CVE-2021-26691
CriticalIn 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
Published Jan 19, 2022 · Updated Sep 8, 2026 · Also known as ALPINE-CVE-2021-26691, BIT-apache-2021-26691, DEBIAN-CVE-2021-26691, UBUNTU-CVE-2021-26691
Key Facts
Patch immediately- CVE-2021-26691 is a critical-severity vulnerability (CVSS 9.8 v3.1) affecting HTTP Server (Apache), Cloud Backup (Netapp), Enterprise Manager Ops Center (Oracle) and 22 more products.
- It is not listed in any known-exploited vulnerabilities catalog. No public exploit code is known.
- EPSS estimates a 68% probability of exploitation in the next 30 days.
- Fixed versions are available, including apache 2.4.47, apache 2.4.47-1, HTTP Server <=2.4.46.
- Brandefense priority: patch immediately, because it is very likely to be exploited soon.
CVSS v3.1
9.8
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS (30-day exploit likelihood)
68%
Higher than 99.3% of all scored CVEs
Exploited in the wild
No known
Not in a known-exploited catalog
Public exploit code
None known
Get the full CVE-2021-26691 brief as a PDF
Every affected product and fixed release, exploitation status and a first 24 hours response checklist, emailed within minutes.
Description
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
Affected Products
345 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| HTTP ServerApache | >= 2.4.46 | 2.4.46 |
| Cloud BackupNetapp | See advisory | n/a |
| Enterprise Manager Ops CenterOracle | 12.4.0.0 | n/a |
| Instantis EnterprisetrackOracle | 17.1, 17.2, 17.3 | n/a |
| Secure BackupOracle | See advisory | 18.1.0.1.0 |
Patched Releases
apache
2.4.47, 2.4.47-1
HTTP Server
<=2.4.46
apache2
2.4.18-2ubuntu3.17+esm1, 2.4.23-29.74.1, 2.4.25-3+deb9u10, 2.4.29-1ubuntu4.16, 2.4.33-3.50.1, 2.4.38-3+deb10u5, 2.4.41-4ubuntu3.3, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1, 2.4.46-6, 2.4.48-r0, 2.4.7-1ubuntu4.22+esm1
apache2-devel
2.4.23-29.74.1, 2.4.33-3.50.1, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1
apache2-doc
2.4.23-29.74.1, 2.4.33-3.50.1, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1
apache2-event
2.4.43-3.22.1, 2.4.43-lp152.2.15.1
apache2-example-pages
2.4.23-29.74.1, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1
apache2-prefork
2.4.23-29.74.1, 2.4.33-3.50.1, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1
apache2-utils
2.4.23-29.74.1, 2.4.33-3.50.1, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1
apache2-worker
2.4.23-29.74.1, 2.4.33-3.50.1, 2.4.43-3.22.1, 2.4.43-lp152.2.15.1
httpd
0:2.4.6-97.el7_9.4, 2.4.37-39.0.1.module+el8.4.0+20372+b87b2deb.1, 2.4.37-39.module+el8.4.0+571+fd70afb1, 2.4.46-10, 2.4.46-5, 2.4.48-1.ph1, 2.4.48-1.ph2, 2.4.48-2.amzn2, 2.4.6-97.0.5.el7_9.4
httpd-devel
2.4.37-39.0.1.module+el8.4.0+20372+b87b2deb.1, 2.4.48-2.amzn2, 2.4.6-97.0.5.el7_9.4
httpd-filesystem
2.4.37-39.0.1.module+el8.4.0+20372+b87b2deb.1, 2.4.48-2.amzn2
httpd-manual
2.4.37-39.0.1.module+el8.4.0+20372+b87b2deb.1, 2.4.48-2.amzn2, 2.4.6-97.0.5.el7_9.4
httpd-tools
2.4.37-39.0.1.module+el8.4.0+20372+b87b2deb.1, 2.4.48-2.amzn2, 2.4.6-97.0.5.el7_9.4
httpd:2.4
8040020211008164252.522a0ee4
httpd24
2.4.48-1.92.amzn1
httpd24-devel
2.4.48-1.92.amzn1
httpd24-manual
2.4.48-1.92.amzn1
httpd24-tools
2.4.48-1.92.amzn1
jbcs-httpd24-apr
0:1.6.3-107.el8jbcs, 0:1.6.3-107.jbcs.el7
jbcs-httpd24-apr-util
0:1.6.1-84.el8jbcs, 0:1.6.1-84.jbcs.el7
jbcs-httpd24-curl
0:7.78.0-2.el8jbcs, 0:7.78.0-2.jbcs.el7
jbcs-httpd24-httpd
0:2.4.37-78.el8jbcs, 0:2.4.37-78.jbcs.el7
jbcs-httpd24-mod_cluster-native
0:1.3.16-9.Final_redhat_2.el8jbcs, 0:1.3.16-9.Final_redhat_2.jbcs.el7
Timeline
Publicly disclosed
Jan 19, 2022
Last updated
Sep 8, 2026
Weakness Type
- CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer
- CWE-122 Heap-based Buffer Overflow
- CWE-787 Out-of-bounds Write
References
- advisoryhttp://access.redhat.com/errata/RHSA-2022:0158
- advisoryhttp://access.redhat.com/errata/RHSA-2022:0162
- advisoryhttp://httpd.apache.org/security/vulnerabilities_24.html
- advisoryRHSA-2021:3816
- advisoryRHSA-2021:4613
FAQ
Common Questions.
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
Affected: HTTP Server (Apache) versions >= 2.4.46; Cloud Backup (Netapp); Enterprise Manager Ops Center (Oracle) versions 12.4.0.0, among others. The full list is in the CVE brief.
It is not listed in any known-exploited vulnerabilities catalog.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
No public exploit code is known.
Fixed versions are available, including apache 2.4.47, apache 2.4.47-1, HTTP Server <=2.4.46.
Brandefense priority: patch immediately, because it is very likely to be exploited soon.
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