Analyzing Mach-O Binaries on macOS

When to Use

• You have a macOS sample (Mach-O) and need a static capability and trust read.
• You must handle a fat/universal binary containing multiple architecture slices.
• You need to inspect linked dylibs, entitlements, and code-signing status.

Do not use Windows PE tooling on Mach-O — the formats differ entirely; use Mach-O-aware parsers.

Prerequisites

• A Mach-O parser (Python stdlib struct, or macholib/LIEF); the sample handled inertly.
• For signing/entitlements on macOS, codesign/otool are authoritative.

Safety & Handling

• Parse statically; never execute the sample, especially on a real macOS host.
• Keep the sample password-protected at rest and reference it by hash.

Workflow

Step 1: Detect fat vs. thin and architecture

Check the magic: 0xCAFEBABE (fat/universal) vs. 0xFEEDFACE/0xFEEDFACF (Mach-O 32/64). For fat binaries, enumerate and analyze each slice.

python scripts/analyst.py header sample.macho

Step 2: Parse load commands

Read load commands for linked dylibs (LC_LOAD_DYLIB), entry point (LC_MAIN), and signing (LC_CODE_SIGNATURE). The dylib list hints at capability (networking, crypto).

Step 3: Inspect entitlements and signing

On macOS, use codesign/otool to read entitlements and verify the signature. Ad-hoc or absent signatures and suspicious entitlements are risk indicators.

Step 4: Infer capability and route

Map linked frameworks/symbols to behaviors and route to disassembly/RE for deeper analysis.

Validation

• Fat binaries are decomposed and each slice is analyzed, not just the first.
• Load commands, dylibs, and signing status are enumerated correctly.
• Capability inferences are corroborated by linked frameworks/symbols.

Pitfalls

• Analyzing only one slice of a universal binary.
• Trusting a present signature without verifying it (ad-hoc signatures verify but aren't trusted).
• Applying PE assumptions (sections/imports) to Mach-O structures.

References

• See references/api-reference.md for the Mach-O header parser.
• Apple Mach-O loader.h and code signing docs (linked in frontmatter).