CVE-2026-112395

Pillow is a Python imaging library. From 8.2.0 until 13.0.0, Pillow's tiled-TIFF decoder can read past the tile buffer while copying four-byte pixels. In _decodeTile, libtiff can return a tile allocation smaller than the required row stride multiplied by the tile length, and Pillow's stride-and-copy loop walks beyond that allocation. An attacker can supply a crafted tiled TIFF to an application that loads it. This can expose adjacent heap data in pixels or crash the process. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 8.2.0 until 13.0.0, Pillow's tiled-TIFF decoder can read past the tile buffer while copying four-byte pixels. In _decodeTile, libtiff can return a tile allocation smaller than the required row stride multiplied by the tile length, and Pillow's stride-and-copy loop walks beyond that allocation. An attacker can supply a crafted tiled TIFF to an application that loads it. This can expose adjacent heap data in pixels or crash the process. This issue is fixed in version 13.0.0.

Summary

Pillow's libImaging _decodeTile decoder, used to decode tiled TIFF images, allocates a per-tile buffer using realloc(state->buffer, TIFFTileSize(tiff)). After each TIFFReadTile it then iterates tile_y from 0 up to current_tile_length and calls the configured row unpacker (here copy4) with the source pointer state->buffer + tile_y * row_byte_size. When libtiff returns a tile_bytes_size smaller than the worst-case row_byte_size * tile_length, the inner stride-and-copy loop walks past the realloc'd region and copy4 performs a 1024-byte memcpy that reads up to (tile_length-1) * row_byte_size + row_byte_size bytes from a buffer sized only to tile_bytes_size. The supplied PoC TIFF triggers a 1024-byte over-read exactly 0 bytes past a 196608-byte allocation. An attacker who can convince a Pillow-using application to decode a crafted TIFF can trigger a process crash (DoS) and may cause adjacent heap memory to be copied into the decoded pixel buffer.

Details

Pillow's TIFF decoder, when libtiff reports the file is tiled, takes the _decodeTile path. The function:

1. Asks libtiff for TIFFTileSize(tiff) (the bytes-per-tile) and TIFFTileRowSize(tiff) (the bytes per row as stored by libtiff).
2. Reallocates state->buffer to exactly tile_bytes_size.
3. For each tile origin (x, y) and each plane, calls TIFFReadTile into state->buffer, then walks the tile row by row, calling the configured ImagingShuffler (here copy4) on each row's worth of source bytes.

The validation done before the loop is:

if (tile_bytes_size > ((tile_length * state->bits / planes + 7) / 8) * tile_width) {
    state->errcode = IMAGING_CODEC_BROKEN;
    return -1;
}

This rejects oversized libtiff tiles, but it does not reject undersized ones — i.e. the case where libtiff returns a tile_bytes_size strictly less than row_byte_size * tile_length. The loop body, however, indexes into the buffer assuming the buffer is at least row_byte_size * tile_length bytes long.

Vulnerable Code

// src/libImaging/TiffDecode.c:446-490 — _decodeTile
new_data = realloc(state->buffer, state->bytes);   // bytes = tile_bytes_size
if (!new_data) { ... }
state->buffer = new_data;

for (y = state->yoff; y < state->ysize; y += tile_length) {
    for (plane = 0; plane < planes; plane++) {
        ImagingShuffler shuffler = unpackers[plane];
        for (x = state->xoff; x < state->xsize; x += tile_width) {
            if (TIFFReadTile(tiff, (tdata_t)state->buffer, x, y, 0, plane) == -1) { ... }

            current_tile_width  = min((INT32)tile_width,  state->xsize - x);
            current_tile_length = min((INT32)tile_length, state->ysize - y);
            for (tile_y = 0; tile_y < current_tile_length; tile_y++) {
                shuffler(
                    (UINT8 *)im->image[tile_y + y] + x * im->pixelsize,
                    state->buffer + tile_y * row_byte_size,   // <-- OOB stride
                    current_tile_width
                );
            }
        }
    }
}

// src/libImaging/Unpack.c:1288-1291 — copy4 (the crash site)
static void
copy4(UINT8 *out, const UINT8 *in, int pixels) {
    /* RGBA, CMYK quadruples */
    memcpy(out, in, 4 * pixels);   // <-- ASAN-detected READ here
}

The PoC's tile metadata yields tile_width = tile_length = 256, row_byte_size = 1024 (4 bytes/pixel × 256 pixels), and tile_bytes_size = 196608 (which is 256·256·3 = three-channel-byte storage). The realloc'd buffer is 196608 bytes. The unpacker iterates tile_y from 0 to 255 and for the highest rows the source pointer state->buffer + tile_y * 1024 already lies at or past offset 196608 — the very last row's memcpy(in=buffer+255·1024, 1024) reads 1024 bytes starting 260096 - 196608 = 63488 bytes past the allocation; ASAN flags the very first byte past the heap region (READ of size 1024 at … 0 bytes after 196608-byte region).

The mismatch is real: libtiff's…

Pillow is a Python imaging library. From 8.2.0 until 13.0.0, Pillow's tiled-TIFF decoder can read past the tile buffer while copying four-byte pixels. In _decodeTile, libtiff can return a tile allocation smaller than the required row stride multiplied by the tile length, and Pillow's stride-and-copy loop walks beyond that allocation. An attacker can supply a crafted tiled TIFF to an application that loads it. This can expose adjacent heap data in pixels or crash the process. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 8.2.0 until 13.0.0, Pillow's tiled-TIFF decoder can read past the tile buffer while copying four-byte pixels. In _decodeTile, libtiff can return a tile allocation smaller than the required row stride multiplied by the tile length, and Pillow's stride-and-copy loop walks beyond that allocation. An attacker can supply a crafted tiled TIFF to an application that loads it. This can expose adjacent heap data in pixels or crash the process. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. From 8.2.0 until 13.0.0, Pillow's tiled-TIFF decoder can read past the tile buffer while copying four-byte pixels. In _decodeTile, libtiff can return a tile allocation smaller than the required row stride multiplied by the tile length, and Pillow's stride-and-copy loop walks beyond that allocation. An attacker can supply a crafted tiled TIFF to an application that loads it. This can expose adjacent heap data in pixels or crash the process. This issue is fixed in version 13.0.0.