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Add STRIDE threat model to security docs (#9562)

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# Security policy
## Reporting a vulnerability
To report sensitive vulnerability information, report it [privately on GitHub](https://github.com/python-pillow/Pillow/security/advisories/new).
If you cannot use GitHub, use the [Tidelift security contact](https://tidelift.com/security). Tidelift will coordinate the fix and disclosure.
If you cannot use GitHub, use the [Tidelift security contact](https://tidelift.com/docs/security). Tidelift will coordinate the fix and disclosure.
DO NOT report sensitive vulnerability information in public.
**DO NOT report sensitive vulnerability information in public.**
## Threat model
Pillow's primary attack surface is parsing untrusted image data. A full STRIDE threat model covering spoofing, tampering, repudiation, information disclosure, denial of service, and elevation of privilege is maintained in the [Security handbook page](https://pillow.readthedocs.io/en/latest/handbook/security.html).
Key risks to be aware of when using Pillow to process untrusted images:
- **Decompression bombs** — do not set `Image.MAX_IMAGE_PIXELS = None` in production.
- **EPS files invoke Ghostscript** — block EPS input at the application layer unless strictly required.
- **`ImageMath.unsafe_eval()`** — never pass user-controlled strings to this function; use `lambda_eval` instead.
- **C extension memory safety** — keep Pillow and its bundled C libraries (libjpeg, libpng, libtiff, libwebp, etc.) up to date.
- **Sandboxing** — for high-risk deployments, run image processing in a sandboxed subprocess.

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tutorial
concepts
appendices
security

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Security
========
Pillow's primary attack surface is **parsing untrusted image data**. This page
documents the threat model for developers integrating Pillow into applications
that handle images from untrusted sources, along with recommended mitigations.
To report a vulnerability see :ref:`security-reporting`.
.. _security-threat-model:
Threat model (STRIDE)
---------------------
The analysis below follows the `STRIDE
<https://en.wikipedia.org/wiki/STRIDE_model>`_ framework and covers the
boundary between untrusted image input and the Pillow API.
.. code-block:: text
┌──────────────────────────────────────────┐
Untrusted zone │ Pillow API │
───────────── │ │
Image files ────►│ Image.open() ──► Format plugins │
Byte streams │ (40+ parsers) (Python + C FFI) │
User metadata │ │
│ ImageMath.unsafe_eval(expr) ───────────┼──► Python eval()
│ ImageShow.show(image) ─────────────────┼──► os.system / subprocess
│ EpsImagePlugin.open(eps) ──────────────┼──► Ghostscript (gs)
└──────────────┬───────────────────────────┘
│ C extensions:
│ _imaging · _imagingft · _imagingcms
│ _webp · _avif · _imagingtk
│ _imagingmath · _imagingmorph
▼
┌──────────────────────────────────────────┐
│ C libraries (bundled or system) │
│ libjpeg · libpng · libtiff · libwebp │
│ openjpeg · freetype · littlecms2 │
└──────────────────────────────────────────┘
Spoofing
^^^^^^^^
**S-1 — Format sniffing bypass**
``Image.open()`` detects format by magic bytes, not file extension or MIME
type. An attacker can name a file ``safe.png`` while its content is TIFF, JPEG
2000, or EPS, causing a different — potentially more dangerous — parser to run.
*Mitigations:* validate MIME type and magic bytes independently before calling
``Image.open()``; pass the ``formats`` argument with an allowlist of accepted
formats.
**S-2 — Plugin registry spoofing**
Pillow's format registry is a global mutable dictionary. A malicious package
installed in the same environment could register a replacement parser for a
well-known format.
*Mitigations:* use isolated virtual environments with pinned, hash-verified
dependencies; audit ``Image.registered_extensions()`` at startup.
Tampering
^^^^^^^^^
**T-1 — Malicious metadata propagation**
Pillow preserves EXIF, XMP, IPTC, ICC profiles, and comments when
round-tripping images. Applications that store or render metadata without
sanitisation are vulnerable to second-order injection (SQLi, XSS, command
injection).
