The Honeypot Project

Scan of the month: november

by Jan Willem Janssen, <janwillem dot janssen at lxtreme dot nl>,
24 november 2002.



Members from the Honeynet.BR team have captured a new worm from the wild. The file (.unlock), was used by the worm to infect the honeypot. Your mission is to analyze the captured file in order to answer the questions below. Be sure you review the submission rules at the SotM challenge page before submitting your results.


After downloading and verifying the MD5 checksum, I used the file command to determine the content type of the given file:

[cow:~/]$ file .unlock
.unlock: gzip compressed data, deflated, last modified:
Fri Sep 20 12:59:04 2002, os: Unix
It turned out to be a gzipped archive, which after unzipping and re-checking with file, contained an tarball archive:
[cow:~/]$ mv .unlock .unlock1.gz
[cow:~/]$ gunzip .unlock1.gz
[cow:~/]$ file .unlock1
.unlock1: GNU tar archive
[cow:~/]$ tar xvf .unlock1
[cow:~/]$ file .*.c
.unlock.c: ASCII English text, with CRLF, LF line terminators
.update.c: ASCII C program text, with CRLF, LF line terminators
So, it appeared the obtained file contains two source files, which based on the findings of file, appears to come from a non-Unix box.
Because I now had all meta information about the files, I continued to look inside the "recovered" source files.

In addition to the conclusion from the latter run file command, we can exactly pinpoint the changes made by the attacker. We can conclude this from the fact that not every line is terminated with the CRLF characters.

One of the first things I noticed was the appearance of two names: contem@efnet and aion. After feeding the first name to my favorite seach-engine [2], it returned a few hundred hits related to this name. One of the higher rated hits linked to the PacketStorm Security site, reveiling that the worm was last modified on September the 12th, 2002.
Searching on both names returned a single Russian site (I don't speak Russian, so it was all jibberish to me), which on itself contained a link to an advisory of ISS[3]. In this advisory, they give some detailed background information about a worm called Slapper. To be more precise, the advisory names aion's version Slapper.B.
So, we now've got a name for our worm.

All this information, and all we did was look briefly into the sources and entered some terms in a search engine! It was time to look deeper into the sources of Slapper.B and start answering the questions.



Question 1

Which is the type of the .unlock file? When was it generated?

The type of the file, according to the file command, is gzip compressed data, deflated. Further investigation results in the archive being a gzipped tarball. It was last modified (by which we can deduce that it was generated) on friday, the 20th of September 2002, at 12:59. The host on which this archive was created was an Unix host.


Question 2

Based on the source code, who is the author of this worm? When it was created? Is it compatible with the date from question 1?

The original author goes by the name contem@efnet, while there are made modifications by -the presumable attacker- who names himself: aion.
The dates of the files are:

The data of the .unlock.c file is not compatible with the creation time of the archive. We could deduce from this -and the fact that both files are sometimes using some CRLF line termination- that the attacker has packed these files on another system (Linux?) than where he made the last file modifications (Windows?).

Question 3

Which process name is used by the worm when it is running?

The worm hides himself under the name of httpd , which would appear as a "normal" Apache deamon to the unwary. But if you look closer, you would see there's a extra space after the name, which should avoid clashes with the already running Apache server.


Question 4

In wich format the worm copies itself to the new infected machine? Which files are created in the whole process? After the worm executes itself, which files remain on the infected machine?

After the worm has found a new victim and has gained access to it, it performs the following steps:

rm -rf /tmp/.unlock.uu /tmp/.unlock.c /tmp/.update.c
       /tmp/httpd /tmp/update /tmp/.unlock
cat > /tmp/.unlock.uu << __eof__
(here the worms' source archive is UU-encoded and sent to the victim host)
uudecode -o /tmp/.unlock /tmp/.unlock.uu
tar xzf /tmp/.unlock -C /tmp/
gcc -o /tmp/httpd  /tmp/.unlock.c -lcrypto
gcc -o /tmp/update /tmp/.update.c
rm -rf /tmp/.unlock.uu /tmp/.unlock.c /tmp/.update.c
       /tmp/httpd /tmp/update
From this, we can see that the only file left here is the .unlock file. It is used to allow the worm to propagate itself to its new victims.

