CVE-2021-31618 | NULL pointer dereference on specially crafted HTTP/2 request

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.

Published: 2021-06-15 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-31618 is rated High Risk (69.6/100): CVSS High severity, with high exploitation likelihood (EPSS 51.21%, 99th percentile). EPSS ranks this CVE among the most likely to be exploited in the near term. EPSS rose +42.50% over the last day, indicating growing attacker interest. High exploitation likelihood—assess exposure and prioritize remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-31618

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 8.70% 51.21% +42.50%
2 2026-06-12 14.97% 8.70% -6.27%
3 2026-06-07 — 14.97% —

Full EPSS history (44 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-31618

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-31618

OS Trackers for CVE-2021-31618

vendor priority summary link
alpine — CVE-2021-31618: 1 source package rows (apache2); 10 state rows across 10 repos (3.10-main, 3.11-main, 3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 10, open 0. https://security.alpinelinux.org/vuln/CVE-2021-31618
debian not yet assigned CVE-2021-31618 not yet assigned priority: Debian including 1 source packages (apache2), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-31618
gentoo low CVE-2021-31618: 1 GLSA(s) (202107-38), 1 atom(s) (www-servers/apache); latest impact low. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-31618
redhat high — https://access.redhat.com/security/cve/CVE-2021-31618
suse high CVE-2021-31618 severity important: SUSE including 272 source package names (amazon/suse-sles-15-sp1-chost-byos-v20210304-hvm-ssd-x86_64, amazon/suse-sles-15-sp1-chost-byos-v20220127-hvm-ssd-x86_64, …), 499 product×package rows across 61 product lines (HPE Helion OpenStack 8, Image SLES15-SP3-EC2-HVM, … (61 product lines)): Fixed 264, Known Affected 231, Known Not Affected 4. https://www.suse.com/security/cve/CVE-2021-31618/
ubuntu medium CVE-2021-31618 medium priority: Ubuntu including 1 source packages (apache2), 9 status rows across 9 suites (bionic, focal, groovy, hirsute, impish, jammy, trusty, upstream, xenial): not-affected 8, released 1. https://ubuntu.com/security/CVE-2021-31618

Affected software / configurations for CVE-2021-31618

Vendor Product Version Raw CPE
apache http_server 1.15.17 cpe:2.3:a:apache:http_server:1.15.17:*:*:*:*:*:*:*
apache http_server 2.4.47 cpe:2.3:a:apache:http_server:2.4.47:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
oracle enterprise_manager_ops_center 12.4.0.0 cpe:2.3:a:oracle:enterprise_manager_ops_center:12.4.0.0:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.1 cpe:2.3:a:oracle:instantis_enterprisetrack:17.1:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.2 cpe:2.3:a:oracle:instantis_enterprisetrack:17.2:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.3 cpe:2.3:a:oracle:instantis_enterprisetrack:17.3:*:*:*:*:*:*:*
oracle zfs_storage_appliance_kit 8.8 cpe:2.3:a:oracle:zfs_storage_appliance_kit:8.8:*:*:*:*:*:*:*

References for CVE-2021-31618

URL Tags
http://httpd.apache.org/security/vulnerabilities_24.html Release Notes Vendor Advisory
http://www.openwall.com/lists/oss-security/2021/06/10/9 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2024/03/13/2
https://lists.apache.org/thread.html/r14b66ef0f4f569fd515a3f96cd4eb58bd9a8ff525cc326bb0359664f%40%3Ccvs.httpd.apache.org%3E
https://lists.apache.org/thread.html/r783b6558abf3305b17ea462bed4bd66d82866438999bf38cef6d11d1%40%3Ccvs.httpd.apache.org%3E
https://lists.debian.org/debian-lts-announce/2021/07/msg00006.html Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2NKJ3ZA3FTSZ2QBBPKS6BYGAWYRABNQQ/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/A73QJ4HPUMU26I6EULG6SCK67TUEXZYR/
https://seclists.org/oss-sec/2021/q2/206 Mailing List Third Party Advisory
https://security.gentoo.org/glsa/202107-38 Third Party Advisory
https://security.netapp.com/advisory/ntap-20210727-0008/ Third Party Advisory
https://www.debian.org/security/2021/dsa-4937 Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.html Patch Third Party Advisory
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