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Fuzzing dictionaries guide fuzzers with domain-specific tokens. Use when fuzzing parsers, protocols, or format-specific code.
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fuzzing-dictionary.SKILL.md
---name: fuzzing-dictionary
type: technique
description: >
Fuzzing dictionaries guide fuzzers with domain-specific tokens.
Use when fuzzing parsers, protocols, or format-specific code.
---# Fuzzing Dictionary
A fuzzing dictionary provides domain-specific tokens to guide the fuzzer toward interesting inputs. Instead of purely random mutations, the fuzzer incorporates known keywords, magic numbers, protocol commands, and format-specific strings that are more likely to reach deeper code paths in parsers, protocol handlers, and file format processors.
## Overview
Dictionaries are text files containing quoted strings that represent meaningful tokens for your target. They help fuzzers bypass early validation checks and explore code paths that would be difficult to reach through blind mutation alone.
### Key Concepts
| Concept | Description |
|---------|-------------|
| **Dictionary Entry** | A quoted string (e.g., `"keyword"`) or key-value pair (e.g., `kw="value"`) |
| **Hex Escapes** | Byte sequences like `"\xF7\xF8"` for non-printable characters |
| **Token Injection** | Fuzzer inserts dictionary entries into generated inputs |
| **Cross-Fuzzer Format** | Dictionary files work with libFuzzer, AFL++, and cargo-fuzz |
## When to Apply
**Apply this technique when:**
| Generate from binary | `strings ./binary \| sed 's/^/"&/; s/$/&"/' > strings.dict` |
## Step-by-Step
### Step 1: Create Dictionary File
Create a text file with quoted strings on each line. Use comments (`#`) for documentation.
**Example dictionary format:**
```conf
# Lines starting with '#' and empty lines are ignored.
# Adds "blah" (w/o quotes) to the dictionary.
kw1="blah"
# Use \\ for backslash and \" for quotes.
kw2="\"ac\\dc\""
# Use \xAB for hex values
kw3="\xF7\xF8"
# the name of the keyword followed by '=' may be omitted:
"foo\x0Abar"
```
### Step 2: Generate Dictionary Content
Choose a generation method based on what's available:
**From LLM:** Prompt ChatGPT or Claude with:
```text
A dictionary can be used to guide the fuzzer. Write me a dictionary file for fuzzing a <PNG parser>. Each line should be a quoted string or key-value pair like kw="value". Include magic bytes, chunk types, and common header values. Use hex escapes like "\xF7\xF8" for binary values.
```
**From header files:**
```bash
grep -o '".*"' header.h > header.dict
```
**From man pages (for CLI tools):**
```bash
man curl | grep -oP '^\s*(--|-)\K\S+' | sed 's/[,.]$//' | sed 's/^/"&/; s/$/&"/' | sort -u > man.dict
```
**From binary strings:**
```bash
strings ./binary | sed 's/^/"&/; s/$/&"/' > strings.dict
```
### Step 3: Pass Dictionary to Fuzzer
Use the appropriate flag for your fuzzer (see Quick Reference above).
## Common Patterns
### Pattern: Protocol Keywords
**Use Case:** Fuzzing HTTP or custom protocol handlers
**Dictionary content:**
```conf
# HTTP methods
"GET"
"POST"
"PUT"
"DELETE"
"HEAD"
# Headers
"Content-Type"
"Authorization"
"Host"
# Protocol markers
"HTTP/1.1"
"HTTP/2.0"
```
### Pattern: Magic Bytes and File Format Headers
**Use Case:** Fuzzing image parsers, media decoders, archive handlers
| Test dictionary effectiveness | Run with and without dict, compare coverage |
### Auto-Generated Dictionaries (AFL++)
When using `afl-clang-lto` compiler, AFL++ automatically extracts dictionary entries from string comparisons in the binary. This happens at compile time via the AUTODICTIONARY feature.
**Enable auto-dictionary:**
```bash
export AFL_LLVM_DICT2FILE=auto.dict
afl-clang-lto++ target.cc -o target
# Dictionary saved to auto.dict
afl-fuzz -x auto.dict -i in -o out -- ./target
```
### Combining Multiple Dictionaries
Some fuzzers support multiple dictionary files:
```bash
# AFL++ with multiple dictionaries
afl-fuzz -x keywords.dict -x formats.dict -i in -o out -- ./target
```
## Anti-Patterns
| Anti-Pattern | Problem | Correct Approach |
|--------------|---------|------------------|
| Including full sentences | Fuzzer needs atomic tokens, not prose | Break into individual keywords |
| Duplicating entries | Wastes mutation budget | Use `sort -u` to deduplicate |
