_gx_binres_pixelmap_table_load() in the GUIX binary resource (theme) loader indexes the pixelmap table with a value read verbatim from the resource file and never bounds it against the table size. A crafted theme drives a write of up to roughly 256 KB past the end of the table.
The overflowed buffer is allocated by GUIX itself, through _gx_system_memory_allocator, rather than supplied by the caller, so the loader corrupts its own arena. The public API gx_binres_theme_load() takes no size parameter, so no amount of caller diligence prevents this.
Reproduces in the shipped default configuration; GX_BINARY_RESOURCE_SUPPORT is enabled by default.