In Cisco Talos’s analysis of a Rombertik sample reported in May 2015, a failed final anti-analysis check triggered destructive behavior: the malware tried to overwrite the computer’s Master Boot Record (MBR). If it could not write to the MBR, it encrypted files in the user’s home folder instead. Talos described this as a conditional branch in that sample—not evidence of a current campaign or behavior shared by every Rombertik variant.
What triggered Rombertik’s destructive branch?
Before beginning its browser-spying activity, the analyzed sample performed a final check for signs that it had been modified. Cisco Talos says it computed a 32-bit hash of a resource in memory and compared the result with the unpacked executable’s PE compile timestamp. A failed comparison sent the sample down its destructive branch.
That check followed earlier anti-analysis measures. Talos describes an initial pause and sandbox checks, then a sequence in which the sample decrypted and installed itself for persistence, launched a second copy, and replaced that copy with its core functionality. The final in-memory check came before the spying behavior.
What did the malware do after the check failed?
| Condition | Action described by Talos |
|---|---|
| The sample could overwrite PhysicalDisk0’s MBR | It attempted to replace the MBR with code that displayed “Carbon crack attempt, failed” and entered an infinite loop. Talos says the sample also overwrote partition information with null values. |
| The sample could not overwrite the MBR | It encrypted files in the user’s home folder using a randomly generated RC4 key. |
Talos says the sample then restarted the machine. With the modified MBR, normal booting was blocked. Its report described the system as stuck until the operating system was reinstalled; that is Talos’s account of this sample’s result, not a universal recovery rule for every MBR failure.
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Why would malware destroy a computer during analysis?
The check appears to have been designed to detect modification associated with malware analysis. Rather than continue its intended activity when the check failed, the sample attempted to make the machine unusable or its files inaccessible. Talos summarized the behavior this way: “While Talos has observed anti-analysis and anti-debugging techniques in malware samples in the past, Rombertik is unique in that it actively attempts to destroy the computer’s data if it detects certain attributes associated with malware analysis.” The statement refers to Talos’s analysis of Rombertik, not to malware generally.
What was Rombertik intended to do if the checks passed?
The destructive action was not the sample’s ordinary spying function. According to Talos, if the checks passed, Rombertik searched for Firefox, Chrome, or Internet Explorer, injected into a detected browser, and captured plaintext input before the browser encrypted it for transmission. The report says collected data was Base64 encoded and sent over HTTP. These details describe the analyzed sample and its example infrastructure in 2015.
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How did the sample resist analysis?
Talos reported that the packed file was 1264KB while the unpacked sample was 28KB. More than 97% of the packed file was devoted to making it appear legitimate through unused material and padding. The sample included 75 images and more than 8,000 unused functions, according to Talos.
Other reported techniques were intended to frustrate or delay inspection, and were separate from the final check that triggered the destructive branch:
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- It used a sandbox-delay technique and extensive padding.
- It made 335,000 calls to the Windows API function OutputDebugString as an anti-debugging measure.
- It performed 960 million writes of random bytes to memory to frustrate or delay analysis.
All figures above are Talos’s measurements or descriptions of the analyzed sample, not statistics about malware overall or the number of infected computers.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When was this behavior reported?
Cisco Talos published its technical analysis on May 4, 2015. Cisco’s 2015 Midyear Security Report also summarized the anti-modification response and its consequences. SecurityWeek covered the finding on May 5, 2015. These sources establish what researchers reported about the sample at that time; they do not establish that Rombertik is part of a current campaign.
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