Gumblar Botnet Resurfaces to Target Windows Users, ScanSafe Says

The Gumblar botnet is back with a new trick up its sleeve.

The goal, however, remains the same—to steal FTP credentials and other data. Gumblar first made the news in May, when attackers compromised legitimate sites and sent visitors on to a malicious site that infected their PCs with malware. Now, attackers are using the legitimate sites they hack to host the malware itself instead of relying on a malicious iFrame that points to the malware.

"In a typical outbreak situation, there are compromised Websites that act as a conduit for malware hosted on an attacker-owned site," explained Mary Landesman, senior security researcher at ScanSafe, on the company's Security Threat Alert Team blog Oct. 15. "But in this case, the malware resides on thousands of legitimate (but compromised) Websites.

"There's no way of determining the total number of sites involved, but judging by the number of blocks in our logs and the research we've done over the past two days, I think it's safe to assume the number is in the low thousands," Landesman continued.

The botnet was named Gumblar based on gumblar.cn, the malware domain involved in the initial attacks. In addition to stealing FTP credentials, user names and passwords, the malware is typically accompanied by another malicious program that installs a backdoor on infected PCs. Meanwhile the stolen FTP credentials are used by the attackers to compromise other Websites. The attackers make configuration changes to the sites to lower security settings and make them vulnerable to further attack even if the password is subsequently changed, Landesman said.

Most of the compromised Websites are small and hosted in non-English-speaking countries. Still, it is a pretty significant number since those pages are actually playing host to the malware, she said.

"The attackers have uploaded the malware to match pre-existing paths/filenames individual to each of the sites," she said. "For example: sitename/images/image.gif will have an accompanying sitename/images/image.php which contains the malware … Thus the name and location of the malicious .php file is unique to each individual compromised Website."

Landesman added that because the attackers are also using forums as a conduit for the malware—and most likely manipulating search results as well—the number of encounters with the malware is actually quite high, currently accounting for 4.8 percent of all ScanSafe Web malware blocks.

So far, the exploits being used by the attackers revolve around Adobe Reader and Adobe Flash Player, as well as the Microsoft Office Web Components vulnerability described in the MS09-043 security bulletin and the Microsoft DirectShow vulnerability described in MS09-011.

"As was the case in the original Gumblar attacks, the malware modifies the following registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Drivers32," Landesman explained to eWEEK. "This causes the malware to load when any sound-enabled application, i.e. any browser, is launched. The malware also takes a read of sqlsodbc.chm, a file targeted by previous Gumblar-delivered malware."