No exploitation reportedFix available

CVE-2021-31618

No exploitation reported by the sources we read.

Published2021-06-15
WeaknessCWE-476

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

Apache HTTP Server protocol handler for the HTTP/2 protocol checks received request headers against the size limitations as configured for the server and used for the HTTP/1 protocol as well. On violation of these restrictions and HTTP response is sent to the client with a status code indicating why the request was rejected. This rejection response was not fully initialised in the HTTP/2 protocol handler if the offending header was the very first one received or appeared in a a footer. This led to a NULL pointer dereference on initialised memory, crashing reliably the child process. Since such a triggering HTTP/2 request is easy to craft and submit, this can be exploited to DoS the server. This issue affected mod_http2 1.15.17 and Apache HTTP Server version 2.4.47 only. Apache HTTP Server 2.4.47 was never released.

Timeline

dates in UTC

2021-06-1509:15CVE publishedNVD
2026-10-2319:23EPSS crossed 10 %FIRST EPSS

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-31618?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-31618?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Oracle instantis enterprisetrack

Branch	Affected	Fixed in	Source
17.1	= 17.1	no fix published	NVD
17.2	= 17.2	no fix published	NVD
17.3	= 17.3	no fix published	NVD

Apache HTTP Server

Branch	Affected	Fixed in	Source
1.15	= 1.15.17	no fix published	NVD
2.4	= 2.4.47	no fix published	NVD

Debian debian linux

Branch	Affected	Fixed in	Source
9.0	= 9.0	no fix published	NVD
10.0	= 10.0	no fix published	NVD

3 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Advisories and patches

httpd.apache.orgRelease Notes · Vendor Advisory
www.oracle.comPatch

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

CISA-ADPSSVC decisionnone2024-05-20
OpenCVESSVC decisionnone—
NVDCVSS 3.17.5read 2026-10-23
NVDCVSS 2.05read 2026-10-23
CNAweakness (CWE)CWE-476read 2026-10-23
NVDweakness (CWE)CWE-476read 2026-10-23
FIRST EPSSEPSS score51.5 %read 2026-10-23

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published: 2021-06-15
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2021-31618?

Is CVE-2021-31618 actively exploited?

Active exploitation of CVE-2021-31618 has not been confirmed. Its EPSS score was 51.5% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-31618?

CVE-2021-31618 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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