A PDF Trojan horse is spreading malware by exploiting a URL-handling vulnerability in Windows XP and Windows Server 2003 running Internet Explorer 7, Symantec warned customers of its DeepSight Alert Services on Oct. 23.
On Oct. 10, Microsoft released Security Advisory 943521 about this vulnerability and public reports of remote code execution. At the time, it said a patch was in the works.
The vulnerability is caused by insufficient validation of URLs. Attackers can leverage the flaw to execute arbitrary commands via maliciously crated URLs.
Symantec noted in its advisory that the issue was originally disclosed in July but initially received scant attention. In light of new research, public exploits and Microsofts advisory, Symantec considers the problem to be more severe.
"With the ease of exploitation, the availability of public proof-of-concept code, and further attention that this vulnerability is receiving, we will likely begin to see more exploitation of this issue in the wild," the company, based in Cupertino, Calif., said in its advisory.
Symantec is calling the malware Trojan.Pidief.A. The rigged PDF file is using the "mailto: option" vulnerability to install a Trojan that in turn is downloading a file that the security firm is detecting as "Downloader." That document is delivered as a piece of spam with a file name such as "BILL.pdf" or "INVOICE.pdf."
Upon execution, the malicious code attempt to disable the Windows Firewall with a "netsh firewall set opmode mode=disable" command, and then downloads a remote file via FTP from 126.96.36.199. Symantec says the remote file is "ldr.exe" and is a Downloader Trojan. As of the afternoon of Oct. 23 when Symantec posted its advisory, the host 188.8.131.52 was alive and still serving "ldr.exe" over FTP.
"This server is known for hosting malicious software," Symantec warned.
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The DeepSight Threat Analyst Team is advising users to block delivery of PDF files in e-mail. The team is also advising organizations to tell employees not to read or execute PDF files from unknown or untrusted sources.
In addition, Symantec is advising users and companies to block access to the network and IP address involved in this attack, as given above, and to apply the patches outlined in Adobe Advisory APSB07-18 as soon as possible. Adobe released fixes on Oct. 22.
In lieu of providing a timetable for the delivery of a patch, Mark Miller, director of Security Response Communications for Microsoft, headquartered in Redmond, Wash., issued a statement explaining that creating security updates is complicated.
"Once the MSRC knows the extent and the severity of the vulnerability, it works to develop an update for every supported version affected. Once the update is built, it must be tested with the different operating systems and applications it affects, then localized for many markets and languages across the globe. In some instances, multiple vendors are affected by the same or similar issues, which require a coordinated release," Miller said.
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