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             AUSCERT External Security Bulletin Redistribution

                               ESB-2015.2094
   Cisco ASA Unicast Reverse Path Forwarding (uRPF) Bypass Vulnerability
                              13 August 2015

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        AusCERT Security Bulletin Summary
        ---------------------------------

Product:           Cisco ASA
Publisher:         Cisco Systems
Operating System:  Cisco
Impact/Access:     Reduced Security -- Remote/Unauthenticated
Resolution:        Mitigation
CVE Names:         CVE-2015-4321  

Original Bulletin: 
   http://tools.cisco.com/security/center/viewAlert.x?alertId=40440

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Vulnerability Alert

Cisco ASA Unicast Reverse Path Forwarding (uRPF) Bypass Vulnerability

Threat Type:		CWE-20: Input Validation
IntelliShield ID:	40440
Version:		1
First Published:	2015 August 12 18:05 GMT
Last Published:		2015 August 12 18:05 GMT
Port:			Not available
CVE:			CVE-2015-4321
Urgency:		Unlikely Use
Credibility:		Confirmed
Severity:		Mild Damage
CVSS Base:		5.0
CVSS Temporal:		4.8

Version Summary: Cisco Adaptive Security Appliance contains a
vulnerability that could allow an unauthenticated, remote attacker to
bypass security restrictions. Updates are not available.

Description
A vulnerability in the Unicast Reverse Path Forwarding (uRPF) feature in
the Cisco Adaptive Security Appliance (ASA) could allow an unauthenticated,
remote attacker to bypass the uRPF validation checks.

The vulnerability is due to incorrect uRPF validation where IP packets from
an outside interface, whose IP address is both in the ASA routing table and
associated with an internal interface, are not dropped. An attacker could
exploit this vulnerability by sending spoofed IP packets to the ASA in a
subnet range that should be dropped. An exploit could allow the attacker
to bypass uRPF validation on the ASA and the packet will be incorrectly
forwarded on the internal network.

Cisco has confirmed the vulnerability; however, software updates are
not available.

Warning Indicators
At the time this alert was first published, Cisco ASA Software Releases
9.3(1.50), 9.3(2.100), 9.3(3), and 9.4(1) were vulnerable. Later releases
of Cisco ASA Software may also be vulnerable.

IntelliShield Analysis
To exploit this vulnerability, the targeted device must have the ip verfiy
reverse-path interface command set on an outside interface and the interface
must have a default route configured. In addition, the attacker must know a
subnet range set on the targeted device that is configured to drop packets
prior to sending spoofed IP packets to the device.

Cisco indicates through the CVSS score that functional exploit code exists;
however, the code is not known to be publicly available.

Vendor Announcements
Cisco has released bug ID CSCuv60724 for registered users, which contains
additional details and an up-to-date list of affected product versions.


Impact

An unauthenticated, remote attacker could exploit this vulnerability to
bypass uRPF validation checks on an affected device. An exploit could
allow the attacker to incorrectly forward packets on the internal network.

Technical Information
The vulnerability is due to incorrect uRPF validation where IP packets
from an outside interface, whose IP address is both in the ASA routing
table and associated with an internal interface, are not dropped.

An unauthenticated, remote attacker could exploit this vulnerability by
sending spoofed IP packets to a targeted ASA in a subnet range that should
be dropped. An exploit could allow the attacker to bypass uRPF validation
on the ASA which would cause packets to be incorrectly forwarded on the
internal network.

Safeguards

Administrators are advised to contact the vendor regarding future updates
and releases.

Administrators can help protect affected systems from external attacks by
using a solid firewall strategy.

Administrators may consider using IP-based access control lists (ACLs)
to allow only trusted systems to access the affected systems.

Administrators are advised to monitor affected systems.

Patches/Software

Software updates are not available.

Alert History

Initial Release

Product Sets

The security vulnerability applies to the following combinations of products.

Primary Products:

Cisco	Cisco Adaptive Security Appliance (ASA) Software	9.3 .3,
(2.100), (1.50) | 9.4 .1

Associated Products:
N/A


LEGAL DISCLAIMER
The urgency and severity ratings of this alert are not tailored to
individual users; users may value alerts differently based upon their
network configurations and circumstances. THE ALERT, AND INFORMATION
CONTAINED THEREIN, ARE PROVIDED ON AN "AS IS" BASIS AND DO NOT IMPLY ANY
KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY
OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE ALERT, AND INFORMATION
CONTAINED THEREIN, OR MATERIALS LINKED FROM THE ALERT, IS AT YOUR OWN
RISK. INFORMATION IN THIS ALERT AND ANY RELATED COMMUNICATIONS IS BASED ON
OUR KNOWLEDGE AT THE TIME OF PUBLICATION AND IS SUBJECT TO CHANGE WITHOUT
NOTICE. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE ALERTS AT ANY TIME.

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