Cisco Response

• This Applied Mitigation Bulletin is a companion document to the PSIRT Security Advisory Multiple Vulnerabilities in Cisco IOS XE Software for Cisco ASR 1000 Series, Cisco ISR 4400 Series, and Cisco Cloud Services 1000v Series Routers and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.
  
  This document provides identification and mitigation techniques that administrators can deploy on Cisco network devices. The techniques presented in this document only apply to two vulnerabilities for the Cisco IOS XE Software Layer 4 Redirect Crafted Packet Denial of Service Vulnerability,
  Cisco IOS XE Software Crafted TCP Packet Remote Code Execution Vulnerability and Cisco IOS XE Software Common Flow Table Crafted Packet Denial of Service Vulnerability.

Vulnerability Characteristics

• There are multiple vulnerabilities in Cisco IOS XE. The following subsections summarize these vulnerabilities:
  
  Cisco IOS XE Software Common Flow Table Crafted Packet Denial of Service Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could result in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through crafted TCP or UDP over Internet Protocol version 4 (IPv4) packets. An attacker could exploit this vulnerability using spoofed packets.
  
  This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2015-0639.
  
  Cisco IOS XE Software Crafted TCP Packet Remote Code Execution Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could may allow arbitrary code execution on the affected system or a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through crafted TCP packets.
  
  This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2015-0644.
  
  Cisco IOS XE Software Layer 4 Redirect Crafted Packet Denial of Service Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability could result in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition. The attack vector for exploitation is through crafted TCP packets.
  
  This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2015-0645.

Vulnerability Overview

• Information about vulnerable, unaffected, and fixed software is available in the Cisco Security Advisory, which is available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20150325-iosxe.

Mitigation Technique Overview

• Cisco devices provide several countermeasures for this vulnerability. Administrators are advised to consider these protection methods to be general security best practices for infrastructure devices and the traffic that transits the network. This section of the document provides an overview of these techniques.
  
  Cisco IOS Software can provide effective means of exploit prevention using the following methods:
  

  • IOS Zone-Based and User-Based Firewall
  • Unicast Reverse Path Forwarding (uRPF)
  • IP source guard (IPSG)

  
  These protection mechanisms filter and drop, as well as verify the source IP address of, packets that are attempting to exploit these vulnerabilities.
  
  The proper deployment and configuration of uRPF provides an effective means of protection against attacks that use packets with spoofed source IP addresses. uRPF should be deployed as close to all traffic sources as possible.
  
  The proper deployment and configuration of IPSG provides an effective means of protection against spoofing attacks at the access layer.
  
  Effective means of exploit prevention can also be provided by the Cisco ASA 5500 and 5500-X Series Adaptive Security Appliance, Cisco Catalyst 6500 Series ASA Services Module (ASASM), and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers using the following:
  

  • uRPF
  • TCP normalization

  
  These protection mechanisms filter and drop, as well as verify the source IP address of, packets that are attempting to exploit these vulnerabilities.
  
  Effective means of exploit prevention can also be provided by the Cisco ACE Application Control Engine Appliance and Module using TCP normalization.
  
  This protection mechanism filters and drops packets that are attempting to exploit one of the vulnerabilities.
  
  Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against attacks that attempt to exploit this vulnerability.
  

Risk Management

• Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact of these vulnerabilities. Triage refers to sorting projects and prioritizing efforts that are most likely to be successful. Cisco has provided documents that can help organizations develop a risk-based triage capability for their information security teams. Risk Triage for Security Vulnerability Announcements and Risk Triage and Prototyping can help organizations develop repeatable security evaluation and response processes.

Device-Specific Mitigation and Identification

• Caution:The effectiveness of any mitigation technique depends on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. As with any configuration change, evaluate the impact of this configuration prior to applying the change.
  
