---
id: "CVE-2026-120251"
kind: "vulnerability"
aliases: ["CVE-2026-120251"]
title: "imagecodecs: crafted CCITT Group 4 stream hangs decoding process indefinitely"
severity: "medium"
cvss: 6.8
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["image-processing", "imagecodecs", "numcodecs", "package", "python"]
not_affected_if: ["no-ccitt-codec", "no-untrusted-input", "version-outside-range"]
published_at: "2026-11-01T01:17:19+00:00"
first_seen_at: "2026-11-01T01:47:22+00:00"
updated_at: "2026-11-01T01:47:39+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-120251", "first_seen_at": "2026-11-01T01:47:22+00:00"}]
---

# imagecodecs: crafted CCITT Group 4 stream hangs decoding process indefinitely

imagecodecs versions 2026.3.6 through 2026.10.10 loop forever in ccitt_fax4_decode_size() when given a crafted CCITT Group 4 image stream. An attacker can pin a CPU core until the calling process is killed.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `python -m pip show imagecodecs` and check whether the version is between `2026.3.6` and `2026.10.10`.
- Search dependency lockfiles and requirements for `imagecodecs` and for `numcodecs` codecs using `Ccittfax4`.

## Fix

- Move to a version of imagecodecs later than `2026.10.10`; the sources do not name the exact fixed release.

## Timeline

- 2026-11-01T01:47:22+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-120251

## From the sources

imagecodecs 2026.3.6 through 2026.10.10 contains an infinite loop vulnerability in ccitt_fax4_decode_size() that allows attackers to hang the calling process by supplying a crafted CCITT Group 4 stream. Attackers can provide an 8-byte G4 stream lacking EOFB to ccittfax4_decode() or the numcodecs Ccittfax4 codec, consuming full CPU until the process is killed.

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