CVE-2026-33721: MapServer is a system for developing web-based GIS applications

This week

MEDIUM 5.3REMOTEUPGRADE

CVE-2026-33721 is a medium-severity flaw (CVSS 5.3) affecting mapserver. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 31%)
CISA exploited list	No
Weakness	CWE-787
Published	2026-03-27
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: osgeo

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-27 01:16 UTCPublished CVE record published
2026-06-17 10:37 UTCUpdated Record last modified

osgeo mapserver UPGRADE

Affected: >= 4.2.0 and < 8.6.1

Fixed in: 8.6.1

Safe to downgrade to: a release before 4.2.0

1. Check installed version

apt-cache policy mapserver || rpm -q mapserver || mapserver --version

2. UPGRADE to 8.6.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade mapserver

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mapserver

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 4.2.0 (Debian / Ubuntu)

apt-cache madison mapserver sudo apt install mapserver=<VERSION_BEFORE_4.2.0> sudo apt-mark hold mapserver

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list mapserver sudo dnf downgrade mapserver-<VERSION_BEFORE_4.2.0> sudo dnf versionlock add mapserver

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-33721?

mapserver: upgrade to 8.6.1. Exact commands for each OS are on this page.

Is CVE-2026-33721 being exploited?

CVE-2026-33721 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-33721 affect?

CVE-2026-33721 affects mapserver. Severity is medium (CVSS 5.3). 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 cr

How urgent is CVE-2026-33721?

Priority 46/100. Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-45104 HIGH 7.5 UPGRADE MapServer is a system for developing web-based GIS applications
CVE-2026-42030 MEDIUM 6.1 UPGRADE MapServer is a system for developing web-based GIS applications

References

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