`CVE-2021-26691` Apache HTTP Server

Affected software	Apache HTTP Server
Severity	9.8 Critical CVSS 3.1 2 scores
Weakness	Out-of-bounds write (CWE-787)
Published	10 Jun 2021
Updated	17 Jun 2026

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

CWE-122

Exploitation status

Forecast only (68% 30-day chance)

No exploit has been reported in the sources we check. EPSS puts this CVE in the top 0.66% of all scored CVEs. That is a model's forecast, not a report of exploitation.

CISA KEV not listed
VulnCheck KEV not listed
CISA Vulnrichment no assessment published
Indexed PoC none indexed
EPSS 68% chance in 30 days A model's forecast, not a report of exploitation. Higher than 99% of all scored CVEs.

Waratek protection

Outside Waratek scope

A Java vulnerability, but not one Waratek protects. It affects only Java on a user's desktop (applets, Web Start, JavaFX, the installer), sits in a product the Waratek agent can't be installed into, or is in a package we don't expect in business applications. The decision below says which.

Protection none
Action not pursued none
Review human Researcher
Record active

Decided by	Manual classification of this CVE in the legacy dataset
Finding	A manual classification set this CVE's status to Outside Waratek scope, but the CVE names a tracked product.

Decided by

Manual classification of this CVE in the legacy dataset

Finding

A manual classification set this CVE's status to Outside Waratek scope, but the CVE names a tracked product.

CVSS

9.8 CRITICAL v3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H 2 metrics

We show the highest score from the newest CVSS version. Scores from different versions can't be compared, and the first score listed is sometimes a 0.0 placeholder.

Version Score Band Vector Assigner Type

CVSS 3.1 9.8 CRITICAL CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H NVD Primary published

CVSS 2.0 7.5 none AV:N/AC:L/Au:N/C:P/I:P/A:P NVD Primary

Version	Score	Band	Vector	Assigner	Type
CVSS 3.1	9.8	CRITICAL	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`	NVD	Primary	published
CVSS 2.0	7.5	none	`AV:N/AC:L/Au:N/C:P/I:P/A:P`	NVD	Primary

Affected software and releases

Oracle advisories

Advisory	Type	Quarter	Products Oracle names in its risk matrix
Oracle Critical Patch Update January 2022 ↗	CPU	2022-Q1	Oracle Secure Backup / Oracle Secure Backup (Apache HTTP Server) (Prior to 18.1.0.1.0)
Oracle Critical Patch Update October 2021 ↗	CPU	2021-Q4	Oracle Construction and Engineering / Core (Apache HTTP Server) (17.1, 17.2, 17.3) Oracle Enterprise Manager / Networking (Apache HTTP Server) (12.4.0.0) Oracle Systems / Operating System Image (8.8)

Advisory

Type

Quarter

Products Oracle names in its risk matrix

Oracle Critical Patch Update January 2022 ↗

CPU

2022-Q1

Oracle Secure Backup / Oracle Secure Backup (Apache HTTP Server) (Prior to 18.1.0.1.0)

Oracle Critical Patch Update October 2021 ↗

CPU

2021-Q4

Oracle Construction and Engineering / Core (Apache HTTP Server) (17.1, 17.2, 17.3)
Oracle Enterprise Manager / Networking (Apache HTTP Server) (12.4.0.0)
Oracle Systems / Operating System Image (8.8)

Oracle components and versions

Family	Component	Versions, as Oracle lists them	Read as
Oracle Construction and Engineering	Core (Apache HTTP Server)	17.1, 17.2, 17.3	17.1 · 17.2 · 17.3
Oracle Enterprise Manager	Networking (Apache HTTP Server)	12.4.0.0	12.4.0.0
Oracle Secure Backup	Oracle Secure Backup (Apache HTTP Server)	Prior to 18.1.0.1.0
Oracle Systems	Operating System Image	8.8	8.8

Family

Component

Versions, as Oracle lists them

Read as

Oracle Construction and Engineering

Core (Apache HTTP Server)

17.1, 17.2, 17.3

17.1 · 17.2 · 17.3

Oracle Enterprise Manager

Networking (Apache HTTP Server)

12.4.0.0

Oracle Secure Backup

Oracle Secure Backup (Apache HTTP Server)

Prior to 18.1.0.1.0

Oracle Systems

Operating System Image

8.8

Products and versions

8 products, 12 version entries

Source	Vendor	Product	Scheme	Affected versions
nvd	apache	http server	generic	≥ 2.4.0 and ≤ 2.4.46
nvd	debian	debian linux	generic	9.0 · 10.0
nvd	fedoraproject	fedora	generic	34 · 35
nvd	netapp	cloud backup	generic	any version
nvd	oracle	enterprise manager ops center	generic	12.4.0.0
nvd	oracle	instantis enterprisetrack	generic	17.1 · 17.2 · 17.3
nvd	oracle	secure backup	generic	＜ 18.1.0.1.0
nvd	oracle	zfs storage appliance kit	generic	8.8

Source

Vendor

Product

Scheme

Affected versions

nvd

apache

http server

generic

≥ 2.4.0 and ≤ 2.4.46

nvd

debian

debian linux

generic

9.0 · 10.0

nvd

fedoraproject

fedora

generic

34 · 35

nvd

netapp

cloud backup

generic

any version

nvd

oracle

enterprise manager ops center

generic

12.4.0.0

nvd

oracle

instantis enterprisetrack

generic

17.1 · 17.2 · 17.3

nvd

oracle

secure backup

generic

＜ 18.1.0.1.0

nvd

oracle

zfs storage appliance kit

generic

8.8

Earlier manual review

A reviewer classified this CVE in an older dataset. We kept the labels and products below, but not the reviewer's reasoning.

