The Anatomy of a Mach-O: Structure, Code Signing, and Pac
8 days ago
- #Mach-O
- #Binary Format
- #Apple
- Mach-O is the binary format used on Apple's operating systems for executables, libraries, and object code.
- Mach-O supports multiple architectures via universal binaries and contains metadata via load commands.
- The Mach-O structure consists of three regions: a header, load commands, and data segments/sections.
- The header identifies the file as Mach-O, specifies the target architecture, and includes flags for options.
- Load commands instruct the OS loader or provide metadata about the binary's layout.
- Segments and sections organize the file's content into memory ranges with specific protections.
- Universal binaries (Fat Mach-O) contain multiple architecture slices, allowing the system to pick the appropriate one at runtime.
- Mach-O integrates with Apple's code signing, requiring all iOS executables to be signed for integrity and origin verification.
- The code signature blob includes components like the Code Directory, entitlements, and a CMS signature for validation.
- Pointer Authentication Codes (PAC) are a security feature on ARM64e devices, adding cryptographic signatures to pointer values.
- ARM64e architecture in Mach-O headers indicates PAC support, ensuring compatibility with PAC-capable processors.
- PAC enhances security by detecting and preventing unauthorized changes to pointers, making exploitation harder.