The patch adds a single line, ANGLE_TRY(stateManager->setPixelUnpackState(context, unpack)), to TextureGL::setCompressedSubImage in ANGLE’s OpenGL backend, immediately after binding the texture and before the code dispatches to the native glCompressedTexSubImage2D/3D path. setCompressedSubImage uploads a block of application-supplied compressed pixel data into an existing texture region. The gl::PixelUnpackState (the unpack parameter passed to the function) carries client-controlled parameters such as GL_UNPACK_ROW_LENGTH, alignment, skip rows/pixels, and, importantly, the bound GL_PIXEL_UNPACK_BUFFER used as the data source. The invariant ANGLE’s GL backend relies on is that before issuing a native texture-upload call, the GL state manager’s cached/native unpack state must be synchronized to the unpack parameters ANGLE resolved for this call; the non-compressed upload paths already do this via setPixelUnpackState, but the compressed path was missing the call. Because of that omission, the driver performed the compressed sub-image upload using stale or default unpack state that did not match the dimensions and byte-count ANGLE validated against, so the amount and layout of data the driver read from the source (client memory or the unpack buffer) could disagree with what was actually available. That mismatch lets the underlying GL driver read past the intended bounds of the source data, i.e. an out-of-bounds read during the upload.

The fix restores the invariant by explicitly programming the pixel-unpack state before the compressed upload, exactly as the uncompressed paths do, so the driver’s read geometry matches the validated buffer. It is worth noting (inference) that the precise faulting read happens inside the platform GL driver, which is not in the diff; the diff establishes only the missing state-synchronization call in ANGLE. Apple characterizes the observable result as an unexpected process crash, consistent with an OOB read rather than a write primitive.

The primitive is an out-of-bounds read during a compressed texture upload inside ANGLE’s GL backend, reachable from any web page via WebGL. Its realistic outcome is an unexpected crash of the GPU process (as Apple describes) with a theoretical, harder-to-control potential for reading adjacent GPU-process memory; the patch shows no write or type-confusion primitive, so escalation to code execution is not directly supported by this diff. The bug is confined to the sandboxed GPU process, so full compromise would require chaining an additional GPU-process-to-system sandbox escape. Overall this is a comparatively weak, mostly denial-of-service-grade primitive relative to a heap-write UAF/OOB-write.