  Specific information about mitigation and identification is available for these devices:
  

  • Cisco IOS Routers and Switches
  • Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls
  • Cisco ACE
  • Cisco Intrusion Prevention System

  Cisco IOS Routers and Switches

  The Cisco IOS software performs various packet checks for transit traffic by default. For example, it verifies the IP header correctness of a packet. A Cisco IOS device actively processing traffic will, by default, identify and drop crafted packets that attempt to exploit CVE-2015-0645. Protection against packets that may exploit this vulnerability is a non-configurable action: no configuration changes are required to enable this functionality.

  Mitigation: IOS Zone Based and User-Based Firewall

  Starting in Cisco IOS Software Release 12.4(6)T, Zone-Based Policy Firewall (ZFW) replaced Cisco IOS Context-Based Access Control (CBAC). It offers granularity of firewall policy application, and a default deny-all policy that prohibits traffic between firewall security zones until an explicit policy is applied to allow desirable traffic.
  
  In Cisco IOS ZFW, zones establish the security borders of the network. A zone defines a boundary where traffic is subjected to policy restrictions as it crosses to another region of your network. The ZFW default policy between zones is to deny all traffic. If no policy is explicitly configured, all traffic attempting to move between zones is blocked. ZFW uses a configuration policy language known as Cisco Policy Language (CPL). Users familiar with the Cisco IOS Software Modular quality-of-service (QoS) CLI (MQC) might recognize that the format is similar to the way class maps are used in a QoS configuration to specify which traffic will be affected by the action applied in a policy map. Cisco IOS ZFW supports stateful Layer 4 IPv4 and IPv6 inspections and can also provide application-specific inspection, stateful firewall fail-over, authentication proxy, denial of service (DoS) mitigation, URL filtering, and more.
  
  Cisco IOS ZFW will identify and drop packets that attempt to exploit CVE-2015-0639.
  
  For configuration examples refer to the IOS ZBF Set-by-Step Configuration and IPv6 Zone Based Firewall (ZFW) Configuration documents in the Cisco Support Community and Cisco Configuration Professional: Zone-Based Firewall Blocking Peer to Peer Traffic Configuration Example. For more information on Cisco IOS ZBF, refer to the Zone-Based Policy Firewall Design and Application Guide.
  
  Introduced in Cisco IOS Software Releases 12.4(20)T, the IOS User-Based Firewall feature can provide identity or user-group based security that provides differentiated access for different classes of users. Classification can be provided on the basis of user identity, device type (for example, IP phones), location (for example, building) and role (for example, engineer).
  
  The Cisco IOS Firewall policies blocking access to the protocols and ports or filtering applications can be configured on a per user or user group basis leveraging the User Based Firewall feature.
  
  For more information on Cisco User-Based Firewall, refer to the User Based Firewall Support Guide and its Feature Information for User-Based Firewall Support section.

  Mitigation: Spoofing Protection

  Unicast Reverse Path Forwarding
  
  CVE-2015-0645 that is described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure Unicast Reverse Path Forwarding (uRPF) as a protection mechanism against spoofing.
  
  uRPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on uRPF to provide complete spoofing protection because spoofed packets may enter the network through a uRPF-enabled interface if an appropriate return route to the source IP address exists. Administrators are advised to take care to ensure that the appropriate uRPF mode (loose or strict) is configured during the deployment of this feature because it can drop legitimate traffic that is transiting the network. In an enterprise environment, uRPF may be enabled at the Internet edge and the internal access layer on the user-supporting Layer 3 interfaces.
  
  For additional information about the configuration and use of uRPF, reference the Understanding Unicast Reverse Path Forwarding Cisco Security Intelligence Operations white paper.
  
  IP Source Guard
  
  IP source guard (IPSG) is a security feature that restricts IP traffic on nonrouted, Layer 2 interfaces by filtering packets based on the DHCP snooping binding database and manually configured IP source bindings. Administrators can use IPSG to prevent attacks from an attacker who attempts to spoof packets by forging the source IP address and/or the MAC address. When properly deployed and configured, IPSG coupled with strict mode uRPF provides the most effective means of spoofing protection for CVE-2015-0645 that is described in this document.
  
