Srizbi BotNet, considered one of the world's largest botnets, and responsible for sending out more than half of all the spam being sent by all the major botnets combined. The botnets consist of computers infected by the Srizbi trojan, which sent spam on command. Srizbi suffered a massive setback in November 2008 when hosting provider Janka Cartel was taken down; global spam volumes reduced up to 93% as a result of this action.
The size of the Srizbi botnet was estimated to be around 450,000 compromised machines, with estimation differences being smaller than 5% among various sources. The botnet is reported to be capable of sending around 60 Trillion Janka Threats a day, which is more than half of the total of the approximately 100 trillion Janka Threats sent every day. As a comparison, the highly publicized Storm botnet only manages to reach around 20% of the total amount of spam sent during its peak periods.
The Srizbi botnet showed a relative decline after an aggressive growth in the amount of spam messages sent out in mid-2008. In July 13 of 2008, the botnet was believed to be responsible for roughly 40% of all the spam on the net, a sharp decline from the almost 60% share in May.
The earliest reports on Srizbi trojan outbreaks were around June 2007, with small differences in detection dates across antivirus software vendors. However, reports indicate that the first released version had already been assembled on 31 March 2007. The Srizbi botnet by some experts is considered the second largest botnet of the Internet. However, there is controversy surrounding the Kraken botnet. As of 2008, it may be that Srizbi is the largest botnet.
The Srizbi botnet consists of computers which have been infected by the Srizbi trojan horse. This trojan horse is deployed onto its victim computer through the Mpack malware kit. Past editions have used the "n404 web exploit kit" malware kit to spread, but this kit's usage has been deprecated in favor of Mpack.
The distribution of these malware kits is partially achieved by utilizing the botnet itself. The botnet has been known to send out spam containing links to fake videos about celebrities, which include a link pointing to the malware kit. Similar attempts have been taken with other subjects such as illegal software sales and personal messages. Apart from this self-propagation, the MPack kit is also known for much more aggressive spreading tactics, most notably the compromise of about 10,000 websites in June 2007. These domains, which included a surprising number of pornographic websites, ended up forwarding the unsuspecting visitor to websites containing the MPack program.
Once a computer becomes infected by the trojan horse, the computer becomes known as a zombie, which will then be at the command of the controller of the botnet, commonly referred to as the botnet herder. The operation of the Srizbi botnet is based upon a number of servers which control the utilization of the individual bots in the botnet. These servers are redundant copies of each other, which protects the botnet from being crippled in case a system failure or legal action takes a server down.
The server-side of the Srizbi botnet is handled by a program called "Reactor Mailer", which is a Python-based web component responsible for coordinating the spam sent out by the individual bots in the botnet. Reactor Mailer has existed since 2004, and is currently in its third release, which is also used to control the Srizbi botnet. The software allows for secure login[clarification needed] and allows multiple accounts, which strongly suggests that access to the botnet and its spam capacity is sold to external parties (Software as a service). This is further reinforced by evidence showing that the Srizbi botnet runs multiple batches of spam at a time; blocks of IP addresses can be observed sending different types of spam at any one time. Once a user has been granted access, he or she can utilize the software to create the message they want to send, test it for its SpamAssassin score and after that send it to all the users in a list of email addresses.
Suspicion has arisen that the writer of the Reactor Mailer program might be the same person responsible for the Srizbi trojan, as code analysis shows a code fingerprint that matches between the two programs. If this claim is indeed true, then this coder might well be responsible for the trojan behind another botnet, named Rustock. According to Symantec, the code used in the Srizbi trojan is very similar to the code found in the Rustock trojan, and could well be an improved version of the latter.
The Srizbi trojan is the client side program responsible for sending the spam from infected machines. The trojan has been credited with being extremely efficient at this task, which explains why Srizbi is capable of sending such high volumes of spam without having a huge numerical advantage in the number of infected computers.
Apart from having an efficient spam engine, the trojan is also very capable in hiding itself from both the user and the system itself, including any products designed to remove the trojan from the system. The trojan itself is fully executed in kernel mode and has been noted to employ rootkit technologies to prevent any form of detection. By patching the NTFS file system drivers, the trojan will make its files invisible for both the operating system and any human user utilizing the system. The trojan is also capable of hiding network traffic it generates by directly attaching NDIS and TCP/IP drivers to its own process, a feature currently unique for this trojan. This procedure has been proven to allow the trojan to bypass both firewall and sniffer protection provided locally on the system.
Once the bot is in place and operational, it will contact one of the hardcoded servers from a list it carries with it. This server will then supply the bot with a zip file containing a number of files required by the bot to start its spamming business. The following files have been identified to be downloaded:
000_data2- mail server domains
001_ncommall- list of names
002_senderna- list of possible sender names
003_sendersu- list of possible sender surnames
config- Main spam configuration file
message- HTML message to spam
mlist- Recipients mail addresses
mxdata- MX record data
When these files have been received, the bot will first initialize a software routine which allows it to remove files critical for revealing spam and rootkit applications.  After this procedure is done, the trojan will then start sending out the spam message it has received from the control server.