We discovered a vulnerability (a signal handler race condition) in
OpenSSH's server (sshd): if a client does not authenticate within
LoginGraceTime seconds (120 by default, 600 in old OpenSSH versions),
then sshd's SIGALRM handler is called asynchronously, but this signal
handler calls various functions that are not async-signal-safe (for
example, syslog()). This race condition affects sshd in its default
configuration.

On investigation, we realized that this vulnerability is in fact a
regression of CVE-2006-5051 ("Signal handler race condition in OpenSSH
before 4.4 allows remote attackers to cause a denial of service (crash),
and possibly execute arbitrary code"), which was reported in 2006 by
Mark Dowd.

This regression was introduced in October 2020 (OpenSSH 8.5p1) by commit
752250c ("revised log infrastructure for OpenSSH"), which accidentally
removed an "#ifdef DO_LOG_SAFE_IN_SIGHAND" from sigdie(), a function
that is directly called by sshd's SIGALRM handler. In other words:

- OpenSSH < 4.4p1 is vulnerable to this signal handler race condition,
  if not backport-patched against CVE-2006-5051, or not patched against
  CVE-2008-4109, which was an incorrect fix for CVE-2006-5051;

- 4.4p1 <= OpenSSH < 8.5p1 is not vulnerable to this signal handler race
  condition (because the "#ifdef DO_LOG_SAFE_IN_SIGHAND" that was added
  to sigdie() by the patch for CVE-2006-5051 transformed this unsafe
  function into a safe _exit(1) call);

- 8.5p1 <= OpenSSH < 9.8p1 is vulnerable again to this signal handler
  race condition (because the "#ifdef DO_LOG_SAFE_IN_SIGHAND" was
  accidentally removed from sigdie()).