Worm:W32/BadTrans is a worm spreading with e-mail messages from Win32 systems. The worm sends email messages with infected attached files, as well as installs a spying trojan component to steal information from infected systems. The worm was discovered in-the-wild on April 12 2001.
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The worm itself is Win32 executable file (PE EXE file). It was found in-the-wild in a compressed form, and is about 13Kb long. Being decompressed the worm's file length increases to about 40Kb.
The worm has a multi-component structure. It consists of two different components that are dropped on a hard disk as three different files and are run as stand-alone programs (email Worm and Trojan). The worm routine is the main component, it keeps trojan program body in its code and installs it into a system while infecting a new machine.
The worm component operates similar to I-Worm.ZippedFiles (aka ExploreZip) worm: by using Windows MAPI functions it gets access to Inbox and "answers" all unread messages. This routine has a bug and may cause transport overload (see below).
The trojan component itself is a variant of already known passwords-stealing trojan (see Trojan.PSW.Hooker). It sends information from infected computers to the email address:
When an infected file is run (when a user clicks on attached file and activates it) the worm code gets control. First of all it drops (installs) its components to the system. The worm copies itself to Windows directory with INETD.EXE name and drops the trojan component to Windows directory with HKK32.EXE name. The trojan component is executed then, it moves itself to Windows system directory with KERN32.EXE name, drops an additional library (key logger) with HKSDLL.DLL name. The worm then registers itself (the INETD.EXE file) in auto-run sections in the system. Under Win9x it writes "run=" command to [windows] section to WIN.INI file, for example:
Under WinNT/2000 the following registry key is created:
The trojan registers itself in the Registry in RunOnce key:
Because this is "run once" key, the trojan on each start rewrites it, and keeps Windows loading trojan file on each restart. To hide its activity when installation into a new machine is complete the worm displays the fake message and exits:
The message looks like:
The worm does not send any messages out of infected machine at first start, it does that on next Windows restarts instead. The spreading routine is activated on next Windows restart when the worm copy is activated from INETD.EXE file (this file is run automatically because it is referred from "run" key in WIN.INI file or system registry). The worm registers itself as hidden (service) process, and sleeps for about 5 minutes before activating its spreading routine.
While spreading the worm gets access to Windows MAPI functions, opens and reads all unread messages, "answers" on them with infected messages. The worm does not terminate, and is active till Windows restart, and sends infected message each time a new message arrives. The infected message has text and attached file. Attached file name is randomly selected from the following variants:
The Subject field in worm messages is the same as in original message with prepended "Re:" prefix. The message body is a "reply" to the original message. For example, if original message is sent from "John Doe" and has two lines like:
the worm will reply with the following text then:
'John Doe' wrote: ==== - message line1 - message line2 > Take a look to the attachment.
If a message has no body (empty message), the worm's "reply" has just one line:
The worm uses a trick to avoid answering the same email twice or more times, and to avoid answering its own messages received from other infected machines. To do that the worm adds two spaces to the end of Subject line, and does not process (reply to) such messages. This "two-spaces" protection works for messages that are already "answered", and worm does not reply to these messages. However, this protection doesn't work for messages that are received from other infected computers. Some email servers (or most of them) simply cut all spaces at the end of Subject line (according to RFC-822 email messages standard).
As a result if an infected message comes to already infected machine it is immediately answered by worm and sent back. So the worm initiates the "looped" traffic with endless number of infected messages. Depending on installed email client the worm also fails to mark "answered" messages.
As a result the worm answers all unread messages ("true" ones and its own messages) in endless loop, and number of sent and received messages grows up to several thousand within a minute. Therefore, the worm can cause email server crash because soon it will not be capable to process all these messages.