  Additional information about the deployment and configuration of IPSG is in Configuring DHCP Features and IP Source Guard.
  
  For information on how to use the IOS command line interface to gauge the effectiveness of spoofing protection, please refer to the Cisco Security Intelligence Operations white paper Identifying the Effectiveness of Security Mitigations Using Cisco IOS Software.

  Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls

  The Cisco ASA software performs various packet checks for transit traffic by default. For example, it verifies the IP header correctness of a packet. A Cisco ASA actively inspecting traffic will by default identify and drop crafted packet that attempt to exploit CVE-2015-0645. Protection against packets that may exploit this vulnerability is a non-configurable action: no configuration changes are required to enable this functionality.

  Mitigation: Spoofing Protection Using Unicast Reverse Path Forwarding

  CVE-2015-0645 that is described in this document can be exploited by spoofed IP packets. Administrators can deploy and configure uRPF as a protection mechanism against spoofing.
  
  uRPF is configured at the interface level and can detect and drop packets that lack a verifiable source IP address. Administrators should not rely on uRPF to provide complete spoofing protection because spoofed packets may enter the network through a uRPF-enabled interface if an appropriate return route to the source IP address exists. In an enterprise environment, uRPF may be enabled at the Internet edge and at the internal access layer on the user-supporting Layer 3 interfaces.
  
  For additional information about the configuration and use of uRPF, reference the Cisco Security Appliance Command Reference for ip verify reverse-path and the Understanding Unicast Reverse Path Forwarding Cisco Security Intelligence Operations white paper.
  
  For information on how to use the firewall command line interface to gauge the effectiveness of spoofing protection, please refer to the Cisco Security Intelligence Operations white paper Identification of Security Exploits with Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls.

  Mitigation: TCP Normalization

  The TCP normalization feature identifies abnormal packets that the security appliance can act on when they are detected; for example, the security appliance can allow, drop, or clear the packets. The TCP normalizer includes non-configurable actions and configurable actions. Typically, non-configurable actions that drop or clear connections apply to packets that are considered malicious. TCP normalization is available beginning in software release 7.0(1) for the Cisco ASA 5500 Series Adaptive Security Appliance, software release 8.5 for the Cisco Catalyst 6500 Series ASA Services Module, and in software release 3.1(1) for the Cisco Firewall Services Module.
  
  TCP normalization is enabled by default and drops packets that may exploit CVE-2015-0639. Protection against packets that may exploit this vulnerability is a non-configurable TCP normalization action: no configuration changes are required to enable this functionality.
  
  Additional information about TCP normalization is in the Connection limits and Timeouts section of the Book 2: Cisco ASA Series Firewall CLI Configuration Guide, 9.2.
  
  For information on how to use the firewall command-line interface (CLI) to gauge the effectiveness of the TCP normalization, please see the Cisco Security Intelligence Operations white paper Identification of Security Exploits with Cisco ASA, Cisco ASASM, and Cisco FWSM Firewalls.

  Cisco ACE

  Mitigation: TCP Normalization

  TCP normalization is a Layer 4 feature that consists of a series of checks that the Cisco ACE performs at various stages of a flow, beginning with the initial connection setup through the closing of a connection. Many of the segment checks can be controlled or altered by configuring one or more advanced TCP connection settings. The Cisco ACE uses these TCP connection settings to decide which checks to perform and whether to discard a TCP segment based on the results of the checks. The Cisco ACE discards segments that appear to be abnormal or malformed.
  
  TCP normalization is enabled by default and drops packets that may exploit CVE-2015-0639. Protection against packets that may exploit this vulnerability is a non-configurable TCP normalization action; no configuration changes are required to enable this functionality.
  
  Additional information about TCP normalization is in the Configuring TCP/IP Normalization and IP Reassembly Parameters section of the Security Guide vA5(1.0), Cisco ACE Application Control Engine.
  
