Use After Free in Libpng
CVE-2026-33416
LIBPNG is a reference library for use in applications that read, create, and manipulate PNG (Portable Network Graphics) raster image files. In versions 1.2.1 through 1.6.55, `png_set_tRNS` and `png_set_PLTE` each alias a heap-allocated buffer between `png_struct` and `png_info`, sharing a single allocation across two structs with independent lifetimes. The `trans_alpha` aliasing has been present since at least libpng 1.0, and the `palette` aliasing since at least 1.2.1. Both affect all prior release lines `png_set_tRNS` sets `png_ptr->trans_alpha = info_ptr->trans_alpha` (256-byte buffer) and `png_set_PLTE` sets `info_ptr->palette = png_ptr->palette` (768-byte buffer). In both cases, calling `png_free_data` (with `PNG_FREE_TRNS` or `PNG_FREE_PLTE`) frees the buffer through `info_ptr` while the corresponding `png_ptr` pointer remains dangling. Subsequent row-transform functions dereference and, in some code paths, write to the freed memory. A second call to `png_set_tRNS` or `png_set_PLTE` has the same effect, because both functions call `png_free_data` internally before reallocating the `info_ptr` buffer. Version 1.6.56 fixes the issue.
Vulnerability class: Use-After-Free
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.005 (41.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 42nd percentile — 41.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.011 · newest: 0.005 · change: -0.005
Affected products
- Libpng
- Pnggroup Libpng — versions >= 1.2.1, < 1.6.56
Weakness classification (CWE)
References
- github.com/pnggroup/libpng/security/advisories/GHSA-m4pc-p4q3-4c7j (Patch, Vendor Advisory)
- github.com/pnggroup/libpng/pull/824 (Exploit, Issue Tracking)
- github.com/pnggroup/libpng/commit/23019269764e35ed8458e517f1897bd3c54820eb (Patch)
- github.com/pnggroup/libpng/commit/7ea9eea884a2328cc7fdcb3c0c00246a50d90667 (Patch)
- github.com/pnggroup/libpng/commit/a3a21443ed12bfa1ef46fa0d4fb2b74a0fa34a25 (Patch)
- github.com/pnggroup/libpng/commit/c1b0318b393c90679e6fa5bc1d329fd5d5012ec1 (Patch)
Frequently asked questions
- What is CVE-2026-33416?
- CVE-2026-33416 is a high-severity vulnerability in Libpng, classified under Use After Free. CVSS score: 7.5/10. Published 2026-03-26.
- How severe is CVE-2026-33416?
- High severity. CVSS v3 base score is 7.5 out of 10.