This fixes the following vulnerability scenario:
* Victim logs into SAFE-HOST on SAFE-IP-ADDRESS regularly.
* Victim is cajoled into connecting to attacker controlled
ATTACK-HOST, on ATTACK-IP-ADDRESS. ATTACK-HOST uses a different host
key type (e.g. Ed25519 vs RSA). The new key is added at the end of
known_hosts.
* Attacker makes DNS system return ATTACK-IP-ADDRESS for SAFE-HOST.
* Victim logs into SAFE-HOST, but is not warned because the host key
matches ATTACK-IP-ADDRESS.
For this attack to work, the key type has to be different, because
knownhosts gives precedence to the first key found for each type. Add
a test that asserts this behavior.
The new semantics simplify the code, but callers that modify
.ssh/known_host interactviely must now take an extra step to remain
OpenSSH compatible: on successful login, the IP address must be
checked without hostname, and if it is not known, added separately to
the known_hosts file, so future logins that use an IP address only
will be protected too.
Thanks to Daniel Parks <security@demonhorse.net> for finding this
vulnerability.
Change-Id: I62b1b60ceb02e2f583a4657213feac1a8885dd42
Reviewed-on: https://go-review.googlesource.com/104939
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Han-Wen Nienhuys
beb2a9779c