Decoding Binary Numeric Expressions, (Mon, Nov 17th)
Source SANS ISC Handlers Diary
Description
In diary entry "Formbook Delivered Through Multiple Scripts", Xavier mentions that the following line:
Impact
For European organizations, the primary impact is on the effectiveness of malware detection and analysis processes. If security teams use outdated or simplistic decoding tools that fail to interpret binary numeric expressions correctly, they may miss indicators of compromise or fail to recognize the use of scripting environments like PowerShell in malware delivery. This can delay incident response and allow malware such as Formbook to persist undetected. While this does not directly compromise systems, it indirectly increases risk by weakening defensive capabilities. Organizations with mature threat intelligence and malware analysis teams will benefit from improved tooling, enabling faster and more accurate identification of threats. Conversely, organizations lacking such capabilities may face increased exposure to script-based malware campaigns. The low severity reflects that this is an analytical challenge rather than a direct exploit or vulnerability affecting system security.
Defensive guidance
European organizations should ensure their malware analysis and detection tools are updated to handle complex obfuscation techniques, including the evaluation of binary numeric expressions within scripts. Specifically, security teams should: 1) Adopt or update static analysis tools to support arithmetic expression evaluation in numeric arrays used in obfuscated scripts. 2) Train analysts to recognize and decode obfuscation patterns involving arithmetic expressions to improve manual and automated analysis accuracy. 3) Integrate enhanced decoding capabilities into automated detection pipelines to reduce false negatives. 4) Collaborate with threat intelligence communities to share updated decoding methods and tools. 5) Regularly review and update detection signatures and heuristics to account for evolving obfuscation tactics. These steps will improve the detection of script-based malware like Formbook and reduce the risk of undetected infections.
Summary
The discussed threat revolves around the decoding of obfuscated malware scripts, specifically those delivering the Formbook malware through multiple scripts. The obfuscation technique involves arrays of numeric expressions, such as '79+1', which represent ASCII character codes when evaluated. Traditional decoding tools like 'numbers-to-hex.py' previously treated these expressions as separate numbers (e.g., '79' and '1'), converting each to hexadecimal independently, which resulted in incorrect ASCII decoding and failure to reconstruct the intended strings like 'POWERSHELL'. This misinterpretation can hinder malware analysts from accurately identifying the use of PowerShell or other scripting components within malicious payloads. The updated tool introduces an evaluation mode (-e option) that processes binary numeric expressions by computing the arithmetic operation before conversion, thus correctly decoding the obfuscated strings. This enhancement improves the accuracy of static malware analysis and aids in the detection of script-based malware delivery. The threat itself is not a vulnerability in software but rather a challenge in malware analysis methodology, emphasizing the need for advanced decoding capabilities to counter increasingly sophisticated obfuscation techniques used by threat actors.