CVE-2026-91186
The PNG chunk reader has no notion of chunk ordering
- Risk score
- 51/100
- CVSS
- 8.4 v4.0
- Exploit chance (30 days)
- 0.2%
- Published
- 20 Oct 2026
Open in the live forecast Today's forecast
What to know
- What it is
- 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 i…
- Affected
- See advisory
- Fix
- Apply the vendor's security update; see the linked advisory.
- Exploitation
- No known exploitation
Why it is Yellow
- CVSS 8.4 high
References
Original description
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.
Last activity 20 Oct 2026. Patch Forecast updates every hour.