Synopsis: Moderate: openssh security, bug fix, and enhancement update
Advisory ID: SLSA-2015:2088-6
Issue Date: 2015-11-19
CVE Numbers: CVE-2015-5600
A flaw was found in the way OpenSSH handled PAM authentication when using
privilege separation. An attacker with valid credentials on the system and
able to fully compromise a non-privileged pre-authentication process using
a different flaw could use this flaw to authenticate as other users.
A use-after-free flaw was found in OpenSSH. An attacker able to fully
compromise a non-privileged pre-authentication process using a different
flaw could possibly cause sshd to crash or execute arbitrary code with
root privileges. (CVE-2015-6564)
It was discovered that the OpenSSH sshd daemon did not check the list of
keyboard-interactive authentication methods for duplicates. A remote
attacker could use this flaw to bypass the MaxAuthTries limit, making it
easier to perform password guessing attacks. (CVE-2015-5600)
It was found that the OpenSSH ssh-agent, a program to hold private keys
used for public key authentication, was vulnerable to password guessing
attacks. An attacker able to connect to the agent could use this flaw to
conduct a brute-force attack to unlock keys in the ssh-agent.
This update fixes the following bugs:
* Previously, the sshd_config(5) man page was misleading and could thus
confuse the user. This update improves the man page text to clearly
describe the AllowGroups feature.
* The limit for the function for restricting the number of files listed
using the wildcard character (*) that prevents the Denial of Service (DoS)
for both server and client was previously set too low. Consequently, the
user reaching the limit was prevented from listing a directory with a
large number of files over Secure File Transfer Protocol (SFTP). This
update increases the aforementioned limit, thus fixing this bug.
* When the ForceCommand option with a pseudoterminal was used and the
MaxSession option was set to “2”, multiplexed SSH connections did not work
as expected. After the user attempted to open a second multiplexed
connection, the attempt failed if the first connection was still open.
This update modifies OpenSSH to issue only one audit message per session,
and the user is thus able to open two multiplexed connections in this
* The ssh-copy-id utility failed if the account on the remote server did
not use an sh-like shell. Remote commands have been modified to run in an
sh-like shell, and ssh-copy-id now works also with non-sh-like shells.
* Due to a race condition between auditing messages and answers when using
ControlMaster multiplexing, one session in the shared connection randomly
and unexpectedly exited the connection. This update fixes the race
condition in the auditing code, and multiplexing connections now work as
expected even with a number of sessions created at once.
In addition, this update adds the following enhancements:
* As not all Lightweight Directory Access Protocol (LDAP) servers possess
a default schema, as expected by the ssh-ldap-helper program, this update
provides the user with an ability to adjust the LDAP query to get public
keys from servers with a different schema, while the default functionality
* With this enhancement update, the administrator is able to set
permissions for files uploaded using Secure File Transfer Protocol (SFTP).
* This update provides the LDAP schema in LDAP Data Interchange Format
(LDIF) format as a complement to the old schema previously accepted by
* With this update, the user can selectively disable the Generic Security
Services API (GSSAPI) key exchange algorithms as any normal key exchange.
– Scientific Linux Development Team