CVE-2026-112401 — Pillow: Quadratic GIF comment concatenation allows DoS in Image.open()

Affected: python-pillow Pillow

Pillow is a Python imaging library. From 5.4.0 until 13.0.0, Pillow performs quadratic copying while parsing GIF comments. GifImageFile._seek() repeatedly appends immutable bytes for comment sub-blocks and multiple comment extensions, and frame zero is parsed during Image.open(). An attacker can submit a small-dimension GIF containing many comment extensions to any application that opens the file. This can consume excessive CPU and deny service before the application can inspect the image. This issue is fixed in version 13.0.0.

GHSA-74r2-7ff9-mjj2 — Pillow: Quadratic GIF comment concatenation allows DoS in Image.open()

Affected: PyPI: Pillow

``` ============================================================================ Quadratic GIF comment concatenation allows DoS in Image.open() Location: src/PIL/GifImagePlugin.py:266 in GifImageFile._seek() Commit: a707c6b21c313880d0771e54965d142dd42358b6 ============================================================================ ```

Opening a crafted GIF file with `Image.open()` takes time that grows with the square of the file's size, so anyone who can submit an image to a service that uses Pillow can keep one worker busy at full processor load for a long time with a small file. A 2 MB file takes about 5 seconds to open, a 4 MB file about 35 seconds, and an 8 MB file about 3 minutes. An ordinary GIF of the same size opens in well under a second. The cost is paid inside `Image.open()` itself, with default settings, before the application has seen the image's size or format.

The bug is in `GifImageFile._seek()` in `src/PIL/GifImagePlugin.py`, which builds `info["comment"]` by appending to an immutable `bytes` object with no size limit. It appends once per 255-byte sub-block (`comment += block`, line 261) and once per comment extension (`info["comment"] += b"\n" + comment`, line 266). Each append copies everything collected so far. A file made of many tiny comment extensions forces a full re-copy for each one. Because `_open()` ends with `self._seek(0)`, any code that only opens an uploaded file to identify it, read its dimensions, or make a thumbnail pays the full cos…

Most recent entries from all

CVE-2026-112401 — Pillow: Quadratic GIF comment concatenation allows DoS in Image.open()

GHSA-74r2-7ff9-mjj2 — Pillow: Quadratic GIF comment concatenation allows DoS in Image.open()