejected, the connection closes cleanly
  (ChannelOpenError: SSH connection closed), and the client does not wedge.

The maintainer (Ron Frederick) independently confirmed the freeze and noted that
even shutting the server down does not break clients out of the loop.

Reproducers available: a focused harness driving the real
SSHChannel._flush_send_buf with _send_pktsize=0, and an end-to-end
malicious_server.py (paramiko) + client.py (real asyncssh) pair. The
end-to-end client repro uses asyncio.new_event_loop() (not get_event_loop())
for Python 3.14 compatibility.

Suggested fix (maintainer's approach)

In connection.py, after each send_pktsize = packet.get_uint32() in
_process_channel_open and _process_channel_open_confirmation:

if send_pktsize == 0:
    raise ProtocolError('Invalid maximum packet size')

CVSS

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (6.5 Medium). An earlier draft
quoted 7.5 High ("100% CPU"); the synchronous loop burns ~100% of one core's
worth of CPU but, being single-threaded, the OS scheduler spreads it across
cores, so the real impact is event-loop / connection freeze, not machine-wide
CPU exhaustion.

References

• RFC 4254 §5.1 (channel "maximum packet size"; behavior for 0 is undefined).
• The same maximum packet size = 0 send-loop wedge was confirmed in several
  other independent SSH implementations (different languages/runtimes) and
  reported to each maintainer separately.

Credits

Reported by zhangph (afldl), 2026-06-20.
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