Simplifying MBA obfuscation with CoBRA
Trail of Bits has open-sourced CoBRA, a highly effective tool designed to deobfuscate Mixed Boolean-Arithmetic (MBA) expressions commonly used by malware authors and software protectors. Available as a CLI tool, C++ library, and LLVM pass, CoBRA successfully simplifies nearly 100% of complex MBA expressions, significantly aiding reverse engineering and malware analysis efforts.
Authors: Trail of Bits
Key Takeaways
- Trail of Bits released CoBRA, an open-source tool for simplifying Mixed Boolean-Arithmetic (MBA) obfuscation.
- CoBRA successfully simplifies 99.86% of over 73,000 tested MBA expressions across linear, semilinear, polynomial, and mixed categories.
- The tool is available as a CLI utility, a C++ library, and an LLVM pass plugin supporting LLVM 19 through 22.
- CoBRA verifies all simplified results against random inputs or proves equivalence using the Z3 theorem prover.
Affected Systems
- LLVM 19-22
- Reverse Engineering Environments
Attack Chain
Malware authors and software protectors utilize Mixed Boolean-Arithmetic (MBA) obfuscation to disguise simple operations behind complex arithmetic and bitwise operators. To counter this, analysts can use CoBRA to ingest these obfuscated expressions. The tool classifies the expression, applies appropriate simplification passes, and verifies the result using Z3 equivalence proofs before outputting the readable, deobfuscated expression.
Detection Availability
- YARA Rules: No
- Sigma Rules: No
- Snort/Suricata Rules: No
- KQL Queries: No
- Splunk SPL Queries: No
- EQL Queries: No
- Other Detection Logic: No
No detection rules are provided as this article discusses a defensive reverse engineering tool.
Detection Engineering Assessment
EDR Visibility: None — This is a reverse engineering tool release, not an active threat generating EDR telemetry. Network Visibility: None — MBA obfuscation occurs at the binary level and does not generate distinct network traffic. Detection Difficulty: Hard — Detecting MBA obfuscation statically requires advanced binary analysis and heuristics to identify unusually complex arithmetic and bitwise instruction blocks.
Hunting Hypotheses
| Hypothesis | Telemetry | ATT&CK Stage | FP Risk |
|---|---|---|---|
| Binaries employing MBA obfuscation will exhibit unusually high concentrations of bitwise and arithmetic instructions within specific functions. | Static Binary Analysis | Defense Evasion | High |
Control Gaps
- Static Analysis Tools lacking MBA simplification capabilities
Key Behavioral Indicators
- High cyclomatic complexity in binary functions
- Dense clusters of bitwise (XOR, AND, OR) and arithmetic (ADD, SUB, MUL) instructions
False Positive Assessment
- Low
Recommendations
Immediate Mitigation
- Integrate CoBRA into existing malware analysis and reverse engineering workflows to handle MBA obfuscation.
Infrastructure Hardening
- N/A
User Protection
- N/A
Security Awareness
- Train reverse engineering and malware analysis teams on utilizing CoBRA and Z3 theorem provers for deobfuscation tasks.
MITRE ATT&CK Mapping
- T1027 - Obfuscated Files or Information
Additional IOCs
- File Paths:
libCobraPass.so- CoBRA LLVM pass plugin libraryDATASETS.md- Documentation file containing benchmark breakdowns
- Command Lines:
- Purpose: Run CoBRA CLI to simplify a basic MBA expression | Tools:
cobra-cli| Stage: Analysis |cobra-cli --mba
- Purpose: Run CoBRA CLI to simplify a basic MBA expression | Tools:
