fuzzing-dictionary

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:

• Fuzzing parsers (JSON, XML, config files)
• Fuzzing protocol implementations (HTTP, DNS, custom protocols)
• Fuzzing file format handlers (PNG, PDF, media codecs)
• Coverage plateaus early without reaching deeper logic
• Target code checks for specific keywords or magic values

Skip this technique when:

• Fuzzing pure algorithms without format expectations
• Target has no keyword-based parsing
• Corpus already achieves high coverage

Quick Reference

Task	Command/Pattern
Use with libFuzzer	./fuzz -dict=./dictionary.dict ...
Use with AFL++	afl-fuzz -x ./dictionary.dict ...
Use with cargo-fuzz	cargo fuzz run fuzz_target -- -dict=./dictionary.dict
Extract from header	grep -o '".*"' header.h > header.dict
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:

# 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