performing-fuzzing-with-aflplusplus - SKILL.md Agent Skill

name: performing-fuzzing-with-aflplusplus description: 'Perform coverage-guided fuzzing of compiled binaries using AFL++ (American Fuzzy Lop Plus Plus) to discover memory corruption, crashes, and security vulnerabilities. The tester instruments target binaries with afl-cc/afl-clang-fast, manages input corpora with afl-cmin and afl-tmin, runs parallel fuzzing campaigns with afl-fuzz, and triages crashes using CASR or GDB scripts. Activates for requests involving binary fuzzing, crash discovery, coverage-guided testing, or AFL++ fuzzing campaigns.

' domain: cybersecurity subdomain: application-security tags:

fuzzing
aflplusplus
coverage-guided
crash-triage
binary-analysis
security-testing version: '1.0' author: mahipal license: Apache-2.0 nist_ai_rmf:
MEASURE-2.7
MAP-5.1
MANAGE-2.4 atlas_techniques:
AML.T0070
AML.T0066
AML.T0082 nist_csf:
PR.PS-01
PR.PS-04
ID.RA-01
PR.DS-10

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

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