From 21945f83864986ded7e35f3faea112aad504109c Mon Sep 17 00:00:00 2001
From: HackTricks News Bot <bot@hacktricks.xyz>
Date: Thu, 5 Mar 2026 19:20:35 +0000
Subject: [PATCH] Add content from: Mutational grammar fuzzing

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 create mode 100644 src/generic-methodologies-and-resources/fuzzing.md

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 # 🤩 Generic Methodologies & Resources
 
 - [Pentesting Methodology](generic-methodologies-and-resources/pentesting-methodology.md)
+- [Fuzzing Methodology](generic-methodologies-and-resources/fuzzing.md)
 - [External Recon Methodology](generic-methodologies-and-resources/external-recon-methodology/README.md)
   - [Wide Source Code Search](generic-methodologies-and-resources/external-recon-methodology/wide-source-code-search.md)
   - [Github Dorks & Leaks](generic-methodologies-and-resources/external-recon-methodology/github-leaked-secrets.md)
diff --git a/src/generic-methodologies-and-resources/fuzzing.md b/src/generic-methodologies-and-resources/fuzzing.md
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+# Fuzzing Methodology
+
+{{#include ../banners/hacktricks-training.md}}
+
+## Mutational Grammar Fuzzing: Coverage vs. Semantics
+
+In **mutational grammar fuzzing**, inputs are mutated while staying **grammar-valid**. In coverage-guided mode, only samples that trigger **new coverage** are saved as corpus seeds. For **language targets** (parsers, interpreters, engines), this can miss bugs that require **semantic/dataflow chains** where the output of one construct becomes the input to another.
+
+**Failure mode:** the fuzzer finds seeds that individually exercise `document()` and `generate-id()` (or similar primitives), but **does not preserve the chained dataflow**, so the “closer-to-bug” sample is dropped because it doesn’t add coverage. With **3+ dependent steps**, random recombination becomes expensive and coverage feedback does not guide search.
+
+**Implication:** for dependency-heavy grammars, consider **hybridizing mutational and generative phases** or biasing generation toward **function chaining** patterns (not just coverage).
+
+## Corpus Diversity Pitfalls
+
+Coverage-guided mutation is **greedy**: a new-coverage sample is saved immediately, often retaining large unchanged regions. Over time, corpora become **near-duplicates** with low structural diversity. Aggressive minimization can remove useful context, so a practical compromise is **grammar-aware minimization** that **stops after a minimum token threshold** (reduce noise while keeping enough surrounding structure to remain mutation-friendly).
+
+## Single-Machine Diversity Trick (Jackalope-Style)
+
+A practical way to hybridize **generative novelty** with **coverage reuse** is to **restart short-lived workers** against a persistent server. Each worker starts from an empty corpus, syncs after `T` seconds, runs another `T` seconds on the combined corpus, syncs again, then exits. This yields **fresh structures each generation** while still leveraging accumulated coverage.
+
+**Server:**
+
+```bash
+/path/to/fuzzer -start_server 127.0.0.1:8337 -out serverout
+```
+
+**Sequential workers (example loop):**
+
+<details>
+<summary>Jackalope worker restart loop</summary>
+
+```python
+import subprocess
+import time
+
+T = 3600
+
+while True:
+  subprocess.run(["rm", "-rf", "workerout"])
+  p = subprocess.Popen([
+      "/path/to/fuzzer",
+      "-grammar", "grammar.txt",
+      "-instrumentation", "sancov",
+      "-in", "empty",
+      "-out", "workerout",
+      "-t", "1000",
+      "-delivery", "shmem",
+      "-iterations", "10000",
+      "-mute_child",
+      "-nthreads", "6",
+      "-server", "127.0.0.1:8337",
+      "-server_update_interval", str(T),
+      "--", "./harness", "-m", "@@",
+  ])
+  time.sleep(T * 2)
+  p.kill()
+```
+
+</details>
+
+**Notes:**
+
+- `-in empty` forces a **fresh corpus** each generation.
+- `-server_update_interval T` approximates **delayed sync** (novelty first, reuse later).
+- In grammar fuzzing mode, **initial server sync is skipped by default** (no need for `-skip_initial_server_sync`).
+- Optimal `T` is **target-dependent**; switching after the worker has found most “easy” coverage tends to work best.
+
+## References
+
+- [Mutational grammar fuzzing](https://projectzero.google/2026/03/mutational-grammar-fuzzing.html)
+- [Jackalope](https://github.com/googleprojectzero/Jackalope)
+
+{{#include ../banners/hacktricks-training.md}}