E-mail users who were slow to update their antivirus software last week may have been surprised to receive a flood of e-mail messages containing .zip files from long lost acquaintances, business partners, and complete strangers.
The e-mail was sent by the recent Mydoom e-mail worm. The .zip attachments were evidence of what antivirus experts say is a new trend in virus writing circles: using compressed .zip files to hide viruses and elude detection by antivirus engines.
.Zip files are containers for one or more compressed files. Using programs like WinZip for Windows or Unzip for Unix, users compact files they want to store or transfer to others. The files must then be decompressed--or "unzipped"--before they can be viewed.
Long a staple of Internet and office communications, the compressed .zip file has become embroiled in an arms race between virus writers and antivirus technology companies, experts say.
"We're definitely seeing a trend," says Alex Shipp, antivirus technology expert at MessageLabs. "It really took off in 2003. As soon as one virus was successful with technology like this, other virus writers took notice."
Virus authors learned long ago to hide their creations in e-mail file attachments, often disguising viruses as Windows screen saver (.scr) files or Windows program information (.pif) files, says Mike Hrabik, chief technology officer of Solutionary, a managed security services company in Omaha, Nebraska.
While .zip files were occasionally used to mask virus payloads, the practice wasn't common in virus writing circles because .zip, unlike .scr and .pif files, required separate software to be installed on the receiving system before the files could be opened and run on ubiquitous Windows machines, he says.
All that changed with the release of Microsoft's Windows XP operating system, which includes native support for opening .zip files. That allows virus writers to count on users being able to unzip their attachment and open the virus file stored inside, Shipp says.
Gerhard Eschelbeck of security vulnerability scanning company Qualys agrees, saying that embedded support for .zips in modern systems makes them a rich target for worms like Mydoom.
In switching to .zips, virus authors were also picking up on trends in legitimate e-mail traffic to hide their own malicious creations, Shipp says.
"When corporations started blocking .exe [executable] files to prevent viruses from coming into their environment, people who wanted to send .exes back and forth started zipping them before they sent them. Virus writers noticed that and took advantage of it," he says.
Unlike .scrs and .pifs, which have no use in legitimate exchanges, .zip files are an important business tool that many individuals and organizations use to transfer large file. That makes it difficult for companies to strip them out of e-mail messages without affecting employees' work, experts say.
"For the most part, .zips are effective ways to send files, so blocking them is not something you want to do because it will break other functionality," says Craig Schmugar, antivirus research manager at Network Associates' McAfee antivirus unit.
The files have other advantages for virus authors, as well, says Vipul Ved Prakash, founder and chief scientist at antispam company Cloudmark of San Francisco.
For mass mailing worms like Mydoom, zipping the virus payload makes it smaller and enables the worm to mail out more copies of itself in the same length of time than it could with uncompressed .scr, .pif, or .exe files, Prakash says.
Zipping also changes the unique signature on the virus attachment, making it harder for antivirus engines to detect the malicious program, he says.
Eighty percent of the Mydoom samples that were submitted to Cloudmark from its SpamNet network of 800,000 users had .zip attachments, Prakash says.
Malicious hackers are also finding other ways to maximize increased .zip file use with viruses.
A recent security advisory from AERAsec Network Services and Security GmbH in Hohenbrunn, Germany, found that many antivirus engines are vulnerable to denial of service attacks from so-called "decompression bombs," in which gigabytes of data are zipped into very small files.
Antivirus engines that try to unzip these bombs often crash when trying to handle the huge amount of data stored in them, AERAsec researchers warn.
While decompression bombs have been around since the 1980s, many software products, including antivirus engines, still do not detect such attacks, says Harald Geiger of AERAsec.
But .zips are not a magic bullet for virus authors. Most antivirus programs can open and analyze the contents of .zip files, flagging any files in a .zip that match known viruses, says Schmugar.
No Easy Answers
In the end, there are no easy answers to the .zip file problem, experts agree.
Solutionary publishes a list of 20 recommended file extensions that should be blocked, including .pif and .scr, Hrabik says.
For others, such as Microsoft Word DOC files and Adobe PDF files, companies should block specific file names that are known to be associated with virus payloads, he says.
Best practices for companies should include scanning inside of .zip files and using extension blocking on files contained in the archives, says NAI's Schmugar.
"Security is always a trade-off," says Cloudmark's Prakash. "You can't just stop receiving .exe and .zip files from people, because most of them are useful."
Companies need to balance business needs with security when setting up policies for files like .zips, he says.
Security policies that attach a trust level to certain e-mail senders outside and inside the company could be effective at blocking malicious .zip attachments. Better user education that addresses bad habits like forwarding executable attachments could also help, Prakash says.