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Fencer Intel
critical 9.8

CVE-2021-26691 in the critical feed json

Apache HTTP Server mod_session response handling heap overflow

published 5y ago modified 2mo ago also BIT-apache-2021-26691 , EUVD-2021-13484

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
68.1%
EPSS 68.1% chance of exploitation in 30 days, percentile 99.3%
public exploits
0
none known

Summary

In Apache HTTP Server versions 2.4.0 to 2.4.46 a specially crafted SessionHeader sent by an origin server could cause a heap overflow

Patch / fix

  • bitnami apache 2.4.47
  • oracle secure_backup 18.1.0.1.0
  • apache2 Debian 14, Debian 13, Debian 12 2.4.46-6

debian advisories DEBIAN-CVE-2021-26691

  • apache2 Ubuntu 18.04 LTS 2.4.29-1ubuntu4.16
  • apache2 Ubuntu 20.04 LTS 2.4.41-4ubuntu3.3
  • apache2 Ubuntu Pro 14.04 LTS 2.4.7-1ubuntu4.22+esm1
  • apache2 Ubuntu Pro 16.04 LTS 2.4.18-2ubuntu3.17+esm1

ubuntu advisories UBUNTU-CVE-2021-26691

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • jbcs-httpd24-apr jboss core services 1 1.6.3-107.jbcs.el7
  • jbcs-httpd24-apr jboss core services 1 1.6.3-107.el8jbcs
  • jbcs-httpd24-curl jboss core services 1 7.78.0-2.jbcs.el7
  • jbcs-httpd24-curl jboss core services 1 7.78.0-2.el8jbcs
  • jbcs-httpd24-httpd jboss core services 1 2.4.37-78.jbcs.el7
  • jbcs-httpd24-httpd jboss core services 1 2.4.37-78.el8jbcs
  • jbcs-httpd24-mod_jk jboss core services 1 1.2.48-20.redhat_1.jbcs.el7
  • jbcs-httpd24-mod_jk jboss core services 1 1.2.48-20.redhat_1.el8jbcs

red hat advisories RHSA-2021:4614RHSA-2022:0143RHSA-2021:3816

  • apache2 SLE Module for Server Applications 15 SP3, SLE Module for Server Applications 15 SP2, SLE Module for Package Hub 15 SP3 +2 more 2.4.43-3.22.1
  • apache2 SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +11 more 2.4.23-29.74.1
  • apache2 SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +10 more 2.4.33-3.50.1

suse advisories SUSE-SU-2021:2127-1SUSE-SU-2021:2006-1SUSE-SU-2021:2004-1

  • apache2 openSUSE Leap 15.3 2.4.43-3.22.1
  • apache2 openSUSE Leap 15.2 2.4.43-lp152.2.15.1

opensuse advisories openSUSE-SU-2021:2127-1openSUSE-SU-2021:0908-1

  • httpd AlmaLinux 8 2.4.37-43.module_el8.5.0+2597+c4b14997.alma
  • mod_md AlmaLinux 8 1:2.0.8-8.module_el8.5.0+2609+b30d9eec
  • mod_md AlmaLinux 8 1:2.0.8-8.module_el8.6.0+2872+fe0ff7aa
  • mod_ssl AlmaLinux 8 1:2.4.37-43.module_el8.5.0+2597+c4b14997.alma
  • mod_ldap AlmaLinux 8 2.4.37-43.module_el8.5.0+2597+c4b14997.alma
  • mod_http2 AlmaLinux 8 1.15.7-3.module_el8.5.0+2609+b30d9eec
  • httpd-tools AlmaLinux 8 2.4.37-43.module_el8.5.0+2597+c4b14997.alma
  • mod_session AlmaLinux 8 2.4.37-43.module_el8.5.0+2597+c4b14997.alma

almalinux advisories ALSA-2021:4537ALSA-2021:3816

  • httpd Rocky Linux 8 2.4.37-39.module+el8.4.0+571+fd70afb1
  • mod_md Rocky Linux 8 1:2.0.8-8.module+el8.5.0+695+1fa8055e
  • mod_md Rocky Linux 8 1:2.0.8-8.module+el8.4.0+553+7a69454b
  • mod_http2 Rocky Linux 8 1.15.7-3.module+el8.4.0+553+7a69454b
  • mod_http2 Rocky Linux 8 1.15.7-3.module+el8.5.0+695+1fa8055e

rocky linux advisories RLSA-2021:3816

  • apache2 Alpine 3.24, Alpine 3.23, Alpine 3.22 +12 more 2.4.48-r0

alpine advisories ALPINE-CVE-2021-26691

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • httpd CBL-Mariner 2.0 2.4.46-10

azure linux advisories AZL-6477

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

  • apache Arch Linux 2.4.47-1

arch linux advisories AVG-2053

Is CVE-2021-26691 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-26691 updated cvss 8.1

