Applied Mitigation Bulletin

This is the initial version of the Cisco Applied Mitigation Bulletin to address the Apache HTTPd Range header denial of service vulnerability (CVE-2011-3192).

Contents

Introduction

Apache HTTP Server contains a vulnerability that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition.

The vulnerability is due to improper processing of certain user-supplied requests by the affected software. An unauthenticated, remote attacker could exploit this vulnerability by sending crafted requests to the system. Processing such requests could cause the application to consume excessive memory, resulting in a DoS condition on the system.

Proof-of-concept code that exploits this vulnerability is publicly available.

Apache has confirmed this vulnerability, but updated software is not available.

Vulnerability Characteristics

Details of this vulnerability are described in IntelliShield Alert 24004.

Mitigation Technique Overview

Effective exploit prevention can be provided by the Cisco ASA 5500 Series Adaptive Security Appliance and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers using application layer protocol inspection.

Cisco ASA and FWSM firewalls can provide visibility through syslog messages and counter values displayed in the output from show commands.

Risk Management

Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact of this vulnerability. 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 ASA and FWSM Firewalls

Cisco ASA and FWSM Firewalls

Mitigation: Application Layer Protocol Inspection

Application layer protocol inspection is available beginning in software release 7.2(1) for the Cisco ASA 5500 Series Adaptive Security Appliance and in software release 4.0(1) for the Firewall Services Module. This advanced security feature performs deep packet inspection of traffic that transits the firewall. Administrators may construct an inspection policy for applications that require special handling through the configuration of inspect class maps and inspect policy maps, which are applied via a global or interface service policy.

Additional information about application layer protocol inspection is in the Applying Application Layer Protocol Inspection section of the Cisco Security Appliance Command Line Configuration Guide.

Caution: Application layer protocol inspection will decrease firewall performance. Administrators are advised to test performance impact in a lab environment before this feature is deployed in production environments.

HTTP Application Inspection

By using the HTTP inspection engine on the Cisco ASA 5500 Series Adaptive Security Appliances and the Firewall Services Module, administrators can configure regular expressions (regexes) for pattern matching and construct inspect class maps and inspect policy maps. This method can help protect against specific vulnerabilities, such as the one described in this document, and other threats that may be associated with HTTP traffic. The following HTTP application inspection configuration uses the Cisco Modular Policy Framework (MPF) to create a policy for inspection of traffic on TCP ports 80, 3128, 8000, 8010, 8080, 8888, and 24326, which are the default ports for the Cisco IPS #WEBPORTS variable.

The following HTTP application inspection policy will only match text strings and log the connections where the Range: or Request-Range: header in an Hypertext Transfer Protocol (HTTP) request contains at least five range values. Additional information about regex syntax is in Creating a Regular Expression.

NOTE: This inspection policy will not drop any HTTP connection, it will only log details about the connections that match the regex for the Range: or Request-Range: header, which should help identify attempts to exploit this vulnerability

!
!-- Configure an object group for the default ports that 
!-- are used by the Cisco IPS #WEBPORTS variable, which 
!-- are TCP ports 80 (www), 3128, 8000, 8010, 8080, 8888, 
!-- and 24326
!
object-group service WEBPORTS tcp
 port-object eq www
 port-object eq 3128
 port-object eq 8000
 port-object eq 8010
 port-object eq 8080
 port-object eq 8888
 port-object eq 24326
!
!
!-- Configure an access list that uses the WEBPORTS object 
!-- group, which will be used to match TCP packets that 
!-- are destined to the #WEBPORTS variable that is used 
!-- by a Cisco IPS device
!
access-list Webports_ACL extended permit tcp any any object-group WEBPORTS 
!
!
!-- Configure a class that uses the above-configured
!-- access list to match TCP packets that are destined
!-- to the ports that are used by the Cisco IPS #WEBPORTS
!-- variable
!
class-map Webports_Class
 match access-list Webports_ACL
!
!
!-- Configure a regex that checks for the presence of at
!-- least five ranges in the "Range:" or "Request-Range:"
!-- header that can be used to exploit the CVE-2011-3192
!-- Range Header DoS vulnerability in Apache HTTPD 1.3/2.x
!
regex CVE-2011-3192 "bytes[\x20\x09]*=[\x20\x09]*([^,]+(,[^,]+){4,}|$)"
!
!
!-- Configure an HTTP inspection class that will match the
!-- regular expression that is configured for CVE-2011-3192
!
class-map type inspect http match-any CVE-2011-3192_http_class
 match request header range regex CVE-2011-3192

