Impact

The Twisted SSH client and server implementation naively accepted an infinite amount of data for the peer's SSH version identifier.

A malicious peer can trivially craft a request that uses all available memory and crash the server, resulting in denial of service. The attack is as simple as nc -rv localhost 22 < /dev/zero.

Patches

The issue was fix in GitHub commit https://github.com/twisted/twisted/commit/98387b39e9f0b21462f6abc7a1325dc370fcdeb1

A fix is available in Twisted 22.2.0.

Workarounds

Limit access to the SSH server only to trusted source IP addresses. Connect over SSH only to trusted destination IP addresses.

References

Reported at https://twistedmatrix.com/trac/ticket/10284 Discussions at https://github.com/twisted/twisted/security/advisories/GHSA-rv6r-3f5q-9rgx

For more information

Found by vin01