## Manual Classification Context **Labels:** NETWORKING (APACHE HTTP SERVER), HTTP, ORACLE SECURE BACKUP (APACHE HTTP SERVER), CORE (APACHE HTTP SERVER), MULTIPLE, NO AUTH REMOTE EXPLOIT, OPERATING SYSTEM IMAGE **Products:** ORACLE SYSTEMS, ORACLE SECURE BACKUP, INSTANTIS ENTERPRISETRACK, ORACLE ENTERPRISE MANAGER, ENTERPRISE MANAGER OPS CENTER, ORACLE CONSTRUCTION AND ENGINEERING, ORACLE ZFS STORAGE APPLIANCE KIT

References

URL	Tags
http://httpd.apache.org/security/vulnerabilities_24.html	Vendor Advisory
http://www.openwall.com/lists/oss-security/2021/06/10/7	Mailing List, Third Party Advisory
https://lists.apache.org/thread.html/r50cae1b71f1e7421069036b213c26da7d8f47dd59874e3bd956959fe%40%3Cannounce.httpd.apache.org%3E
https://lists.apache.org/thread.html/r7f2b70b621651548f4b6f027552f1dd91705d7111bb5d15cda0a68dd%40%3Cdev.httpd.apache.org%3E
https://lists.apache.org/thread.html/re026d3da9d7824bd93b9f871c0fdda978d960c7e62d8c43cba8d0bf3%40%3Ccvs.httpd.apache.org%3E	Mailing List, Release Notes, Vendor Advisory
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/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
https://security.gentoo.org/glsa/202107-38	Third Party Advisory
https://security.netapp.com/advisory/ntap-20210702-0001/	Third Party Advisory

URL

Timeline

Published	2021-06-10	By the CVE Program.
NVD record modified	2026-06-17	NVD's own last-modified date for this record.

Published

2021-06-10

By the CVE Program.

NVD record modified

2026-06-17

NVD's own last-modified date for this record.

How VRT decided (5 of 24 rules ran)

5 of 24 rules ran. The rest didn't run because an earlier rule had already settled the CVE.

# Rule Outcome Status Detail

identity: Does this identifier describe a vulnerability at all? the first rule that matches decides

1 rejected no match CVE Program withdrawal of this identifier

scope: Is it a vulnerability Waratek protects? the first rule that matches decides

2 armr-patch-file no match An ARMR patch file on disk for this CVE

3 armr-secure-rule-file no match An ARMR security-rule template on disk for this CVE

4 manual-classification no match A researcher's recorded decision for this specific CVE

5 manual classification decided it matched Outside Waratek scope Found in legacy manual classifications

6 non-java-upstream not reached A root cause in non-Java code: an OS kernel, a C library, or a bundled runtime

7 oracle-component not reached The defect being in a component of a tracked product the agent does not reach: JavaFX, the Java SE installer, the JInitiator browser plug-in, the native WebLogic proxy plug-in, or a native part of Oracle Application Server or Collaboration Suite (HTTP Server, Web Cache, Internet Directory, mail and calendar servers). Oracle gives these no CPE of their own and names them only in the description.

8 java-client-side not reached Oracle's own risk-matrix note saying the CVE is reached only through untrusted code in the Java sandbox (applets, Web Start) or only through a client installation of Java, so a server running its own trusted code is not exposed

9 osv-maven not reached An OSV.dev record of this CVE affecting a package published to Maven Central. When every such package is one business applications do not use (Minecraft mods and launchers, Fastjson) and no tracked product is named by CPE, it settles the CVE as Outside Waratek scope instead

10 cna-maven-package not reached An affected Maven artifact named by the CNA itself

11 known-cpe-product not reached An NVD CPE naming a product ARMR tracks

12 cna-cpe-product not reached A CPE the CNA published naming a tracked product. Oracle publishes these on advisory day, months before NVD.

13 oracle-advisory-family not reached A tracked product family named in Oracle's own risk matrix. CPE cannot carry a suite name, only a module.

14 cna-named-product not reached A tracked product named in words by the CVE's own CNA, on a record with no CPE yet. Apache and Eclipse publish these on advisory day, before NVD adds CPE data.

15 non-deployable-product not reached The only affected product being a sealed appliance image, where Java runs but no -javaagent can be attached

16 unmatched-product not reached No tracked product matched above, and none present at all

disposition: What do we do about one that is? every rule runs

17 patch-hint not reached A reference pointing at the upstream fix: a commit, a PR, a Red Hat bug

18 open-source-maven not reached The affected package being open source on Maven, so the fix can be read

19 poc-derivable not reached An indexed exploit clearing the evidence bar to reverse-engineer, where there is no source to read

20 no-exploit not reached Nothing to build a rule from yet: no published exploit, no CISA record of exploitation and no source-level change to read

21 secure-rule-match not reached A vulnerability class already blocked by an ARMR security rule

22 exploited-no-fix-or-poc not reached Somebody exploiting this CVE (a CISA or VulnCheck KEV listing, or a CISA verdict of active exploitation), with no fix, source code or PoC to build a rule from

23 below-action-threshold not reached Medium or Low severity, no CISA or VulnCheck KEV listing, no PoC clearing the evidence bar, no CISA verdict, no Oracle advisory

24 no-exploit-published not reached Exploit code for this candidate, published in a reviewed catalogue or a GitHub repository with 5 or more stars, or in use per CISA or VulnCheck; not only a CISA verdict, a forecast, or nothing at all

NVD ↗ · CVE.org ↗