---
id: CVE-2026-91186
title: >-
  PNG decoder: a duplicate IHDR chunk after IDAT resizes the image after
  allocation, causing a heap out-of-bounds write
summary: >-
  The PNG chunk reader has no notion of chunk ordering. It records whether an
  IDAT has been seen and whether a tRNS has already supplied transparency, but
  nothing prevents an IHDR being read a second time.




  A PNG carries exactly one IHDR…
severity: high
cvss: 8.4
cvssVector: 'CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N'
cvssSource: cna
cwe:
  - CWE-125
  - CWE-787
vendor: Eclipse Foundation
product: GUIX
affected:
  - GUIX <= 6.5.1.202602a
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-10-20T15:28:30.798866Z'
published: '2026-10-20'
updated: '2026-10-20'
sourceUpdated: '2026-10-20T15:28:59.237Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-91186'
references:
  - url: >-
      https://github.com/eclipse-threadx/guix/security/advisories/GHSA-gx8f-4rpc-4fjw
tags:
  - cve.org
ingestedAt: '2026-10-20T15:09:24.116Z'
---

## Overview

The PNG chunk reader has no notion of chunk ordering. It records whether an IDAT has been seen and whether a tRNS has already supplied transparency, but nothing prevents an IHDR being read a second time.



A PNG carries exactly one IHDR and it is the first chunk. A file that places a second one after IDAT rewrites the image width, height, bit depth and pixel depth *after* the decoded-data buffer has already been allocated and filled from the original values. The pass that reverses the PNG row filters then derives its own extent from the new dimensions and walks the old allocation, moving image bytes past the end of it.



The buffer is allocated by GUIX rather than supplied by the caller, so the application has no way to size it defensively. The decoder is compiled in by default.

## Affected

- `GUIX <= 6.5.1.202602a`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.