M-125: Apache/mod_ssl Worm

[***** Start CERTŪ  Advisory CA-2002-27 *****]

CERTŪ Advisory CA-2002-27 Apache/mod_ssl Worm
Original release date: September 14, 2002
Last revised: September 16, 2002 11:19 EDT (UTC-0500)
Source: CERT/CC

A complete revision history can be found at the end of this file.



Systems Affected
Linux systems running Apache with mod_ssl accessing SSLv2-enabled OpenSSL 0.9.6d or 
earlier on Intel x86 architectures 


Overview
The CERT/CC has received reports of self-propagating malicious code which exploits a 
vulnerability (VU#102795) in OpenSSL. This malicious code has been referred to as 
Apache/mod_ssl worm, linux.slapper.worm and bugtraq.c worm. Reports received by the 
CERT/CC indicate that the Apache/mod_ssl worm has already infected thousands of systems.



I. Description
The Apache/mod_ssl worm is self-propagating malicious code that exploits the OpenSSL 
vulnerability described in VU#102795. This vulnerability was the among the topics 
discussed in CA-2002-23 Multiple Vulnerabilities In OpenSSL. While this OpenSSL server 
vulnerability exists on a wide variety of platforms, the Apache/mod_ssl worm appears to 
work only on Linux systems running Apache with the OpenSSL module (mod_ssl) on Intel 
architectures.

The Apache/mod_ssl worm scans for potentially vulnerable systems on 80/tcp using an 
invalid HTTP GET request. 

GET /mod_ssl:error:HTTP-request HTTP/1.0

When an Apache system is detected, it attempts to send exploit code to the SSL service 
via 443/tcp. If successful, a copy of the malicious source code is then placed on the 
victim server, where the attacking system tries to compile and run it. Once infected, 
the victim server begins scanning for additional hosts to continue the worm's 
propagation. 

Additionally, the Apache/mod_ssl worm can act as an attack platform for distributed 
denial-of-service (DDoS) attacks against other sites by building a network of infected 
hosts. During the infection process, the attacking host instructs the newly-infected 
victim to initiate traffic on 2002/udp back to the attacker. Once this communications 
channel has been established, the infected system becomes part of the Apache/mod_ssl 
worm's DDoS network. Infected hosts can then share information on other infected systems 
as well as attack instructions. Thus, the 2002/udp traffic can be used by a remote 
attacker as a communications channel between infected systems to coordinate attacks on 
other sites.

Reports to the CERT/CC indicate that the high volume of 2002/udp traffic generated 
between hosts infected with the Apache/mod_ssl worm may itself lead to performance 
issues on networks with infected hosts. Furthermore, since repairing an infected host 
does not remove its IP address from the Apache/mod_ssl worm's Peer-to-Peer network, 
sites that have had hosts infected with the Apache/mod_ssl worm and subsequently patched 
them may continue to see significant levels of 2002/udp traffic directed at those 
formerly infected systems.

Identifying infected hosts
Reports indicate that the Apache/mod_ssl worm's source code is placed in /tmp/.bugtraq.c 
on infected systems. It is compiled with gcc, resulting in the executable binary being 
stored at /tmp/.bugtraq; therefore, presence of any of the following files on Linux 
systems running Apache with OpenSSL is indicative of compromise. 

/tmp/.bugtraq.c 
/tmp/.bugtraq 
The probing phase of the attack may show up in web server logs as shown in the example 
below. Actual log entries may vary from system to system, but will generally include an 
"SSL handshake failed" followed by an OpenSSL library error. It is important to note 
that there may be other causes of such log entries, so the appearance of entries 
matching (or similar to) these in a web server log should not be construed as evidence 
of compromise. Rather, their presence is indicative that further investigation may be 
warranted. For example, : 

GET /mod_ssl:error:HTTP-request HTTP/1.0


Reports received by the CERT/CC indicate that Apache systems may subsequently log 
messages similar to the following: 

[error] SSL handshake failed: HTTP spoken on HTTPS port; trying to send HTML error page 
(OpenSSL library error follows)
[error] OpenSSL: error:1407609C:SSL routines:SSL23_GET_CLIENT_HELLO:http request [Hint: 
speaking HTTP to HTTPS port!?]

Hosts found to be listening for or transmitting data on 2002/udp are also indicative of 
compromise by the Apache/mod_ssl worm.

