Hi,
Regarding the race condition fixed in OpenSSH 9.8. A mitigation to
prevent exploitation of this bug is to disable the login grace timer
by setting LoginGraceTime=0 in sshd_config. This will however make
it much easier for an attacker to deny service to sshd.
Similarly, the much more minor keystroke timing bug can be avoided
by disabling the feature using ObscureKeystrokeTiming=0.
Some users will understandably prefer to patch their OpenSSH rather
than upgrade to the newest version, so here are minimal patches for
both problems.
1) Critical race condition in sshd
diff --git a/log.c b/log.c
index 9fc1a2e2e..191ff4a5a 100644
--- a/log.c
+++ b/log.c
@@ -451,12 +451,14 @@ void
sshsigdie(const char *file, const char *func, int line, int showfunc,
LogLevel level, const char *suffix, const char *fmt, ...)
{
+#ifdef SYSLOG_R_SAFE_IN_SIGHAND
va_list args;
va_start(args, fmt);
sshlogv(file, func, line, showfunc, SYSLOG_LEVEL_FATAL,
suffix, fmt, args);
va_end(args);
+#endif
_exit(1);
}
2) Minor logic error in ObscureKeystrokeTiming
diff --git a/clientloop.c b/clientloop.c
index 8ec36af94..6dcd6c853 100644
--- a/clientloop.c
+++ b/clientloop.c
@@ -608,8 +608,9 @@ obfuscate_keystroke_timing(struct ssh *ssh, struct timespec *timeout,
if (timespeccmp(&now, &chaff_until, >=)) {
/* Stop if there have been no keystrokes for a while */
stop_reason = "chaff time expired";
- } else if (timespeccmp(&now, &next_interval, >=)) {
- /* Otherwise if we were due to send, then send chaff */
+ } else if (timespeccmp(&now, &next_interval, >=) &&
+ !ssh_packet_have_data_to_write(ssh)) {
+ /* If due to send but have no data, then send chaff */
if (send_chaff(ssh))
nchaff++;
}
Thanks,
Damien Miller
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