Imported from trailofbits/skills (plugins/testing-handbook-skills/skills/fuzzing-dictionary/SKILL.md) via skills.sh. Install upstream with npx skills add trailofbits/skills --skill fuzzing-dictionary. Copyright stays with the author.

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

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

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

Choose a generation method based on what's available:

From LLM: Prompt ChatGPT or Claude with:

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:

grep -o '".*"' header.h > header.dict

From man pages (for CLI tools):

man curl | grep -oP '^\s*(--|-)\K\S+' | sed 's/[,.]$//' | sed 's/^/"&/; s/$/&"/' | sort -u > man.dict

From binary strings:

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:

# 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

Dictionary content:

# PNG magic bytes and chunks
png_magic="\x89PNG\r\n\x1a\n"
ihdr="IHDR"
plte="PLTE"
idat="IDAT"
iend="IEND"

# JPEG markers
jpeg_soi="\xFF\xD8"
jpeg_eoi="\xFF\xD9"

Pattern: Configuration File Keywords

Use Case: Fuzzing config file parsers (YAML, TOML, INI)

Dictionary content:

# Common config keywords
"true"
"false"
"null"
"version"
"enabled"
"disabled"

# Section headers
"[general]"
"[network]"
"[security]"

Advanced Usage

Tips and Tricks

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:

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:

# AFL++ with multiple dictionaries
afl-fuzz -x keywords.dict -x formats.dict -i in -o out -- ./target

Anti-Patterns

Tool-Specific Guidance

libFuzzer

clang++ -fsanitize=fuzzer,address harness.cc -o fuzz
./fuzz -dict=./dictionary.dict corpus/

Integration tips:

• Dictionary tokens are inserted/replaced during mutations
• Combine with -max_len to control input size
• Use -print_final_stats=1 to see dictionary effectiveness metrics
• Dictionary entries longer than -max_len are ignored

AFL++

afl-fuzz -x ./dictionary.dict -i input/ -o output/ -- ./target @@

Integration tips:

• AFL++ supports multiple -x flags for multiple dictionaries
• Use AFL_LLVM_DICT2FILE with afl-clang-lto for auto-generated dictionaries
• Dictionary effectiveness shown in fuzzer stats UI
• Tokens are used during deterministic and havoc stages

cargo-fuzz (Rust)

cargo fuzz run fuzz_target -- -dict=./dictionary.dict

Integration tips:

• cargo-fuzz uses libFuzzer backend, so all libFuzzer dict flags work
• Place dictionary file in fuzz/ directory alongside harness
• Reference from harness directory: cargo fuzz run target -- -dict=../dictionary.dict

go-fuzz (Go)

go-fuzz does not have built-in dictionary support, but you can manually seed the corpus with dictionary entries:

# Convert dictionary to corpus files
grep -o '".*"' dict.txt | while read line; do
    echo -n "$line" | base64 > corpus/$(echo "$line" | md5sum | cut -d' ' -f1)
done

go-fuzz -bin=./target-fuzz.zip -workdir=.

Troubleshooting

Related Skills

Tools That Use This Technique

Related Techniques

Resources

Key External Resources

AFL++ Dictionaries Pre-built dictionaries for common formats (HTML, XML, JSON, SQL, etc.). Good starting point for format-specific fuzzing.

libFuzzer Dictionary Documentation Official libFuzzer documentation on dictionary format and usage. Explains token insertion strategy and performance implications.

Additional Examples

OSS-Fuzz Dictionaries Real-world dictionaries from Google's continuous fuzzing service. Search project directories for *.dict files to see production examples.