CVE-2021-28040Public exploitNetwork attack vector

Process crash via unrestricted XML recursion

Published Mar 5, 2021 · Updated Aug 3, 2024

Uncontrolled recursion in OSSEC 3.6.0 allows remote attackers to crash an affected process by supplying deeply nested XML input. The _ReadElem function in os_xml.c recursively parses opening and closing tags without a depth limit, exhausting the stack until execution reaches unmapped memory and raises SIGSEGV. The demonstrated payload requires XML to reach the affected parser; the bounded consequence is process termination and service interruption, with no reported data disclosure or modification.

CVSS severity7.5
High
EPSS probability1.19%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Uncontrolled recursion in OSSEC 3.6.0 allows remote attackers to crash an affected process by supplying deeply nested XML input. The _ReadElem function in os_xml.c recursively parses opening and closing tags without a depth limit, exhausting the stack until execution reaches unmapped memory and raises SIGSEGV. The demonstrated payload requires XML to reach the affected parser; the bounded consequence is process termination and service interruption, with no reported data disclosure or modification.

The record

CVE
CVE-2021-28040
Published
Mar 5, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-674, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-674Uncontrolled Recursion
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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