Conditional DDE

Author: b

Here’s a little trick we’d like to share in the end-of-year rush:

DDE is the new black, malware authors quickly adopted the technique and so did pentesters and red teams in order to simulate the latest attacks. According to our experience trivial DDE payloads (like fully readable PowerShell scripts) slip through conventional detections, but process monitoring can cause some headache: powershell.exe launched from Office is surely an obvious indicator of something phishy.

Malware sandboxes (that execute incoming files in virtualized environments to learn more about their purpose) are an example of defensive tools that implement such detection. And although they are commonly seen as all-in-one APT stoppers, these tools are in fact quite limited in terms of simulating an actual target, that provides a broad venue for their bypass.  Evasion is generally performed by conditional checks to determine if the payload would run in the right domain, timezone, etc. If the condition is not met, the payload remains dormant so the instrumentation in the sandbox won’t catch anything suspicious.

So how do we implement this with DDE? Looking at some public obfuscation techniques it’s easy to spot the IF field code, that allows conditional parsing of other fields in the document. We can combine this with the DATE or TIME codes to construct a document with time-based execution:

{SET P {IF {TIME \@ "m"} > 13 "C:\\Winows\\System32\\calc.exe" ""}}
{DDEAUTO {REP P} "s2"}

The above DDE construct only executes calc.exe if the minutes of the hour are past 13. Suppose you send attachments that only execute code after 9:00 AM during the night – by the time someone opens the bait, the analyzer already marked it safe hours ago. Or better yet, you can rely on the resource constraints of the sandbox and make it cache/whitelist your first shot before you send the rest. These methods can be further refined with the use of fields like USERNAME or even FILENAME.

By the way, is DDE Turing-complete?


An update on MD5 poisoning

Author: b

Last year we published a proof-of-concept tool to demonstrate bypasses against security products that still rely on the obsolete MD5 cryptographic hash function.

Summary: The method allows bypassing malicious executable detection and whitelists by creating two executables with colliding MD5 hashes. One of the executables (“sheep”) is harmless and can even perform some useful task and is expected to be categorized as goodware by the victim. After the sheep is accepted by the victim, the colliding malicious version (“wolf”) is sent. Because affected products rely solely on the MD5 fingerprints to identify known good executables, wolf is already whitelisted and can run.

Although the reception of the research was generally positive, some were skeptical about the extent and even the validity of the issue. Although in the meantime we received information about more affected products, NDA’s prevented us from further demonstrating that the problem indeed exists and affects multiple vendors. 

Today we are able to share a demonstration of the problem affecting Panda Adaptive Defense 360. The issue is demonstrated against the stricter “Lock mode” of the product meaning that the Panda agent only allows known good executables to run (application whitelisting). For the sake of this video we manually unblock the harmless executable version (sheep4.exe) to speed up the process, as otherwise the analysis could take several hours to complete (it was confirmed that the “sheep” executables aren’t detected as malicious by the cloud scanner in case they are not manually unblocked):


(You can skip 01:00-01:55 if you are not interested in the policy update)

We notified Panda Security about this issue through their Hungarian partner (see the timeline at the end of this post). Panda responded that this is a known issue that is expected to be fixed in the next major version, but no ETA was provided. Panda stated that MD5 was used because of performance reasons. We informed Panda that the BLAKE2 hash function can provide higher level of security at better performance than MD5 (thanks to Tony Arcieri for this update!).

We’d like to stress that this research is not about individual vendors but about bad practices prevalent in the security industry. We now know of at least four vendors affected by the above problem and several others still provide MD5 fingerprints only in their tools and public reports. It is shameful that while hard work is put into phasing out SHA-1, in the security industry it is still generally accepted to use MD5, even after it was exploited in a real-world incident. We understand that there are more straightforward ways for evasion, but think that this issue is a good indicator of how security product development is often approached.

We should do better than this!

Timeline

2016.08.30: Sending technical information to vendor.
2016.09.05: Vendor requests more information, including PCOPInfo logs collected during retest.
2016.09.06: Sending demo video and identification information about product instance. Requesting more information about PCOPInfo usage.
2016.09.06: Vendor responds with instructions about PCOPInfo.
2016.09.08: Sending PCOPInfo logs to vendor.
2016.09.19: Vendor responds that this is a known issue, replacement algorithm is expected to be implemented in the next version.
2016.09.27: Requesting negotiation about issue publication date.
2016.10.12: Requesting negotiation about issue publication date. Including notification about 90-day disclosure deadline in case no agreement would be achieved.
2016.10.19: Vendor responds, internal discussion is still in progress.
2016.11.16: Requesting information about acceptance of publication date.
2016.11.28: Public release.