CVE-2021-41524Network attack vector

Service interruption via HTTP/2 NULL dereference

Published Oct 5, 2021 · Updated Aug 4, 2024

NULL pointer dereference in Apache HTTP Server 2.4.49 allows remote attackers to interrupt service via a crafted HTTP/2 request. HTTP/2 processing dereferences a NULL pointer reached by the request, but Apache has not published the affected function or input field. HTTP/2 must be enabled; the published record reports denial of service only and does not distinguish a worker crash from a full server outage.

CVSS severity7.5
High
EPSS probability25.17%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

NULL pointer dereference in Apache HTTP Server 2.4.49 allows remote attackers to interrupt service via a crafted HTTP/2 request. HTTP/2 processing dereferences a NULL pointer reached by the request, but Apache has not published the affected function or input field. HTTP/2 must be enabled; the published record reports denial of service only and does not distinguish a worker crash from a full server outage.

The record

CVE
CVE-2021-41524
Published
Oct 5, 2021
Updated
Aug 4, 2024
Vendor
The Apache Software Foundation
Product
Apache HTTP Server
Classifications
CWE-476, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2.4.49

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache HTTP Serverversion=2.4.49

Published CVSS scores

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

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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-476NULL Pointer Dereference
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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