Defeating string and api obfuscation

Defeating String and API Obfuscation

When to Use

• A binary has few readable strings because they are XOR/stack-encoded or built at runtime.
• The import table is sparse because APIs are resolved by hash at runtime.
• You need to recover C2 URLs, paths, and the real API set to understand behavior.

Do not use plain strings and conclude "no indicators" — modern malware hides strings; absence of readable strings is itself a sign of obfuscation.

Prerequisites

• A disassembler/decompiler to read the decode routine, plus the bundled XOR/hash tooling.
• Optionally FLOSS for automated stack/decoded-string recovery.

Workflow

Step 1: Recognize the obfuscation type

Stack strings  : bytes mov'd to the stack one/few at a time, then used
Single-byte XOR: a loop XORing a buffer with a constant
Multi-byte/RC4 : a keyed stream over a blob
API hashing    : a hash compared against export-name hashes to resolve functions

Step 2: Recover XOR-encoded strings

If you find the key and ciphertext, decode directly. The script brute-forces single-byte XOR and surfaces readable results:

python scripts/analyst.py xor-strings sample.bin

Step 3: Reconstruct stack strings

Read the decompiler to collect the byte sequence assembled on the stack and reassemble it. For volume, FLOSS emulates and extracts these automatically.

Step 4: Resolve API hashing

Identify the hash algorithm (often ROR13/ROR7 additive, or djb2). Precompute hashes for known export names and match the constants in the binary back to functions:

python scripts/analyst.py api-hash --algo ror13 --hash 0x726774C

Step 5: Reannotate and extract IOCs

Apply recovered strings and API names back in the disassembler and extract the now-visible URLs, paths, and behavior.

Validation

• Decoded strings are meaningful (URLs, DLL/API names, paths), not random bytes.
• Resolved API names match the calls' usage in the surrounding code.
• The recovered API set explains behavior seen dynamically.

Pitfalls

• Assuming single-byte XOR when it is keyed/RC4; check key length and the decode loop.
• Using the wrong hash algorithm/seed and getting no matches — verify against a known function.
• Recovering strings but not feeding them back into the analysis.

References

• See references/api-reference.md for the XOR and API-hash tools.
• FLOSS and API-hashing references (linked in frontmatter).