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Category:   Application (VPN)  >   OpenSSH Vendors:   OpenSSH.org
(Red Hat Issues Fix) OpenSSH 'KbdInteractiveDevices' Lets Remote Users Bypass Security Restrictions on the Target System
SecurityTracker Alert ID:  1034188
SecurityTracker URL:  http://securitytracker.com/id/1034188
CVE Reference:   CVE-2015-5600   (Links to External Site)
Date:  Nov 20 2015
Impact:   Host/resource access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  

Description:   A vulnerability was reported in OpenSSH. A remote user can bypass authentication attempt limits on the target system.

A remote user can request the keyboard-interactive authentication option ('KbdInteractiveDevices') to open a large number of keyboard-interactive devices on the target server and perform a brute-force password guessing attack against the target sshd service. The number of password attempts can effectively exceed the 'MaxAuthTries' limit and are permitted to occur until the 'LoginGraceTime' limit is reached or the number of keyboard-interactive devices are used.

Servers that have keyboard-interactive authentication enabled (e.g., FreeBSD in the default configuration) are affected.

A demonstration exploit command is provided:

ssh -l[username] -oKbdInteractiveDevices=`perl -e 'print "pam," x 10000'` [target]

The original advisory is available at:

https://kingcope.wordpress.com/2015/07/16/openssh-keyboard-interactive-authentication-brute-force-vulnerability-maxauthtries-bypass/

kingcope reported this vulnerability.

Impact:   A remote user can bypass authentication attempt limits on the target system.
Solution:   Red Hat has issued a fix.

The Red Hat advisory is available at:

https://rhn.redhat.com/errata/RHSA-2015-2088.html

Vendor URL:  rhn.redhat.com/errata/RHSA-2015-2088.html (Links to External Site)
Cause:   Access control error, State error
Underlying OS Comments:  7

Message History:   This archive entry is a follow-up to the message listed below.
Jul 20 2015 OpenSSH 'KbdInteractiveDevices' Lets Remote Users Bypass Security Restrictions on the Target System



 Source Message Contents

Subject:  [RHSA-2015:2088-06] Moderate: openssh security, bug fix, and enhancement update

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=====================================================================
                   Red Hat Security Advisory

Synopsis:          Moderate: openssh security, bug fix, and enhancement update
Advisory ID:       RHSA-2015:2088-06
Product:           Red Hat Enterprise Linux
Advisory URL:      https://rhn.redhat.com/errata/RHSA-2015-2088.html
Issue date:        2015-11-19
CVE Names:         CVE-2015-5600 CVE-2015-6563 CVE-2015-6564 
=====================================================================

1. Summary:

Updated openssh packages that fix multiple security issues, several bugs,
and add various enhancements are now available for Red Hat Enterprise
Linux 7.

Red Hat Product Security has rated this update as having Moderate security
impact. Common Vulnerability Scoring System (CVSS) base scores, which give
detailed severity ratings, are available for each vulnerability from the
CVE links in the References section.

2. Relevant releases/architectures:

Red Hat Enterprise Linux Client (v. 7) - x86_64
Red Hat Enterprise Linux Client Optional (v. 7) - x86_64
Red Hat Enterprise Linux ComputeNode (v. 7) - x86_64
Red Hat Enterprise Linux ComputeNode Optional (v. 7) - x86_64
Red Hat Enterprise Linux Server (v. 7) - aarch64, ppc64, ppc64le, s390x, x86_64
Red Hat Enterprise Linux Server Optional (v. 7) - aarch64, ppc64, ppc64le, s390x, x86_64
Red Hat Enterprise Linux Workstation (v. 7) - x86_64
Red Hat Enterprise Linux Workstation Optional (v. 7) - x86_64

3. Description:

OpenSSH is OpenBSD's SSH (Secure Shell) protocol implementation. These
packages include the core files necessary for both the OpenSSH client and
server.

