GHSA-g842-9rc4-69pw BIT-pillow-2026-96637 CVE-2026-96637 PYSEC-2026-4338
Pillow: Decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS
- Severity
- 0.0 / 10
- Medium · CVSS 4.0
- Exploit chance (EPSS)
- 0.37%
- Chance of exploitation in the next 30 days. Higher than 31.27% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-400 Uncontrolled Resource Consumption
- CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)
- Fix
13.0.0- Upgrade pillow to this version or later
Description
Summary
SpiderImagePlugin's seek() bypasses the decompression bomb check, allowing a tiny (<1KB) crafted SPIDER stack file to trigger a multi-gigabyte allocation on im.load().
Affected versions
All versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84)
Root cause
Image.open() runs Image._decompression_bomb_check(im.size) exactly once, in _open_core (src/PIL/Image.py:3668), right after the initial _open().
SPIDER stack files are multi-frame: SpiderImageFile.seek(frame) (SpiderImagePlugin.py:179-190) computes an attacker-influenced stkoffset and calls self._open() AGAIN directly (line 190) — NOT through Image.open(). _open() re-parses a fresh 27-float header at that offset and reassigns:
self._size = int(h[12]), int(h[2]) # SpiderImagePlugin.py:131
straight from unvalidated header floats (isSpiderHeader only checks internal labrec*lenbyt==labbyt consistency, never bounds nsam/nrow). No bomb check runs on this new size.
On the subsequent load(), ImageFile.load_prepare() (ImageFile.py:436-437) executes self.im = Image.core.new(self.mode, self.size) with the unchecked, attacker-chosen size. Mode is always "F" (4 bytes/pixel), so e.g. 40000x40000 = 6.4 GB.
Why this is a real bug
Sibling multi-frame plugins DO re-check on their alternate paths:
- GifImagePlugin.py:286 - Image._decompression_bomb_check(self._size) (in seek)
- TiffImagePlugin.py:1309 - Image._decompression_bomb_check(self._tile_size)
- IcnsImagePlugin.py:121/138, IcoImagePlugin.py:236/240 (sub-image selection)
SpiderImagePlugin has NO bomb check anywhere. This is the same class of bug just fixed for GdImageFile (GHSA-phj9-mv4w-65pm) and the FontFile classes (GHSA-5x94/8v84/45hq): an allocation reached on a non-Image.open() path sized by an attacker header field, independent of the one-time pixel bomb check.
Reproduction
Python 3.13, Pillow HEAD e7a7ea84
Attached: pillow_poc_spider_bomb.py
Sets Image.MAX_IMAGE_PIXELS=5000, then:
- CONTROL: Image.open(standalone 100x100 SPIDER) -> DecompressionBombWarning fires correctly.
- BYPASS: Image.open(stack, frame0=1x1) passes; im.seek(1) sets size (100,100) with NO DecompressionBombWarning/Error -> the check is bypassed.
Real-world impact: a large frame-1 nsam/nrow (e.g. 40000x40000) causes im.load() to allocate ~6.4 GB (mode F) with the bomb check fully bypassed, from a file under 1 KB.
Suggested fix
Call Image._decompression_bomb_check(self.size) at the end of SpiderImageFile._open() (or in seek() after _open()), mirroring GifImagePlugin / TiffImagePlugin. Optionally validate nsam/nrow are positive and sane in _open().
Credit: Yazan Balawneh - CyStack Security Team pillow_poc_spider_bomb.py
Affected versions
PyPI
| Package | Vulnerable | Fixed in |
|---|---|---|
| pillow | < 13.0.0 |
13.0.0 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
pillow PyPI → 13.0.0
- pip
pip install --upgrade "pillow>=13.0.0"- poetry
poetry add "pillow>=13.0.0"- uv
uv add "pillow>=13.0.0"
Severity breakdown
CVSS 4.0 0.0
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- Attack vector
- Network
- Attack complexity
- Low
- Attack requirements
- None
- Privileges required
- None
- User interaction
- Passive
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- None
- Availability (vulnerable system)
- High
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
CVSS 3.1 6.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Weakness
CWE-400: Uncontrolled Resource Consumption
The product does not properly control the allocation and maintenance of a limited resource.
How to prevent it
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Implementation: Ensure that all failures in resource allocation place the system into a safe posture.
CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)
The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| PYSEC-2023-175 Pillow versions before v10.0.1 bundled libwebp binaries in wheels that are vulnerable to CVE-2023-5129 (previously CVE-2023-4863). | High Known exploited | Also affects pillow |
| GHSA-j7hp-h8jx-5ppr libwebp: OOB write in BuildHuffmanTable | High Known exploited | Also affects pillow |
| GHSA-78g4-xqwv-q3g8 Pillow: Bundled libopenjp2 2.5.4 in all wheels since 11.1.0 carries OSV-2025-219 heap-buffer-overflow write reachable from Image.open().load() | High | Also affects pillow |
| GHSA-654x-cwwv-5j9c Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1 | Medium | Also affects pillow |
| GHSA-xvh5-g6gv-559v Pillow: Memory disclosure and segfault in libtiff encoder via crafted TIFF tags | High | Also affects pillow |
| GHSA-g7cr-3p24-7wf7 Pillow: Heap buffer overflow writing JPEG comment after ICC profile fills buffer | High | Also affects pillow |
| GHSA-9v9v-9qvf-h2hj Pillow: Heap over-read in WMF/EMF rendering via 16-bit dimension truncation (Windows builds) | High | Also affects pillow |
| GHSA-g4qv-2jxx-769h Pillow: PNG eXIf chunk makes EXIF parsing run in quadratic time | Medium | Also affects pillow |
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