Revised: Portable OpenSSH security advisory: portable-keysign-rand-helper.adv May 03 2011 12:48AM
Damien Miller (djm cvs openbsd org)
OpenSSH Security Advisory: portable-keysign-rand-helper.adv

This document may be found at:

1. Vulnerability

Portable OpenSSH's ssh-keysign utility may allow unauthorised
local access to host keys on platforms if ssh-rand-helper is

2. Affected configurations

Portable OpenSSH prior to version 5.8p2 only on platforms
that are configured to use ssh-rand-helper for entropy

ssh-rand-helper is enabled at configure time when it is
detected that OpenSSL does not have a built-in source of
randomness, and only used at runtime if this condition
remains. Platforms that support /dev/random or otherwise
configure OpenSSL with a random number provider are not

In particular, *BSD, OS X, Cygwin and Linux are not

3. Mitigation

If host-based authentication is not in use (enabled using
HostBasedAuthentication or RhostsRSAAuthentication in
sshd_config), then remove the setuid bit from ssh-keysign.

4. Details

ssh-keysign is a setuid helper program that is used to mediate
access to the host's private host keys during host-based
authentication. It would use its elevated privilege to open
the keys and then immediately drop privileges to complete its
cryptographic signing operations.

After privilege was dropped, ssh-keysign would ensure that
the OpenSSL random number generator that it depends upon was
adequately prepared. On configurations that lacked a built-in
source of entropy in OpenSSL, ssh-keysign would execute the
ssh-rand-helper program to attempt to retrieve some from the
system environment.

However, the file descriptors to the host private key files
were not closed prior to executing ssh-rand-helper. Since this
process was "born unprivileged" and inherited the sensitive
file descriptors, there was no protection against an attacker
using ptrace(2) to attach to it and instructing it to read out
the private keys.

5. Credit

This issue was privately reported by Tomas Mraz on April 26,

6. Fix

OpenSSH 5.8p2 contains a fix for this vulnerability.

Future releases of portable OpenSSH will remove support for
ssh-rand-helper - in 2011, there is no excuse for not
providing a /dev/random-like interface as part of the OS.
Users stuck on one of these platforms may use PRNGd
(http://prngd.sf.net) to provide a host-wide random pool.

Users of older versions that do not wish to upgrade
immediately may apply this patch:

Index: ssh-keysign.c
RCS file: /var/cvs/openssh/ssh-keysign.c,v
retrieving revision 1.43
diff -u -p -r1.43 ssh-keysign.c
--- ssh-keysign.c 10 Sep 2010 01:12:09 -0000 1.43
+++ ssh-keysign.c 29 Apr 2011 01:25:55 -0000
@@ -167,6 +167,9 @@ main(int argc, char **argv)

key_fd[0] = open(_PATH_HOST_RSA_KEY_FILE, O_RDONLY);
key_fd[1] = open(_PATH_HOST_DSA_KEY_FILE, O_RDONLY);
+ if (fcntl(key_fd[0], F_SETFD, FD_CLOEXEC) != 0 ||
+ fcntl(key_fd[1], F_SETFD, FD_CLOEXEC) != 0)
+ fatal("fcntl failed");

original_real_uid = getuid(); /* XXX readconf.c needs this */
if ((pw = getpwuid(original_real_uid)) == NULL)

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