CVE-2026-96638 python-pillow CVE debrief

Who should care

Defenders responsible for systems using Pillow, especially those processing untrusted images, should assess exposure and prioritize patching or mitigating this vulnerability. This includes operators of platforms where Pillow is used to process images, vulnerability management teams, and security teams responsible for monitoring and incident response. Additionally, defenders should review and update incident response plans to address potential denial-of-s

Why it matters

A memory-amplification denial-of-service vulnerability exists in Pillow's BMP RLE decoder. Defenders should prioritize patching or mitigating this vulnerability, especially in environments where Pillow is used to process untrusted images.

• Denial-of-service condition due to excessive memory allocation
• Potential for exploitation via crafted BMP files
• Need for patching or mitigating the vulnerability to prevent exploitation

Technical summary

The BmpRleDecoder in Pillow is vulnerable to a memory-amplification denial-of-service attack. An attacker can craft a BMP file to cause the decoder to allocate gigabytes of memory, leading to a denial-of-service condition. This is due to the decoder's inability to properly handle certain types of BMP files, leading to unbounded memory growth. The decoder's loop condition is only checked at the top, and individual branches can append attacker-controlled amounts of data, leading to a severe memory-amplification DoS. The vulnerability can be triggered by a tiny RLE stream combined with a large-but-legal xsize, resulting in a multi-GB `data` bytearray.

Defensive priority

Defenders should prioritize patching or mitigating this vulnerability, especially in environments where Pillow is used to process untrusted images.

Recommended defensive actions

• Patch Pillow to version 13.0.0 or later
• Restrict access to Pillow's BMP RLE decoder to trusted users
• Monitor Pillow's usage and detect potential denial-of-service attempts
• Implement additional security measures such as input validation and monitoring
• Conduct regular security audits and vulnerability assessments
• Review and update incident response plans to address potential denial-of-service attacks
• Track and analyze potential exploitation attempts

Evidence notes

The vulnerability is caused by the BmpRleDecoder's inability to properly handle certain types of BMP files, leading to unbounded memory growth. The decoder's loop condition is only checked at the top, and individual branches can append attacker-controlled amounts of data, leading to a severe memory-amplification DoS.

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.