CVE-2021-26691

69

FAUCET Score

CVE-2021-26691 is a critical heap overflow vulnerability affecting Apache HTTP Server versions 2.4.0 to 2.4.46, as well as products from Debian, Fedora, NetApp, and Oracle. This flaw allows a remote attacker to trigger a heap overflow by sending a specially crafted SessionHeader from an origin server. With a CVSS score of 9.8, it poses a severe risk, enabling full compromise of confidentiality, integrity, and availability. While there is no evidence of active exploitation, nor public exploit code, the vulnerability has garnered significant community discussion and media coverage, indicating high awareness.

Impacted Technologies

Vendor	Product	Version(s)	CPE
		>= 2.4.0, <= 2.4.46CPE matchmatch criteria	cpe:2.3:a:apache:http_server::::::::
		9.0CPE matchmatch criteria	cpe:2.3:o:debian:debian_linux:9.0:::::::*
		10.0CPE matchmatch criteria	cpe:2.3:o:debian:debian_linux:10.0:::::::*
		34CPE matchmatch criteria	cpe:2.3:o:fedoraproject:fedora:34:::::::*
		35CPE matchmatch criteria	cpe:2.3:o:fedoraproject:fedora:35:::::::*

CVSS Data

CVSS version used by this source: 3.1

9.8CRITICAL

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

Exploitability Score

3.9

Impact Score

5.9

CvssVersion

3.1

Exploit Intelligence

EPSS Score

68.07%

Probability of exploitation in next 30 days

EPSS Percentile

99.3%

Percentile rank of EPSS score among Peer Group

As of 2026-07-27

Model: v2026.06.15

This CVE's current EPSS score of 0.6807 is in the 97th percentile among its peer group of 36,897 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.1 Reddit, 0.3 Bluesky, 0.3 Mastodon, and 2.4 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.3 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Patch Available

Vendor Patches (36)

apachepatch availablevia llm_extracted

Fixed in: 2.4

microsoftpatch availablevia msrc

Product: cm1 httpd 2.4.46-5 on CBL Mariner 1.0Fixed in: 2.4.46-5

microsoftpatch availablevia msrc

Product: 19037-16820Fixed in: 2.4.46-5

microsoftpatch availablevia msrc

Product: 19038-16823Fixed in: 2.4.46-10

microsoftpatch availablevia msrc

Product: cbl2 httpd 2.4.46-10 on CBL Mariner 2.0Fixed in: 2.4.46-10

oraclepatch availablevia nvd_reference

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-apr-0:1.6.3-107.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-httpd-0:2.4.37-78.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-mod_cluster-native-0:1.3.16-9.Final_redhat_2.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-mod_http2-0:1.15.7-21.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-mod_jk-0:1.2.48-20.redhat_1.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-mod_md-1:2.0.8-40.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-mod_security-0:2.9.2-67.GA.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-nghttp2-0:1.39.2-39.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-openssl-1:1.1.1g-8.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-openssl-chil-0:1.0.0-7.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-openssl-pkcs11-0:0.4.10-22.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-apr-0:1.6.3-107.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-apr-util-0:1.6.1-84.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-curl-0:7.78.0-2.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-httpd-0:2.4.37-78.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-mod_cluster-native-0:1.3.16-9.Final_redhat_2.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-mod_http2-0:1.15.7-21.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-mod_jk-0:1.2.48-20.redhat_1.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-mod_md-1:2.0.8-40.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-mod_security-0:2.9.2-67.GA.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-nghttp2-0:1.39.2-39.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-openssl-1:1.1.1g-8.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-openssl-chil-0:1.0.0-7.jbcs.el7

redhatpatch availablevia redhat_api

Product: JBoss Core Services on RHEL 7Fixed in: jbcs-httpd24-openssl-pkcs11-0:0.4.10-22.jbcs.el7

redhatpatch availablevia redhat_api

Product: Red Hat Enterprise Linux 7Fixed in: httpd-0:2.4.6-97.el7_9.4

redhatpatch availablevia redhat_api

Product: Red Hat Enterprise Linux 8Fixed in: httpd:2.4-8040020211008164252.522a0ee4

redhatpatch availablevia redhat_api

Product: Text-Only JBCSFixed in: httpd

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-curl-0:7.78.0-2.el8jbcs

redhatpatch availablevia redhat_api

Product: JBoss Core Services for RHEL 8Fixed in: jbcs-httpd24-apr-util-0:1.6.1-84.el8jbcs

redhatend of lifevia redhat_api

Product: Red Hat Software CollectionsFixed in: httpd24-httpd

Vendor Advisories (5)

apachellm-apache-f398f8ed28802aa3LOW

Apache HTTP Server 2.4 vulnerabilities - The Apache HTTP Server Project

apachellm-apache-a7a91ec4c0e9421dHIGH

Apache HTTP Server 2.4 vulnerabilities - The Apache HTTP Server Project

microsoft2021-Jun/CVE-2021-26691Critical

Apache HTTP Server mod_session response handling heap overflow

redhatCVE-2021-26691Moderate

httpd: mod_session: Heap overflow via a crafted SessionHeader value

apachellm-apache-684e4d0003611bd4LOW

Apache HTTP Server 2.4 vulnerabilities - The Apache HTTP Server Project

References

httpd.apache.org / security/vulnerabilities_24.html

Vendor Advisory

lists.apache.org / thread.html/r50cae1b71f1e7421069036b213c26da7d8f47dd59874e3bd956959fe%40%3Cannounce.httpd.apache.org%3E

lists.apache.org / thread.html/r7f2b70b621651548f4b6f027552f1dd91705d7111bb5d15cda0a68dd%40%3Cdev.httpd.apache.org%3E

lists.apache.org / thread.html/re026d3da9d7824bd93b9f871c0fdda978d960c7e62d8c43cba8d0bf3%40%3Ccvs.httpd.apache.org%3E

Mailing ListRelease NotesVendor Advisory

lists.debian.org / debian-lts-announce/2021/07/msg00006.html

Mailing ListThird Party Advisory

lists.fedoraproject.org / archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD

lists.fedoraproject.org / archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R

security.gentoo.org / glsa/202107-38

Third Party Advisory

security.netapp.com / advisory/ntap-20210702-0001

Third Party Advisory

debian.org / security/2021/dsa-4937

Mailing ListThird Party Advisory

oracle.com / security-alerts/cpujan2022.html

PatchThird Party Advisory

oracle.com / security-alerts/cpuoct2021.html

PatchThird Party Advisory

openwall.com / lists/oss-security/2021/06/10/7

Mailing ListThird Party Advisory