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.
(CVE-2015-6563)

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. (BZ#1238238)

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. (BZ#1150007)

* 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. (BZ#1160377)

* 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
situation. (BZ#1199112)

* 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.
(BZ#1201758)

* 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. (BZ#1240613)

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
stays untouched. (BZ#1201753)

* With this enhancement update, the administrator is able to set
permissions for files uploaded using Secure File Transfer Protocol (SFTP).
(BZ#1197989)

* This update provides the LDAP schema in LDAP Data Interchange Format
(LDIF) format as a complement to the old schema previously accepted
by OpenLDAP. (BZ#1184938)

* With this update, the user can selectively disable the Generic Security
Services API (GSSAPI) key exchange algorithms as any normal key exchange.
(BZ#1253062)

Users of openssh are advised to upgrade to these updated packages, which
correct these issues and add these enhancements.

4. Solution:

Before applying this update, make sure all previously released errata
relevant to your system have been applied.

For details on how to apply this update, refer to:

https://access.redhat.com/articles/11258

5. Bugs fixed (https://bugzilla.redhat.com/):

1125110 - pam_namespace usage is not consistent across system-wide PAM configuration
1160377 - sftp is failing using wildcards and many files
1178116 - Default selinux policy prevents ssh-ldap-helper from connecting to LDAP server
1181591 - No Documentation= line in the sshd.service file
1184938 - Provide LDIF version of LPK schema
1187597 - sshd -T does not show all (default) options, inconsistency
1197666 - ssh client using HostbasedAuthentication aborts in FIPS mode
1197989 - RFE: option to let openssh/sftp force the exact permissions on newly uploaded files
1238238 - openssh: weakness of agent locking (ssh-add -x) to password guessing
1245969 - CVE-2015-5600 openssh: MaxAuthTries limit bypass via duplicates in KbdInteractiveDevices
1252844 - CVE-2015-6563 openssh: Privilege separation weakness related to PAM support
1252852 - CVE-2015-6564 openssh: Use-after-free bug related to PAM support

6. Package List:

Red Hat Enterprise Linux Client (v. 7):

Source:
openssh-6.6.1p1-22.el7.src.rpm

x86_64:
openssh-6.6.1p1-22.el7.x86_64.rpm
openssh-askpass-6.6.1p1-22.el7.x86_64.rpm
openssh-clients-6.6.1p1-22.el7.x86_64.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-keycat-6.6.1p1-22.el7.x86_64.rpm
openssh-server-6.6.1p1-22.el7.x86_64.rpm

Red Hat Enterprise Linux Client Optional (v. 7):

x86_64:
openssh-debuginfo-6.6.1p1-22.el7.i686.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-ldap-6.6.1p1-22.el7.x86_64.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.x86_64.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.i686.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.x86_64.rpm

Red Hat Enterprise Linux ComputeNode (v. 7):

Source:
openssh-6.6.1p1-22.el7.src.rpm

x86_64:
openssh-6.6.1p1-22.el7.x86_64.rpm
openssh-clients-6.6.1p1-22.el7.x86_64.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-keycat-6.6.1p1-22.el7.x86_64.rpm
openssh-server-6.6.1p1-22.el7.x86_64.rpm

Red Hat Enterprise Linux ComputeNode Optional (v. 7):

x86_64:
openssh-askpass-6.6.1p1-22.el7.x86_64.rpm
openssh-debuginfo-6.6.1p1-22.el7.i686.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-ldap-6.6.1p1-22.el7.x86_64.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.x86_64.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.i686.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.x86_64.rpm

Red Hat Enterprise Linux Server (v. 7):

Source:
openssh-6.6.1p1-22.el7.src.rpm

aarch64:
openssh-6.6.1p1-22.el7.aarch64.rpm
openssh-clients-6.6.1p1-22.el7.aarch64.rpm
openssh-debuginfo-6.6.1p1-22.el7.aarch64.rpm
openssh-keycat-6.6.1p1-22.el7.aarch64.rpm
openssh-server-6.6.1p1-22.el7.aarch64.rpm

ppc64:
openssh-6.6.1p1-22.el7.ppc64.rpm
openssh-askpass-6.6.1p1-22.el7.ppc64.rpm
openssh-clients-6.6.1p1-22.el7.ppc64.rpm
openssh-debuginfo-6.6.1p1-22.el7.ppc64.rpm
openssh-keycat-6.6.1p1-22.el7.ppc64.rpm
openssh-server-6.6.1p1-22.el7.ppc64.rpm

