CVE-2021-31618Network attack vector

Child-process crash via uninitialized HTTP/2 rejection response

Published Jun 15, 2021 · Updated Aug 3, 2024

Denial of service in mod_http2 in Apache HTTP Server 2.4.47 allows remote attackers to crash a child process via an oversized header. On the first request header or a trailer that exceeds configured HTTP/1 size limits, the HTTP/2 handler builds an incompletely initialized rejection response and dereferences NULL. Exposure requires HTTP/2 and the unreleased 2.4.47 build; a crafted request reliably terminates a child process rather than the entire server.

CVSS severity7.5
High
EPSS probability51.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial of service in mod_http2 in Apache HTTP Server 2.4.47 allows remote attackers to crash a child process via an oversized header. On the first request header or a trailer that exceeds configured HTTP/1 size limits, the HTTP/2 handler builds an incompletely initialized rejection response and dereferences NULL. Exposure requires HTTP/2 and the unreleased 2.4.47 build; a crafted request reliably terminates a child process rather than the entire server.

The record

CVE
CVE-2021-31618
Published
Jun 15, 2021
Updated
Aug 3, 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. Jun 15, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=2.4.47

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache HTTP Serverversion=2.4.47

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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.

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo