CVE-2026-112399 python-pillow CVE debrief

Who should care

Defenders responsible for maintaining and securing applications that use Pillow should be aware of this vulnerability and take steps to prevent potential memory spikes. This includes verifying and updating Pillow to version 13.0.0 or later, reviewing and restricting image file uploads, and monitoring system memory usage. Additionally, defenders should review compensating controls for exposed systems, check relevant monitoring and logs, and track exceptions

Why it matters

Defenders should prioritize verifying and updating Pillow to version 13.0.0 or later to prevent potential memory spikes. This vulnerability can cause performance issues and potentially lead to denial-of-service conditions. Verification of Pillow version and restriction of image file uploads are necessary to prevent exploitation.

• Memory spikes can cause performance issues and potentially lead to denial-of-service conditions
• Successful exploitation can lead to increased memory usage, potentially causing system instability
• Verification of Pillow version and restriction of image file uploads are necessary to prevent exploitation

Technical summary

The vulnerability exists in Pillow's handling of animated PNG files. When the `Image.open()` function is called with a specially crafted APNG file, it can cause a multi-gigabyte memory spike. This is due to the allocation and filling of memory based on the image dimensions claimed by the file, before Pillow's built-in check for oversized images runs. The bug is in `PngImageFile._seek()` in `src/PIL/PngImagePlugin.py` (around line 1000). For any APNG, `_open()` calls `self._seek(0)`. When the first `fcTL` chunk requests `OP_BACKGROUND` disposal, `_seek()` calls `Image.core.fill(self.mode, self.size)` with the full IHDR size, which allocates and zero-fills memory sized by the image dimensions the file claims.

Defensive priority

Defenders should prioritize verifying and updating Pillow to version 13.0.0 or later to prevent potential memory spikes.

Recommended defensive actions

• Verify and update Pillow to version 13.0.0 or later
• Review and restrict image file uploads to prevent malicious APNG files from being processed
• Monitor system memory usage and implement limits on memory-intensive operations
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The vulnerability is caused by the allocation and filling of memory based on the image dimensions claimed by the APNG file, before Pillow's built-in check for oversized images runs. This can lead to a multi-gigabyte memory spike.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.