GHSA-5gjj-6r7v-ph3x: pillow-heif: Integer Overflow in Encode Path Buffer Validation Leads to Heap Out-of-Bounds Read

Summary An integer overflow in the encode path buffer validation of _pillow_heif.c allows an attacker to bypass bounds checks by providing large image dimensions, resulting in a heap out-of-bounds read. This can lead to information disclosure (server heap memory leaking into encoded images) or denial of service (process crash). No special configuration is required — this triggers under default settings. Details The buffer validation in _CtxWriteImage_add_plane(), _CtxWriteImage_add_plane_la(), and _CtxWriteImage_add_plane_l() uses 32-bit int multiplication to check whether the input buffer is large enough: // _pillow_heif.c, lines 158, 344, 449 if (stride_in * height > buffer.len) { PyBuffer_Release(&buffer); PyErr_SetString(PyExc_ValueError, "image plane does not contain enough data"); return NULL; } Both stride_in and height are declared as int (32-bit signed). When their product exceeds INT_MAX (2,147,483,647), the multiplication overflows before the comparison with buffer.len (which is Py_ssize_t, 64-bit). The overflowed value wraps to zero or a negative number, causing the bounds check to pass incorrectly. For example, with stride_in = 196608 (65536 × 3 for RGB) and height = 65536: - True product: 12,884,901,888 - int32 product: 0 (wraps around) - Comparison: 0 > buffer.len → false → check bypassed After the check is bypassed, the subsequent loop reads beyond the input buffer: for (int i = 0; i < height; i++) memcpy(out + stride_out * i, in + stride_in * i, real_stride); Additionally, real_stride = width * n_channels (e.g., line 148: real_stride = width * 3) is also computed as int * int, which can independently overflow for large width values. This vulnerability exists in the encode path, which is distinct from the decode path: - The decode path is partially guarded by libheif's built-in security limits - The encode path has no such guards — DISABLE_SECURITY_LIMITS is irrelevant - The encode path is reachable whenever an application calls pillow_heif