*Mitigations:* treat all values from ``image.info``, ``image._getexif()``,
``image.getexif()``, and ``image.text`` as untrusted; sanitise before storing
or rendering; strip metadata when it is not required.
**T-2 — Covert data channel (steganography)**
Pillow does not remove hidden data (JPEG comments, PNG text chunks) when
re-saving. An attacker can embed data that survives the
encode-decode cycle invisibly.
*Mitigations:* to guarantee a clean output when saving, create a new image instance via
``image.copy()`` and delete the ``image.info`` contents.
**T-3 — Supply chain tampering**
Pre-compiled wheels bundle libjpeg-turbo, libpng, libtiff, libwebp, openjpeg,
freetype, littlecms2, and other libraries. A compromised PyPI release or build pipeline
could ship malicious binaries.
*Mitigations:* pin with hash verification
(``python3 -m pip install --require-hashes``); monitor `Pillow security advisories
<https://github.com/python-pillow/Pillow/security/advisories>`_; use
Dependabot or OSV-Scanner for bundled C library CVEs.
Repudiation
^^^^^^^^^^^
**R-1 — No structured audit trail**
Without application-level logging there is no record of which images were
opened, what formats were detected, or what operations were performed, making
forensic investigation harder after an incident.
*Mitigations:* log the filename/hash, detected format, and dimensions of every
image processed; log and alert on ``Image.DecompressionBombWarning``,
``Image.DecompressionBombError``, and ``PIL.UnidentifiedImageError``.
Information disclosure
^^^^^^^^^^^^^^^^^^^^^^
**I-1 — Metadata in saved images**
GPS coordinates, author names, software version strings, and ICC profiles can
be inadvertently included in output images served publicly.
*Mitigations:* explicitly strip EXIF and XMP on save (set ``exif=b""``,
``icc_profile=None``, omit ``pnginfo``); verify output with ``exiftool`` in CI.
**I-2 — Temporary file exposure**
Several code paths write pixel data to temporary files via
``tempfile.mkstemp()``. Exception paths can leave these files behind on shared
filesystems.
*Mitigations:* files are created with mode ``0o600``; mount ``/tmp`` as a
per-container ``tmpfs``; ensure ``try/finally`` cleanup is in place.
Denial of service
^^^^^^^^^^^^^^^^^
**D-1 — Decompression bomb**
A small compressed image can expand to gigabytes in memory.
:py:data:`PIL.Image.MAX_IMAGE_PIXELS` raises
``Image.DecompressionBombError`` at 2× the limit and
``Image.DecompressionBombWarning`` at 1×. PNG text chunks are
separately capped by ``PngImagePlugin.MAX_TEXT_CHUNK`` and
``MAX_TEXT_MEMORY``. Check the values in your installed Pillow version at
runtime or in the reference/source for the current defaults.
*Mitigations:* **never** set ``Image.MAX_IMAGE_PIXELS = None`` in production;
treat ``Image.DecompressionBombWarning`` as an error; set OS/container memory limits
per worker.
**D-2 — CPU exhaustion**
Large-but-legal images (within ``MAX_IMAGE_PIXELS``) can still saturate CPU
through high-quality resampling, convolution filters, or complex draw
operations.
*Mitigations:* apply per-request CPU time limits; set a practical dimension
ceiling below ``MAX_IMAGE_PIXELS``; rate-limit processing requests.
**D-3 — Algorithmic complexity in parsers**
Formats such as TIFF (nested IFD chains), animated GIF/WebP (many frames), and
PNG (many text chunks) can exhaust CPU or memory before pixel data is decoded.
*Mitigations:* restrict accepted formats to the minimum required; enforce a
file-size limit before passing data to Pillow; use per-request timeouts.
Elevation of privilege
^^^^^^^^^^^^^^^^^^^^^^
**E-1 — C extension memory corruption (RCE)**
Pillow's ~87 C source files and its bundled C libraries process
attacker-controlled bytes. Historical CVEs include buffer overflows, integer
overflows, and use-after-free vulnerabilities that allow arbitrary code
execution.