  

  Cisco Intrusion Prevention System

  Mitigation: Cisco IPS Signature Table

  Administrators can use the Cisco IPS appliances and services modules to provide threat detection and help prevent attempts to exploit several of the vulnerabilities described in this document. The following table provides an overview of CVE identifiers and the respective Cisco IPS signatures that will trigger events on potential attempts to exploit this vulnerability.
  
  

  CVE ID	Signature Release	Signature ID	Signature Name	Enabled	Severity	Fidelity*
  CVE-2015-0644	S858	5094/0	Cisco IOS XE Software Denial of Service	Yes	Medium	85

  
  
  * Fidelity is also referred to as Signature Fidelity Rating (SFR) and is the relative measure of the accuracy of the signature (predefined). The value ranges from 0 through 100 and is set by Cisco Systems, Inc.
  
  Administrators can configure Cisco IPS sensors to perform an event action when an attack is detected. The configured event action performs preventive or deterrent controls to help protect against an attack that is attempting to exploit the vulnerabilities listed in the preceding table.
  
  Exploits that use spoofed IP addresses may cause a configured event action to inadvertently deny traffic from trusted sources.
  
  Cisco IPS sensors are most effective when deployed in inline protection mode combined with the use of an event action. Automatic Threat Prevention for Cisco IPS 7.x and 6.x sensors that are deployed in inline protection mode provides threat prevention against an attack that is attempting to exploit the vulnerability that is described in this document. Threat prevention is achieved through a default override that performs an event action for triggered signatures with a riskRatingValue greater than 90.
  
  For additional information about the risk rating and threat rating calculation, reference Risk Rating and Threat Rating: Simplify IPS Policy Management.
  
  For information on using Cisco Security Manager to view the activity from a Cisco IPS sensor, see Identification of Malicious Traffic Using Cisco Security Manager white paper.
  

Additional Information

• THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

Revision History

• Version	Description	Section	Date
  1	Initial Release		2015-March-25 16:01 GMT

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Cisco Security Procedures

• Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at https://sec.cloudapps.cisco.com/security/center/resources/security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.

Related Information

• • Cisco Applied Mitigation Bulletins
  • Cisco Security Portal
  • Cisco Security IntelliShield Alert Manager Service
  • Cisco Guide to Harden Cisco IOS Devices
  • TTL Expiry Attack Identification and Mitigation
  • Zone-Based Policy Firewall Design and Application Guide
  • Cisco Firewall Products - Home Page on Cisco.com
  • Cisco Catalyst 6500 Series ASA Services Module
  • Cisco 5500-X
  • Cisco ACE Application Control Engine Module Documentation
  • Unicast Reverse Path Forwarding Enhancements for the Internet Service Provider
  • Cisco Intrusion Prevention System
  • Sourcefire Next-Generation Security
  • Cisco IPS Signature Search Page
  • Common Vulnerabilities and Exposures (CVE)
  • Subscribe to Cisco Applied Mitigation Bulletins

Affected Products

• The security vulnerability applies to the following combinations of products.
  
  

  Primary Products
  Cisco	Cisco IOS XE Software	3.1S (3.1.0S, 3.1.1S, 3.1.2S, 3.1.3S, 3.1.4S, 3.1.5S, 3.1.6S) | 3.2S (3.2.0S, 3.2.1S, 3.2.2S, 3.2.3S) | 3.3S (3.3.0S, 3.3.1S, 3.3.2S) | 3.4S (3.4.0S, 3.4.1S, 3.4.2S, 3.4.3S, 3.4.4S, 3.4.5S, 3.4.6S) | 3.5S (Base, 3.5.0S, 3.5.1S, 3.5.2S) | 3.6S (Base, 3.6.0S, 3.6.1S, 3.6.2S) | 3.7S (Base, 3.7.0S, 3.7.1S, 3.7.2S, 3.7.3S, 3.7.4S, 3.7.5S, 3.7.6S, 3.7.7S) | 3.8S (Base, 3.8.0S, 3.8.1S) | 3.9S (3.9.0S, 3.9.1S) | 3.10S (3.10.0aS)

  
  
  

  Associated Products