# CVE-2026-120251: imagecodecs 2026.3.6 through 2026.10.10 Infinite Loop via ccittfax4_decode()

Status: published. Published: 2026-11-01. Updated: 2026-11-06.

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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- cgohlke / imagecodecs (imagecodecs): ≥ 2026.3.6; ≤ 2026.10.10 (affected, custom). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 6.8 (medium); CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 5.5 (medium); CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H; source: CNA: VulnCheck

- CVSS 4.0: 6.8 (medium); CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: disclosure@vulncheck.com

- CVSS 3.1: 5.5 (medium); CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H; source: NVD: disclosure@vulncheck.com

- CISA KEV: No listing in collected sources

- EPSS: 0.12% as of 2026-11-06; percentile 2.14%

- SSVC: exploitation poc; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-835

## Exploit evidence

- exploit: https://github.com/cgohlke/imagecodecs/issues/158 (ADP: CISA-ADP)

## References

- https://github.com/cgohlke/imagecodecs/issues/158 (CNA: VulnCheck; issue-tracking)

- https://github.com/cgohlke/imagecodecs/blob/v2026.10.10/3rdparty/ccitt/ccitt.c#L1449 (CNA: VulnCheck; technical-description)

- https://github.com/cgohlke/imagecodecs (CNA: VulnCheck; product)

- https://www.vulncheck.com/advisories/imagecodecs-2026.3.6-through-2026.10.10-infinite-loop-via-ccittfax4-decode (CNA: VulnCheck; third-party-advisory)

- https://github.com/cgohlke/imagecodecs/issues/158 (ADP: CISA-ADP; exploit)

- https://github.com/cgohlke/imagecodecs (disclosure@vulncheck.com)

- https://github.com/cgohlke/imagecodecs/blob/v2026.10.10/3rdparty/ccitt/ccitt.c#L1449 (disclosure@vulncheck.com)

- https://github.com/cgohlke/imagecodecs/issues/158 (disclosure@vulncheck.com)

- https://www.vulncheck.com/advisories/imagecodecs-2026.3.6-through-2026.10.10-infinite-loop-via-ccittfax4-decode (disclosure@vulncheck.com)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-120251; retrieved 2026-11-06; source updated 2026-11-06

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-120251; retrieved 2026-11-03; source updated 2026-11-02

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-120251; retrieved 2026-11-06; source updated Not reported

Revision: dc9c1b643dcddfa36606d7d76d01a341b70d31c1842549cd08c1dc77e5e4f5f4

Canonical: https://y2.dev/cve/CVE-2026-120251

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