*Mitigations:* keep Pillow and all C libraries up to date; compile with
hardening flags (ASLR, stack canaries, PIE, ``_FORTIFY_SOURCE=2``); run image
processing in a sandboxed subprocess (seccomp-bpf, AppArmor, or a restricted
container).
**E-2 — Ghostscript exploitation via EPS (RCE)**
Opening an EPS file invokes the system Ghostscript binary (``gs``) via
``subprocess``. Ghostscript has a long history of sandbox-escape CVEs
permitting arbitrary code execution from malicious PostScript.
*Mitigations:* **block EPS files** at the application input layer before
passing files to Pillow; if EPS must be supported, run Ghostscript in a fully
isolated sandbox with no network and no sensitive mounts. Pillow does not
provide a stable public API for unregistering individual format plugins, so do
not rely on mutating internal registries such as ``Image.OPEN`` as a security
control.
**E-3 — ImageMath.unsafe_eval() code injection**
:py:meth:`~PIL.ImageMath.unsafe_eval` calls Python's built-in ``eval()`` with
only a minimal ``__builtins__`` restriction, which can be bypassed via
introspection. Any user-controlled string passed to this function results in
arbitrary code execution.
*Mitigations:* **never** pass user-controlled strings to
``ImageMath.unsafe_eval()``; use :py:meth:`~PIL.ImageMath.lambda_eval` instead,
which accepts a Python callable and never calls ``eval``.
**E-4 — Font path traversal via ImageFont**
``ImageFont.truetype(font, size)`` passes the filename to the FreeType C
library. If font paths are constructed from user input without
canonicalisation, an attacker may supply a path like
``../../../../etc/passwd``.
*Mitigations:* never construct font paths from user input; if font selection
must be user-driven, resolve names against an explicit allowlist of
pre-validated absolute paths.
.. _security-recommendations:
Recommendations
---------------
The following mitigations are listed in priority order.
1. **Sandbox image processing** — run Pillow workers in a seccomp/AppArmor
restricted subprocess, isolated from the main application process.
2. **Block or sandbox EPS** — reject EPS at the application boundary, or run
Ghostscript in an isolated container.
3. **Never use** ``ImageMath.unsafe_eval()`` **with user input** — migrate all
callers to :py:meth:`~PIL.ImageMath.lambda_eval`.
4. **Keep all dependencies current** — Pillow and its C library dependencies
(including libjpeg, libpng, libtiff, libwebp, openjpeg, freetype,
littlecms2, Ghostscript, and others). Subscribe to `Pillow security
advisories <https://github.com/python-pillow/Pillow/security/advisories>`_.
5. **Enforce** ``MAX_IMAGE_PIXELS`` — never set it to ``None``; treat
``Image.DecompressionBombWarning`` as an error.
6. **Allowlist image formats** — restrict accepted formats when opening
images, for example with ``Image.open(..., formats=...)``, and isolate
installs/environments if you need to minimise supported formats.
7. **Strip metadata on output** — never pass through EXIF/XMP/ICC from user
uploads to publicly served images.
8. **Sanitise all metadata** returned by Pillow before using it downstream.
9. **Pin dependencies with hash verification** — use
``pip install --require-hashes`` and lockfiles.
10. **Log and alert** on ``Image.DecompressionBombWarning``,
``Image.DecompressionBombError``, ``PIL.UnidentifiedImageError``,
and all exceptions from ``Image.open()``.
.. _security-reporting:
Reporting a vulnerability
-------------------------
To report sensitive vulnerability information, report it `privately on GitHub
<https://github.com/python-pillow/Pillow/security/advisories/new>`_.
If you cannot use GitHub, use the `Tidelift security contact
<https://tidelift.com/docs/security>`_. Tidelift will coordinate the fix and
disclosure.
**Do not report sensitive vulnerability information in public.**