Add STRIDE threat model to security docs

.github/SECURITY.md

@@ -1,7 +1,21 @@
 # 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.**
+
+## 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:
 
-DO NOT report sensitive vulnerability information in public.
+- **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.

docs/handbook/index.rst

@@ -8,3 +8,4 @@ Handbook
   tutorial
   concepts
   appendices
+  security

docs/handbook/security.rst

@@ -0,0 +1,259 @@
+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.**