ppc64le:
openssh-6.6.1p1-22.el7.ppc64le.rpm
openssh-askpass-6.6.1p1-22.el7.ppc64le.rpm
openssh-clients-6.6.1p1-22.el7.ppc64le.rpm
openssh-debuginfo-6.6.1p1-22.el7.ppc64le.rpm
openssh-keycat-6.6.1p1-22.el7.ppc64le.rpm
openssh-server-6.6.1p1-22.el7.ppc64le.rpm

s390x:
openssh-6.6.1p1-22.el7.s390x.rpm
openssh-askpass-6.6.1p1-22.el7.s390x.rpm
openssh-clients-6.6.1p1-22.el7.s390x.rpm
openssh-debuginfo-6.6.1p1-22.el7.s390x.rpm
openssh-keycat-6.6.1p1-22.el7.s390x.rpm
openssh-server-6.6.1p1-22.el7.s390x.rpm

x86_64:
openssh-6.6.1p1-22.el7.x86_64.rpm
openssh-askpass-6.6.1p1-22.el7.x86_64.rpm
openssh-clients-6.6.1p1-22.el7.x86_64.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-keycat-6.6.1p1-22.el7.x86_64.rpm
openssh-server-6.6.1p1-22.el7.x86_64.rpm

Red Hat Enterprise Linux Server Optional (v. 7):

aarch64:
openssh-askpass-6.6.1p1-22.el7.aarch64.rpm
openssh-debuginfo-6.6.1p1-22.el7.aarch64.rpm
openssh-ldap-6.6.1p1-22.el7.aarch64.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.aarch64.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.aarch64.rpm

ppc64:
openssh-debuginfo-6.6.1p1-22.el7.ppc.rpm
openssh-debuginfo-6.6.1p1-22.el7.ppc64.rpm
openssh-ldap-6.6.1p1-22.el7.ppc64.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.ppc64.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.ppc.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.ppc64.rpm

ppc64le:
openssh-debuginfo-6.6.1p1-22.el7.ppc64le.rpm
openssh-ldap-6.6.1p1-22.el7.ppc64le.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.ppc64le.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.ppc64le.rpm

s390x:
openssh-debuginfo-6.6.1p1-22.el7.s390.rpm
openssh-debuginfo-6.6.1p1-22.el7.s390x.rpm
openssh-ldap-6.6.1p1-22.el7.s390x.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.s390x.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.s390.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.s390x.rpm

x86_64:
openssh-debuginfo-6.6.1p1-22.el7.i686.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-ldap-6.6.1p1-22.el7.x86_64.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.x86_64.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.i686.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.x86_64.rpm

Red Hat Enterprise Linux Workstation (v. 7):

Source:
openssh-6.6.1p1-22.el7.src.rpm

x86_64:
openssh-6.6.1p1-22.el7.x86_64.rpm
openssh-askpass-6.6.1p1-22.el7.x86_64.rpm
openssh-clients-6.6.1p1-22.el7.x86_64.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-keycat-6.6.1p1-22.el7.x86_64.rpm
openssh-server-6.6.1p1-22.el7.x86_64.rpm

Red Hat Enterprise Linux Workstation Optional (v. 7):

x86_64:
openssh-debuginfo-6.6.1p1-22.el7.i686.rpm
openssh-debuginfo-6.6.1p1-22.el7.x86_64.rpm
openssh-ldap-6.6.1p1-22.el7.x86_64.rpm
openssh-server-sysvinit-6.6.1p1-22.el7.x86_64.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.i686.rpm
pam_ssh_agent_auth-0.9.3-9.22.el7.x86_64.rpm

These packages are GPG signed by Red Hat for security.  Our key and
details on how to verify the signature are available from
https://access.redhat.com/security/team/key/

7. References:

https://access.redhat.com/security/cve/CVE-2015-5600
https://access.redhat.com/security/cve/CVE-2015-6563
https://access.redhat.com/security/cve/CVE-2015-6564
https://access.redhat.com/security/updates/classification/#moderate

8. Contact:

The Red Hat security contact is <secalert@redhat.com>. More contact
details at https://access.redhat.com/security/team/contact/

Copyright 2015 Red Hat, Inc.
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