Question 5

Which port is scanned by the worm?

The worm scans hosts for open ports 80. If it can connect to a Apache server, it pulls apart the server fingerprint (e.g. Server: Apache/2.0.39 (Unix) mod_ssl/2.0.39 OpenSSL/0.9.6b) and from the information in this string, it determines wheter to try to exploit the OpenSSL implementation at port 443.


Question 6

Which vulnerability the worm tries to exploit? In which architectures?

The worm has already appeared in the wild and was reported on the Bugtraq mailing list as vulnerability under the ID #5363, with the name: OpenSSL SSLv2 Malformed Client Key Remote Buffer Overflow Vulnerability. The worm tries to exploit vulnerable OpenSSL modules of Apache webservers running on Linux hosts.
The versions of OpenSSL which are vulnerable to this exploit are:

OpenSSL versions up to and including 0.9.6d and 0.9.7 beta1

The versions of Apache that are vulnerable / scanned for are:

Gentoo: Apache ?,
Debian: Apache 1.3.26
Red-Hat: Apache 1.3.6, 1.3.9, 1.3.12, 1.3.19, 1.3.20,
         1.3.22, 1.3.23, 1.3.26
SuSe: Apache 1.3.12, 1.3.17, 1.3.19, 1.3.20, 1.3.23
Mandrake: 1.3.14, 1.3.19, 1.3.20, 1.3.23
Slackware: Apache 1.3.26

Gentoo persumably hides the exact version number of their Apache, but this (of course) wouldn't mean that all Gentoo hosts are vulnerable to this worm.
Also funny to mention is that if the worm isn't able to determine the exact architecture from the servers' fingerprint, it just assumes it's dealing with a Red-Hat machine using Apache 1.3.23 (which is probably a default RH7.0 installation).


Question 7

What kind of information is sent by the worm by email? To which account?

The account used is hosted on, a free Ukrainian mail service. The account used is

Question 8

Which port (and protocol) is used by the worm to communicate to other infected machines?

The worm uses UDP port 4156 to communicate with all other machines. The protocol creates a decentral peer-to-peer network of victim hosts, which can communicate and forward messages for each other.


Question 9

Name 3 functionalities built in the worm to attack other networks.

The attacker is able to start three kinds of flooding attacks (additional information regarding the specific attack-details is taken from ISS' advisory):

Furthermore, although not a network attack, the worm is able to scan local machines for email addresses. The results are send back to the attacker.

Question 10

What is the purpose of the .update.c program? Which port does it use?

The .update.c program opens port 1052 and waits for a password (aion1981). After it has received this password, it opens a backdoor -by means of an interactive shell- which allows the attacker access to the machine.
This allows the attacker to access each node of his DDOS network individually to gain knowledge about its host or to start other attacks.


Question 11

Bonus Question: What is the purpose of the SLEEPTIME and UPTIME values in the .update.c program?

The trojan program tries to hide itself from getting unwanted attention by only listening for 10 seconds each 5 minutes. If it doesn't receive a connect call within these 10 seconds, it closes the port and sleeps for 5 minutes. After which he tries again and again until the attacker connects to it.



Based on the answer of question 7 and the password of the trojan (.update.c), we can deduce that the attacker is probably an Russian speaking person, probably from the Ukraine, and 21 years old. He re-used sources, which date from the 11th of September 2002 (and was published on PacketStorm Security[1] under the name pud), and made his last modification on the 20th of September 2002 (according to both the timestamp and version string).
From observing the source code, the attacker is familiar with C programming, though probably only from observating other persons' code rather than crafting it himself (he treats all pointers as arrays). Nevertheless, he's smart and has enough knowledge to glue all parts together to get a working DDOS worm.

This particular variant of the Slapper worm can easily be defeated, by creating an empty, root-owned, read-only, immutable .unlock.uu file. Since the worm tries to write his source archive into this file while not checking for any errors, the propagation will fail.
Another solution would be placing the /tmp on a separate partition which is mounted without execution rights (e.g. mount /tmp -o noexec).


Tools used