CVE-2021-28040: An issue was discovered in OSSEC 3.6.0

This week

HIGH 7.5REMOTEDOWNGRADE

CVE-2021-28040 is a high-severity flaw (CVSS 7.5) affecting ossec. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in OSSEC 3.6.0. An uncontrolled recursion vulnerability in os_xml.c occurs when a large number of opening and closing XML tags is used. Because recursion is used in _ReadElem without restriction, an attacker can trigger a segmentation fault once unmapped memory is reached.

Why this priority

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

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 32%)
CISA exploited list	No
Weakness	CWE-674
Published	2021-03-05
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: ossec

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

Am I affected?

Timeline

2021-03-05 18:15 UTCPublished CVE record published
2026-06-17 03:45 UTCUpdated Record last modified

ossec ossec DOWNGRADE

Affected: 3.6.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.6.0

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 3.6.0 (Debian / Ubuntu)

apt-cache madison ossec sudo apt install ossec=<VERSION_BEFORE_3.6.0> sudo apt-mark hold ossec

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list ossec sudo dnf downgrade ossec-<VERSION_BEFORE_3.6.0> sudo dnf versionlock add ossec

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-2021-28040?

ossec: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-28040 being exploited?

CVE-2021-28040 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-2021-28040 affect?

CVE-2021-28040 affects ossec. Severity is high (CVSS 7.5). An issue was discovered in OSSEC 3.6.0. An uncontrolled recursion vulnerability in os_xml.c occurs when a large number of opening and closing XML tags is used. Because recursion is used in _ReadElem without restriction, an attacker can trig

How urgent is CVE-2021-28040?

Priority 53/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

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References

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