Agent Skills: Performing Fuzzing with AFL++

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

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