8.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-40192

CVE-2026-40192: Pillow is vulnerable to a FITS GZIP decompression bomb

Pillow is a Python imaging library. Versions 10.3.0 through 12.1.1 did not limit the amount of GZIP-compressed data read when decoding a FITS image, making them vulnerable to decompression bomb attacks. A specially crafted FITS file could cause unbounded memory consumption, leading to denial of service (OOM crash or severe performance degradation). If users are unable to immediately upgrade, they should only open specific image formats, excluding FITS, as a workaround.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-40192

CVE-2026-40192:

8.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
pillow	pip	>= 10.3.0, < 12.2.0	12.2.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the FitsGzipDecoder.decode function within the src/PIL/FitsImagePlugin.py file of the Pillow library. The function was responsible for decompressing GZIP-compressed data from FITS image files. The vulnerability existed because the code read the entire compressed data stream into memory before decompression, without any limits. This made it susceptible to a decompression bomb attack, where a small, heavily compressed file could expand to an enormous size, leading to excessive memory consumption and a denial-of-service (DoS) condition.

The patch addresses this issue by changing the way the data is read and decompressed. Instead of reading the entire file at once, the patched code uses gzip.open to handle the compressed data as a stream. It then reads only the necessary amount of data based on the image's dimensions (self.state.xsize * self.state.ysize * 4). This ensures that the amount of memory allocated is bounded by the expected size of the image, effectively neutralizing the decompression bomb threat. The identified vulnerable function, FitsGzipDecoder.decode, is the exact location where this flawed logic existed and was subsequently fixed.

Vulnerable functions

FitsGzipDecoder.decode

src/PIL/FitsImagePlugin.py

The function `FitsGzipDecoder.decode` in `src/PIL/FitsImagePlugin.py` is vulnerable to a decompression bomb attack. The original code `value = gzip.decompress(self.fd.read())` reads the entire compressed data from the file descriptor `self.fd` into memory and then decompresses it. A malicious FITS file could provide a small, highly compressed data stream that, when decompressed, expands to a very large size, consuming excessive memory and causing a denial of service. The patch mitigates this by using `gzip.open` to create a file-like object and then reading a limited number of bytes from the decompressed stream, calculated from the image dimensions (`self.state.xsize * self.state.ysize * 4`). This prevents an attacker from causing unbounded memory allocation.

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WAF Protection Rules

WAF Rule

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Reasoning

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