CVE-2026-112401

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Quadratic GIF comment concatenation allows DoS in Image.open()
Location: src/PIL/GifImagePlugin.py:266  in GifImageFile._seek()
Commit:   a707c6b21c313880d0771e54965d142dd42358b6
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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 cost. The decompression-bomb check does not help, since the image is 1×1. Memory use stays linear, so the effect is processor exhaustion and latency, bounded by whatever upload size the service allows (about 1 second at 1 MB). This is the same kind of issue as the EPS open-phase slowdown fixed as CVE-2021-28677, here in the GIF comment handling. The quadratic pattern was introduced in commit b2b737b68b06 ("Allow arbitrary number of comment extension subblocks", November 2018) and has shipped since Pillow 5.4.0. The attached script builds 2 MB and 4 MB test files and compares each against a byte-identical control whose extensions are labelled as plain text instead of comment. The 4 MB comment file took about 34 seconds to open, against 0.26 seconds for its control. A patch that collects the pieces in a list and joins them once is also attached.

Details

GifImageFile._seek() collects GIF comment extensions into info["comment"] using bytes concatenation, once per 255-byte sub-block and once per comment extension, with no limit on the total. bytes is immutable, so every += allocates a new object and copies everything accumulated so far; the work done by Image.open() is therefore quadratic in the amount of comment data in frame 0.

A 1×1 GIF89a whose body is nothing but the 5-byte comment extension 21 FE 01 41 00 repeated makes this directly observable. Image.open(BytesIO(data)) on such a file, stock build at this commit, 2-CPU machine:

| file size | comment extensions (21 FE …) | same bytes labelled plain-text extension (21 01 …) | |---|---|---| | 1 MB | 1.2 s | 0.09 s | | 2 MB | 4.9 s | 0.13 s | | 4 MB | 35 s | 0.26 s | | 8 MB | 183 s | — |

The control column is the identical byte stream with the extension label changed from 0xFE to 0x01; it goes through the same loop and the same self.data() reads but is discarded instead of concatenated, and scales linearly. No load() is needed and no warning or exception is raised; im.info["comment"] simply ends up holding the multi-megabyte blob.

The code, src/PIL/GifImagePlugin.py:229-281 (trimmed):

            elif s == b"!":
                s = self.fp.read(1)
                block = self.data()
                …
                elif s[0] == 254:
                    #
                    # comment extension
                    #
                    comment = b""

                    # Read this comment block
                    while block:
                      …

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.

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.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 11.3.0.post11+tuxcare.

This update for python-Pillow fixes the following issues:

• CVE-2026-96636: decompression-bomb protection bypass via multi-frame seek() in MPO and DCX (bsc#1286802).
• CVE-2026-96637: decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS (bsc#1286801).
• CVE-2026-96640: PDF append accepts malformed shared page-tree DAG, which can cause resource exhaustion (bsc#1286798).
• CVE-2026-96641: reachable undefined behavior in native integer operations can lead to denial of service or logic integrity compromise (bsc#1286797).
• CVE-2026-96642: TarIO negative size field causes infinite CPU-spin loop (bsc#1286796).
• CVE-2026-112390: Denial of Service via infinite loop during crafted MIC image parsing (bsc#1286256).
• CVE-2026-112391: Denial of Service via unbounded recursion in PDF parser (bsc#1286273).
• CVE-2026-112392: Denial of Service via zero-size images in rank filters (bsc#1286287).
• CVE-2026-112393: heap buffer overflow and stale-heap disclosure in _tiffWriteProc when saving compressed TIFF data (bsc#1286325).
• CVE-2026-112394: P;2L and P;4L decoders may read 1 to 3 bytes past the passed buffer size (bsc#1286311).
• CVE-2026-112397: out-of-bounds access in _imagingmorph on a zero-width image (bsc#1286314).
• CVE-2026-112398: heap buffer overflow in FPX JPEG decoding can lead to code execution (bsc#1286406).
• CVE-2026-112399: Denial of service via crafted APNG image (bsc#1286368).
• CVE-2026-112400: Denial of Service via memory exhaustion from crafted GIF image (bsc#1286370).
• CVE-2026-112401: Denial of Service via crafted GIF comments (bsc#1286372).
• CVE-2026-112402: Denial of service via double free during image transformation (bsc#1286381).
• CVE-2026-112403: heap buffer overflow in ImagingJpegDecode allows for code execution (bsc#1286405).
• CVE-2026-112405: NULL pointer dereference in Image.merge via an unvalidated band 0 (bsc#1286404).
• CVE-2026-112406: infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (bsc#1286403).
• CVE-2026-112891: quadratic GIF comment copying during save (bsc#1286402).
• CVE-2026-112892: MSP RLE decoder expands a tiny file into unbounded memory (bsc#1286401).
• CVE-2026-112894: Denial of Service via crafted multi-frame image conversion (bsc#1286408).
• CVE-2026-112895: Denial of service via crafted PDF during append operations (bsc#1286411).
• CVE-2026-112896: TIFF encoder ColorMap plane-size mismatch causes a heap out-of-bounds read (bsc#1286400).
• CVE-2026-112897: TiffDecode returns uninitialized memory as pixels for short JPEG strips (bsc#1286398).
• CVE-2026-112899: EPS load(scale) lets Ghostscript render past the decompression bomb limit (bsc#1286489).
• CVE-2026-112900: argument Injection in Ghostscript call can lead to remote code execution (bsc#1286397).
• CVE-2026-112901: ReDoS in XbmImagePlugin can be triggered by 512-byte file (bsc#1286396).
• CVE-2026-112902: memory disclosure and segfault in libtiff encoder via crafted TIFF tags (bsc#1286804).