!
!
!-- Configure an HTTP inspection policy that will match and
!-- log HTTP requests that match the regex configured for the
!-- HTTP inspection class "CVE-2011-3192_http_class"
!
policy-map type inspect http CVE-2011-3192_policy
 parameters
 class CVE-2011-3192_http_class
  log
!
!
!-- Add the above-configured "Webports-Class" that matches 
!-- TCP packets that are destined to the default ports  
!-- that are used by the Cisco IPS #WEBPORTS variable to  
!-- the default policy "global_policy" and use it to 
!-- inspect HTTP traffic that transits the firewall
! 
policy-map global_policy
 class Webports_Class
  inspect http CVE-2011-3192_policy 
!
!
!-- By default, the policy "global_policy" is applied 
!-- globally, which results in the inspection of 
!-- traffic that enters the firewall from all interfaces 
!
service-policy global_policy global
!

For additional information about the configuration and use of object groups, reference the Cisco Security Appliance Command Reference for object-group.

Additional information about HTTP application inspection and the MPF is in the HTTP Inspection Overview section of the Cisco Security Appliance Command Line Configuration Guide.

Identification: Application Layer Protocol Inspection

Firewall syslog message 415008 will be generated when a user-specified field in the HTTP message header matches the regular expression in an HTTP request. The syslog message will identify the corresponding HTTP class and HTTP policy and indicate the action applied to the HTTP connection. Additional information about this syslog message is in Cisco Security Appliance System Log Message - 415008.

Information about configuring syslog for the Cisco ASA 5500 Series Adaptive Security Appliance is in Monitoring the Security Appliance - Configuring and Managing Logs. Information about configuring syslog on the FWSM for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers is in Monitoring the Firewall Services Module.

In the following example, the show logging | grep regex command extracts syslog messages from the logging buffer on the firewall. These messages provide additional information about denied packets that could indicate attempts to exploit this vulnerability. Administrators can use different regular expressions with the grep keyword to search for specific data in the logged messages.

Additional information about regular expression syntax is in Creating a Regular Expression.

HTTP Application Inspection

firewall#show logging | grep 415008
%ASA-5-415008: HTTP - matched Class 22: CVE-2011-3192_http_class in
  policy-map CVE-2011-3192_policy, header matched from
  outside:10.0.1.21/52208 to inside:172.16.10.3/80
%ASA-5-415008: HTTP - matched Class 22: CVE-2011-3192_http_class in
  policy-map CVE-2011-3192_policy, header matched from
  outside:10.0.1.21/52209 to inside:172.16.10.3/80

With HTTP application inspection enabled, the show service-policy inspect protocol command will identify the number of HTTP packets that are inspected and logged by this feature. The following example shows output for show service-policy inspect http:

firewall# 
firewall# show service-policy inspect http

Global policy: 
  Service-policy: global_policy
    Class-map: inspection_default
    Class-map: Webports_Class
      Inspect: http CVE-2011-3192_policy, packet 158, drop 0, reset-drop 0
        protocol violations
          packet 6
        class CVE-2011-3192_http_class (match-any) 
          Match: request header range regex CVE-2011-3192, 4 packets
          log, packet 4
firewall# 
firewall# 

In the preceding example, 158 HTTP packets have been inspected and 4 HTTP packets have been logged.

Additional Information

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Cisco Security Procedures

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Initial Release