CVE-2026-96638 is a medium-severity (CVSS 6.5) Uncontrolled Resource Consumption vulnerability in pillow. A fix is available for pillow — see the affected versions and patch details below.
Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-96638 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
pillowReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
BmpRleDecoder can be forced to allocate gigabytes of memory from a kilobyte-sized crafted BMP file via unbounded growth in the delta escape and absolute mode branches.
Affected versions
All versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84)
Root cause
BmpRleDecoder.decode() (BmpImagePlugin.py:327-395) builds a Python bytearray data with dest_length = xsize*ysize, looping while len(data) < dest_length. The loop condition is only checked at the TOP; individual branches append attacker-controlled amounts UNBOUNDED by dest_length:
- delta escape (line 370): data += b"\x00" * (right + up * self.state.xsize) with right,up in 0..255 -> up to ~255*xsize bytes in ONE iteration.
- absolute mode (line 385): data += bytes_read (up to 255).
Because a single delta can push len(data) to ~256 * (row size), a tiny RLE stream combined with a large-but-legal xsize (xsize*ysize just under MAX_IMAGE_PIXELS, ~89M) yields a multi-GB data bytearray (then copied again by bytes(data) and set_as_raw). No heap OOB (bytearray/set_as_raw are bounds-safe), but a severe memory-amplification DoS: the attacker file stays tiny.
Reproduction
Python 3.13, Pillow HEAD e7a7ea84
Attached: pillow_poc_bmp_rle_dos.py
A 1084-byte BMP (BI_RLE8, width=3,000,000, height=1 -> 3M px, passes bomb check) with one delta escape (00 02 00 150) produces ~429 MB data; measured peak RSS 911 MB. Scaling width toward the bomb limit and up=255 produces tens of GB from a ~1 KB file.
Suggested fix
Cap growth at dest_length in the delta and absolute branches, e.g. clamp the delta fill to (dest_length - len(data)) and stop appending once len(data) >= dest_length; reject deltas that would move the cursor beyond the image, matching the existing per-row clamp at line 341-343.
Credit: Yazan Balawneh - CyStack Security Team (GitHub: sonicnew)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pillow | ≥ 9.1.0&&< 13.0.0 | 13.0.0pip install --upgrade 'pillow==13.0.0' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pillow, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pillow to 13.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-96638 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2026-96638 in your dependencies?
Find it across PyPI, including transitive dependencies.