performing-fuzzing-with-aflplusplus

Performing Fuzzing with AFL++

Overview

AFL++ is a community-maintained fork of American Fuzzy Lop (AFL) that provides coverage-guided fuzzing for compiled binaries. It instruments targets at compile time or via QEMU/Unicorn mode for binary-only fuzzing, then mutates input corpora to discover new code paths. AFL++ includes advanced scheduling (MOpt, rare), custom mutators, CMPLOG for input-to-state comparison solving, and persistent mode for high-throughput fuzzing.

When to Use

• When conducting security assessments that involve performing fuzzing with aflplusplus
• When following incident response procedures for related security events
• When performing scheduled security testing or auditing activities
• When validating security controls through hands-on testing

Common Misconfigurations & Verification

A campaign that runs for hours with zero crashes is usually a setup problem, not a secure target.

• No sanitizer, so bugs go silent: instrument with AFL_USE_ASAN=1 afl-cc (or afl-clang-fast). Without ASAN/UBSAN a heap overflow corrupts memory without crashing and AFL++ never flags it.
• Empty or trivial seed corpus: starting -i with one tiny file leaves the fuzzer unable to reach parser logic. Provide diverse valid inputs, then prune with afl-cmin, and shrink each with afl-tmin.
• No dictionary for structured formats: without -x dict/ (magic bytes, keywords, tokens) AFL++ wastes cycles guessing format headers. Supply a format dictionary.
• Single-core run wastes the box: use -M main + several -S sec1 sec2 ... secondaries to fuzz in parallel and share finds via the sync dir.
• Coverage stall ignored: map density flat and pending near zero for hours means saturation — add CMPLOG (-c), MOpt, or new seeds rather than letting it spin.

Verify the harness can actually find bugs: compile a build with a planted bug (e.g. an unchecked strcpy on input) and confirm AFL++ surfaces it in crashes/ within minutes and that afl-tmin + CASR/GDB reproduce it. If the planted bug is never found, fix instrumentation/seeds before trusting a clean run.

Prerequisites

• AFL++ installed (apt install afl++ or build from source)
• Target binary source code (for compile-time instrumentation) or QEMU mode for binary-only
• Initial seed corpus of valid inputs for the target format
• Linux system with /proc/sys/kernel/core_pattern configured

Steps

1. Instrument the target binary with afl-cc or afl-clang-fast
2. Prepare seed corpus directory with minimal valid inputs
3. Minimize corpus with afl-cmin to remove redundant seeds
4. Run afl-fuzz with appropriate flags (-i input -o output)
5. Monitor fuzzing progress via afl-whatsup and UI stats
6. Triage crashes with afl-tmin minimization and CASR/GDB analysis
7. Report unique crashes with reproduction steps

Expected Output

+++ Findings +++
  unique crashes: 12
  unique hangs: 3
  last crash: 00:02:15 ago
+++ Coverage +++
  map density: 4.23% / 8.41%
  paths found: 1847
  exec speed: 2145/sec