PEB-Walking API Resolution

Runtime API resolution technique used by the blackmatter / unattributed MSVC 14.12 reflective-loader cluster (40+ confirmed siblings). The binary resolves ~30+ threat APIs at runtime by walking the PEB InMemoryOrderModuleList (starting from fs:[0x30]), iterating export tables, and matching export names by hash or direct string comparison. Resolved pointers are cached in a pseudo-import table inside the .data section (typically around 0x425000).

Detection / Fingerprint

• Minimal static import table: only GDI32, USER32, KERNEL32 GUI functions.
• No imports for VirtualAlloc, CreateThread, InternetOpen, CryptAcquireContext, FindFirstFile, etc.
• Heavy use of indirect calls through .data slots (e.g., call dword ptr [0x42551c]).
• PEB access pattern: mov eax, fs:[0x30] followed by mov eax, [eax+0x0C] / mov eax, [eax+0x14] (InMemoryOrderModuleList).

Implementation Patterns Observed

The blackmatter cluster uses a two-stage resolver:

1. Push a single ASCII character (e.g., 'C' = 0x43) to slot 0x4256c4 to trigger module loading (kernel32).
2. A dedicated walker function iterates exports, hashes names, and writes resolved pointers into the .data slot array.

Defensive Countermeasures

• API hooking / EDR inline hooks can intercept the resolved function pointers post-resolution.
• Memory-protections on .data can block the write stage if the resolver uses RWX allocation.
• String-based detections for the PEB-access assembly pattern are high-signal.

Pages Where Observed

• blackmatter — 40+ confirmed siblings, all using identical PEB-walking stub.
• unattributed — Original cluster analysis (sample 136b5750).