Detecting Apache/mod_ssl worm activity on the network
Infected systems are readily identifiable on a network by the following traffic 
characteristics: 

Probing -- Scanning on 80/tcp 
Propagation -- Connections to 443/tcp 
DDoS -- Transmitting or receiving datagrams with both source and destination ports 
2002/udp. This traffic is used as a communications channel between infected systems to 
coordinate attacks on other sites. 
Additionally, infected hosts that are actively participating in DDoS attacks against 
other systems may generate unusually high volumes of attack traffic using various 
protocols (e.g., TCP, UDP, ICMP)



II. Impact
Compromise by the Apache/mod_ssl worm indicates that a remote attacker can execute 
arbitrary code as the apache user on the victim system. It may be possible for an 
attacker to subsequently leverage a local privilege escalation exploit in order to gain 
root access to the victim system. The high volume of 2002/udp traffic generated between 
hosts infected with the Apache/mod_ssl worm may itself lead to performance issues on 
networks with infected or formerly infected hosts. Furthermore, the DDoS capabilities 
included in the Apache/mod_ssl worm allow victim systems to be used as platforms to 
attack other systems. 



III. Solution
Apply a patch
Administrators of all systems running OpenSSL are encouraged to review CA-2002-23 and 
VU#102795 for detailed vendor recommendations regarding patches.

Note that while the vulnerability exploited by the Apache/mod_ssl worm was fixed 
beginning with OpenSSL version 0.9.6e, as of this writing the latest version of OpenSSL 
is 0.9.6g. Administrators may wish to upgrade to that version instead.

The following is reproduced in part from CA-2002-23

Upgrade to version 0.9.6e of OpenSSL
Upgrade to version 0.9.6e of OpenSSL to resolve the issues addressed in this advisory. 
As noted in the OpenSSL advisory, separate patches are available:


Combined patches for OpenSSL 0.9.6d:
http://www.openssl.org/news/patch_20020730_0_9_6d.txt 
After either applying the patches above or upgrading to 0.9.6e, recompile all 
applications using OpenSSL to support SSL or TLS services, and restart said services or 
systems. This will eliminate all known vulnerable code. 

Sites running OpenSSL pre-release version 0.9.7-beta2 may wish to upgrade to 0.9.7-
beta3, which corrects these vulnerabilities. Separate patches are available as well:


Combined patches for OpenSSL 0.9.7 beta 2:
http://www.openssl.org/news/patch_20020730_0_9_7.txt 

Disable SSLv2
Disabling SSLv2 handshaking will prevent exploitation of VU#102795. CERT/CC recomends 
consulting the mod_ssl documentation for a complete description of the options but one 
method for disabling SSLv2 is to remove SSLv2 as a supported cipher in the 
SSLCipherSuite directive in the configuration file. For example: 
SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+SSLv2 
which allows SSLv2 can be changed to

SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:!SSLv2 
which will disable SSLv2. Note the changing of +SSLv2 to !SSLv2. 
However, systems may still be susceptible to the other vulnerabilities described in CA-
2002-23.

Ingress/Egress filtering
The following steps are only effective in limiting the damage that systems already 
infected with the Apache/mod_ssl worm can do. They provide no protection whatsoever 
against the initial infection of systems. As a result, these steps are only recommended 
in addition to the preventative steps outlined above, not in lieu thereof.

Ingress filtering manages the flow of traffic as it enters a network under your 
administrative control. Servers are typically the only machines that need to accept 
inbound traffic from the public Internet. In the network usage policy of many sites, 
external hosts are only permitted to initiate inbound traffic to machines that provide 
public services on specific ports. Thus, ingress filtering should be performed at the 
border to prohibit externally initiated inbound traffic to non-authorized services.

Egress filtering manages the flow of traffic as it leaves a network under your 
administrative control. There is typically limited need for machines providing public 
services to initiate outbound connections to the Internet. 

In the case of the Apache/mod_ssl worm, employing ingress and egress filtering can help 
prevent systems on your network from participating in the worm's DDoS network and 
attacking systems elsewhere. Blocking UDP datagrams with both source and destination 
port 2002 from entering or leaving your network reduces the risk of external infected 
systems communicating with infected hosts inside your network.

Recovering from a system compromise
If you believe a system under your administrative control has been compromised, please 
follow the steps outlined in

Steps for Recovering from a UNIX or NT System Compromise
Reporting
The CERT/CC is interested in receiving reports of this activity. If machines under your 
administrative control are compromised, please send mail to cert@cert.org with the 
following text included in the subject line: "[CERT#23820]".

   [  DOE sites should report any activity to ciac@ciac.org  ]

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Feedback can be directed to the author: Allen Householder 


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This document is available from: http://www.cert.org/advisories/CA-2002-27.html  


[***** End CERTŪ  Advisory CA-2002-27 *****]

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