httpd: mod_session: Heap overflow via a crafted SessionHeader value

  • Red Hat JBoss Enterprise Application Platform 6 httpd22 httpd22 Red Hat JBoss Enterprise Web Server 2 httpd22 httpd22
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Software Collections httpd24-httpd, httpd24-mod_md, httpd24-mod_ssl, httpd24-mod_ldap, httpd24-httpd-tools, httpd24-mod_session, +2 more
    affected: workaround, no fix planned “Will not fix”
  • Red Hat Enterprise Linux AppStream (v. 8) mod_md 2.0.8-8.module+el8.3.0+6814+67d1e611, httpd 2.4.37-39.module+el8.4.0+12865+a7065a39.1, mod_http2 1.15.7-3.module+el8.4.0+8625+d397f3da, mod_ssl 2.4.37-39.module+el8.4.0+12865+a7065a39.1, mod_ldap 2.4.37-39.module+el8.4.0+12865+a7065a39.1, httpd-tools 2.4.37-39.module+el8.4.0+12865+a7065a39.1, +4 more
    fixed: vendor fix RHSA-2021:3816
  • Red Hat Enterprise Linux Client Optional (v. 7) httpd 2.4.6-97.el7_9.4, mod_ssl 2.4.6-97.el7_9.4, mod_ldap 2.4.6-97.el7_9.4, httpd-tools 2.4.6-97.el7_9.4, mod_session 2.4.6-97.el7_9.4, httpd-manual 2.4.6-97.el7_9.4, +1 more Red Hat Enterprise Linux ComputeNode Optional (v. 7) httpd 2.4.6-97.el7_9.4, mod_ssl 2.4.6-97.el7_9.4, mod_ldap 2.4.6-97.el7_9.4, httpd-tools 2.4.6-97.el7_9.4, mod_session 2.4.6-97.el7_9.4, httpd-manual 2.4.6-97.el7_9.4, +1 more Red Hat Enterprise Linux Server (v. 7) httpd 2.4.6-97.el7_9.4, mod_ssl 2.4.6-97.el7_9.4, httpd-tools 2.4.6-97.el7_9.4, mod_session 2.4.6-97.el7_9.4, httpd-manual 2.4.6-97.el7_9.4 Red Hat Enterprise Linux Server Optional (v. 7) mod_ldap 2.4.6-97.el7_9.4, mod_proxy_html 2.4.6-97.el7_9.4 Red Hat Enterprise Linux Workstation (v. 7) httpd 2.4.6-97.el7_9.4, mod_ssl 2.4.6-97.el7_9.4, httpd-tools 2.4.6-97.el7_9.4, mod_session 2.4.6-97.el7_9.4, httpd-manual 2.4.6-97.el7_9.4 Red Hat Enterprise Linux Workstation Optional (v. 7) mod_ldap 2.4.6-97.el7_9.4, mod_proxy_html 2.4.6-97.el7_9.4
    fixed: vendor fix RHSA-2022:0143
  • Red Hat JBoss Core Services
    fixed: vendor fix RHSA-2021:4613
  • Red Hat JBoss Core Services on RHEL 7 Server jbcs-httpd24-apr 1.6.3-107.jbcs.el7, jbcs-httpd24-curl 7.78.0-2.jbcs.el7, jbcs-httpd24-httpd 2.4.37-78.jbcs.el7, jbcs-httpd24-mod_md 2.0.8-40.jbcs.el7, jbcs-httpd24-libcurl 7.78.0-2.jbcs.el7, jbcs-httpd24-openssl 1.1.1g-8.jbcs.el7, +26 more Red Hat JBoss Core Services on RHEL 8 jbcs-httpd24-apr 1.6.3-107.el8jbcs, jbcs-httpd24-curl 7.78.0-2.el8jbcs, jbcs-httpd24-httpd 2.4.37-78.el8jbcs, jbcs-httpd24-mod_md 2.0.8-40.el8jbcs, jbcs-httpd24-libcurl 7.78.0-2.el8jbcs, jbcs-httpd24-openssl 1.1.1g-8.el8jbcs, +26 more
    fixed: vendor fix RHSA-2021:4614
  • Red Hat Enterprise Linux 6 httpd, mod_ssl, httpd-tools, httpd-manual Red Hat Enterprise Linux 9 httpd, mod_lua, mod_ssl, mod_ldap, httpd-core, httpd-tools, +4 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

cisa ICSA-22-132-02 updated cvss 9.8

Mitsubishi Electric MELSOFT iQ AppPortal

  • Mitsubishi Electric MELSOFT iQ AppPortal (SW1DND-IQAPL-M) >= 1.00A, <= 1.26C
    affected: mitigation “Update to 1.29F or later. Unzip the downloaded file (zip format) and execute the file “setup.exe.”” mitsubishielectric.com

From CISA ICS and IT advisories (CSAF, https://github.com/cisagov/CSAF), a U.S. Government work; changed: reduced to products, statuses and remediations. CISA notification · csaf

Exploit prediction

68.07% chance of exploitation in the next 30 days, higher than 99.3% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P
OSV 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected packages

ecosystem package affected fixed in
bitnami apache
>=2.4.0, <2.4.47
2.4.47

Affected products

  • Apache Software Foundation Apache HTTP Server

Classification

Weakness
Technology feeds

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.