The volume of email containing polymorphic malware -- malicious software that can change its characteristics to evade detection by anti-virus defences -- has increased dramatically, according to security vendor Symantec.
Overall the amount of malicious email is relatively stable at one in every 281 emails. However 23.7 percent of malicious emails now contain polymorphic malware, double the figure six months ago, according to the Symantec Intelligence Report: July 2011 (PDF).
Over the same time, the number of variants, or different strains, of malware involved in each attack has grown 25 times.
This indicates a "much more aggressive strategy" on the part of the criminals responsible, according to Symantec's senior intelligence analyst Paul Wood. "Perhaps greater use of automation has enabled them to increase their output to this extent."
Wood believes this "alarming" proliferation almost certainly heightens the risk for many organisations.
"This new breed of malware is likely to be causing a great deal of pain for a great number of traditional anti-virus companies that rely on signatures, heuristics and software emulation in order to detect malicious activity," Wood wrote.
The most recent malware samples detected by Symantec were specifically designed to evade the software emulation used by some anti-virus software to analyse the code of potential malware.
"This new breed of malware includes a series of unnecessary 'jump' instructions in the startup code, which are introduced in between the real instructions specifically to confound the anti-virus engine detection," said the report.
In one example, every instruction had five or six extra jumps that added nothing to the functionality.
The malware is typically contained in an executable file inside a ZIP archive attached to the email. This is often disguised as a PDF file or a Microsoft Office document by relying on hidden file extensions. In Windows, if the "Hide extensions for known file types" is turned on, the file name "malware.pdf.exe" is displayed as "malware.pdf".
Symantec also reports that the number if websites harbouring malware and other potentially unwanted programs such as spyware and adware has increased 25.5 percent since June.