Linux.Adore.Worm

SUMMARY

Linux.Adore.Worm is a worm that spreads on Linux systems. The worm targets vulnerabilities commonly found on default installations of Linux. Using these vulnerabilities, the worm gains root access to the system, downloads and executes itself, and then searches for new systems to infect.

NOTE: The Linux rootkit known as Adore is unrelated to this worm.

Protection

• Virus Definitions (Intelligent Updater) April 5, 2001

Threat Assessment

Wild

• Wild Level: Medium
• Number of Infections: 50 - 999
• Number of Sites: More than 10
• Geographical Distribution: Medium
• Threat Containment: Moderate
• Removal: Moderate

Damage

• Damage Level: Medium
• Modifies Files: Replaces ps and klogd
• Releases Confidential Info: Emails system information to anonymous addresses
• Compromises Security Settings: Creates a root shell backdoor

Distribution

• Distribution Level: Medium
• Target of Infection: Linux systems with vulnerable wuftpd, bind, lprng, or statd

TECHNICAL DETAILS

Once gaining access to a system, the worm attempts to download a tar file from go.163.com. This site appears to have been closed, causing the worm to no longer be effective. If it is able to download the file, it does the following:

1. The worm untars the file to /usr/lib/lib and executes a script, which begins the worm routine.
2. It replaces ps with a Trojanized version, and backs up the original to /usr/bin/adore.
3. Next, the worm adds a script to the daily cron job, which kills all of its processes except the installed backdoor by rebooting or using killall on the appropriate processes. The script also replaces the Trojanized ps. This allows the worm to propagate for a limited amount of time, but reduces the chances of being detected.
4. Linux.Adore.Worm then adds the users ftp and anonymous to /etc/ftpusers, blocking the wuftpd hole, which is exploited. The worm also kills the rpc.statd, rpc.rstatd, and lpd processes, preventing those vulnerabilities from being exploited.
5. Next, the worm replaces klogd (kernel message logger) with a backdoor program that uses ICMP instead of the traditional TCP or UDP methods. The backdoor allows root shell access.
6. The worm then sends information to two of four email addresses located in China. The ISP has been notified accordingly. The information includes the IP address of the compromised computer, the process list, the history, hosts file, and shadow password file.
7. Finally, the worm executes the routines to find new systems to compromise. The worm generates random class-B IP addresses and checks to see if they are vulnerable to the common statd, lprng, wuftp, and bind vulnerabilities. If vulnerable, the worm exploits the vulnerability to gain access to the system.

Information on patching the four vulnerabilities including links to patches can be found at:

• LPRng: http://www.cert.org/advisories/CA-2000-22.html
• wu-ftpd 2.6: http://www.cert.org/advisories/CA-2000-13.html
• Bind: http://www.cert.org/advisories/CA-2001-02.html
• rpc.statd: http://www.cert.org/advisories/CA-2000-17.html

Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
• Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. Perform a forensic analysis and restore the computers using trusted media.
• Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.

REMOVAL

Because infected systems contain commonly exploited vulnerabilities, the system is likely to have been previously compromised. SARC recommends the system be imaged to a standalone system for future forensic analysis, and replaced with a clean installation with the latest security patches.