Feeds

Camouflaged code threatens security apps

Evil twin hash bash

Next gen security for virtualised datacentres

Antivirus firms are concerned about the emergence of techniques that could render meaningless the use of checksums to mark applications as safe.

The issue concerns hash functions - one way mathematical functions that produce a small fixed length checksum or message digest from a much longer batch of code or email message. When two different input values produce the same output value this is called a collision.

Weaknesses in hashing algorithms, such as MD5, that allowed the discovery of collisions much more quickly than would be possible using brute-force attacks have been known about by cryptographic researchers for more than three years.

Previous techniques meant one type of junk message might be mistaken for another junk message, a weakness of interest to cryptographers but that carried little sting in practice. In addition, high speed computers were needed to discover collisions.

But a recent post on a full disclosure list explains a method to append a few thousand bytes to two arbitrary files such that both files have the same MD5 value. One of the arbitrary files might be malicious. Not only that but the researchers - Marc Stevens, Arjen K. Lenstra, and Benne de Weger - produced their proof-of-concept files using a single PC in less than two days.

Symantec reports that the approach threatens to undermine the use of hash functions to identify applications as safe (whitelisting). Malware authors might get harmless code, which generates the same MD5 output as a companion (malicious) app, whitelisted by submitting it to a classification server. Such a technique would clear the way to later distribute a companion malicious application that generates a MD5 result previously flagged as safe.

The approach is far from trivial but creates a means to smuggle malicious apps past whitelisting tools. Both the malicious and harmless apps might be digitally signed to make the malware look even more harmless.

"While what they have achieved is not the same as producing an identical MD5 for an existing file, it's still not a good thing. In particular it causes serious trouble for application white-listing implementations," Symantec notes.

Looking for extra bytes might be a common sense means of detecting the trick. But the extra bytes may look like compressed data in an installer application, or some kind of signature, so that approach to solving the problem is unreliable.

MD5 is not the only hashing function known to have cryptographic weaknesses. SHA-1 is also known to produce collisions and is thus potentially subject to the same kinds of trickery. The solution might be to move towards more robust hashing algorithms such as SHA-2, Symantec researcher Peter Ferrie concludes. ®

The essential guide to IT transformation

More from The Register

next story
Goog says patch⁵⁰ your Chrome
64-bit browser loads cat vids FIFTEEN PERCENT faster!
Chinese hackers spied on investigators of Flight MH370 - report
Classified data on flight's disappearance pinched
NIST to sysadmins: clean up your SSH mess
Too many keys, too badly managed
Scratched PC-dispatch patch patched, hatched in batch rematch
Windows security update fixed after triggering blue screens (and screams) of death
Researchers camouflage haxxor traps with fake application traffic
Honeypots sweetened to resemble actual workloads, complete with 'secure' logins
Attack flogged through shiny-clicky social media buttons
66,000 users popped by malicious Flash fudging add-on
prev story

Whitepapers

Top 10 endpoint backup mistakes
Avoid the ten endpoint backup mistakes to ensure that your critical corporate data is protected and end user productivity is improved.
Implementing global e-invoicing with guaranteed legal certainty
Explaining the role local tax compliance plays in successful supply chain management and e-business and how leading global brands are addressing this.
Backing up distributed data
Eliminating the redundant use of bandwidth and storage capacity and application consolidation in the modern data center.
The essential guide to IT transformation
ServiceNow discusses three IT transformations that can help CIOs automate IT services to transform IT and the enterprise
Next gen security for virtualised datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.