CVE-2026-33416 — LIBPNG has use-after-free via pointer aliasing in `png_set_tRNS` and `png_set_PLTE`

Affected: pnggroup libpng

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.

USN-8251-1 — libpng1.6 vulnerabilities

Affected: Ubuntu:22.04:LTS: libpng1.6, Ubuntu:24.04:LTS: libpng1.6, Ubuntu:25.10: libpng1.6

It was discovered that libpng incorrectly handled memory when processing certain PNG files. If a user or automated system were tricked into opening a specially crafted PNG file, an attacker could use this issue to cause libpng to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2026-33416)

It was discovered that libpng incorrectly handled expanding 8-bit paletted rows to RGB or RGBA on ARM processors. If a user or automated system were tricked into opening a specially crafted PNG file, an attacker could use this issue to cause libpng to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2026-33636)

It was discovered that libpng incorrectly handled certain setter APIs. An attacker could possibly use this issue to obtain sensitive information. (CVE-2026-34757)

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CVE-2026-33416 — LIBPNG has use-after-free via pointer aliasing in `png_set_tRNS` and `png_set_PLTE`

USN-8251-1 — libpng1.6 vulnerabilities