What’s Vulnerable › Vulnerabilities

CVE-2026-33721: MapServer has heap buffer overflow in SLD `Categorize` Threshold parsing

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.2% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-03-27 · updated 2026-06-17
• Weakness: CWE-787

Description

MapServer is a system for developing web-based GIS applications. Starting in version 4.2 and prior to version 8.6.1, a heap-buffer-overflow write in MapServer’s SLD (Styled Layer Descriptor) parser lets a remote, unauthenticated attacker crash the MapServer process by sending a crafted SLD with more than 100 Threshold elements inside a ColorMap/Categorize structure (commonly reachable via WMS GetMap with SLD_BODY). Version 8.6.1 patches the issue.

Affected products

• MapServer: from 4.2.0 before 8.6.1 — fixed in 8.6.1

How to fix

Upgrade to a fixed version: MapServer 8.6.1. Check the vendor advisory for mitigations.

References

• https://github.com/MapServer/MapServer/releases/tag/rel-8-6-1
• https://github.com/MapServer/MapServer/security/advisories/GHSA-cv4m-mr84-fgjp
• https://lists.debian.org/debian-lts-announce/2026/04/msg00017.html

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