PatchTheCVECVE-2021-26691: In Apache HTTP Server versions 2.4.0 to 2.4.46 a specially crafted SessionHeader sent by an origin server coul
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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE
CVE-2021-26691 is a critical-severity flaw (CVSS 9.8) affecting http server, debian linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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
Why this priority
- Critical severity
- EPSS top 1%: very likely to be exploited
- Remotely exploitable, no login, no user action
- On your watchlist
A fixed release or distro patch exists.
Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
apache http server UPGRADE
Affected: >= 2.4.0 and <= 2.4.46
Fixed in: newer than 2.4.46
Safe to downgrade to: a release before 2.4.0
1. Check installed version
apt-cache policy http-server || rpm -q http-server || http-server --version
2. UPGRADE to newer than 2.4.46 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade http-server
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade http-server
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 2.4.0 (Debian / Ubuntu)
apt-cache madison http-server
sudo apt install http-server=<VERSION_BEFORE_2.4.0>
sudo apt-mark hold http-server
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list http-server
sudo dnf downgrade http-server-<VERSION_BEFORE_2.4.0>
sudo dnf versionlock add http-server
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
debian debian linux MITIGATE
Affected: 9.0 · 10.0
Fixed in: no fixed release known
Safe to downgrade to: a release before 9.0
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
fedoraproject fedora MITIGATE
Affected: 34 · 35
Fixed in: no fixed release known
Safe to downgrade to: a release before 34
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
oracle enterprise manager ops center DOWNGRADE
Affected: 12.4.0.0
Fixed in: no fixed release known
Safe to downgrade to: a release before 12.4.0.0
1. Check installed version
apt-cache policy enterprise-manager-ops-center || rpm -q enterprise-manager-ops-center || enterprise-manager-ops-center --version
2. No fixed release: DOWNGRADE to a release before 12.4.0.0 (Debian / Ubuntu)
apt-cache madison enterprise-manager-ops-center
sudo apt install enterprise-manager-ops-center=<VERSION_BEFORE_12.4.0.0>
sudo apt-mark hold enterprise-manager-ops-center
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list enterprise-manager-ops-center
sudo dnf downgrade enterprise-manager-ops-center-<VERSION_BEFORE_12.4.0.0>
sudo dnf versionlock add enterprise-manager-ops-center
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
oracle instantis enterprisetrack DOWNGRADE
Affected: 17.1 · 17.2 · 17.3
Fixed in: no fixed release known
Safe to downgrade to: a release before 17.1
1. Check installed version
apt-cache policy instantis-enterprisetrack || rpm -q instantis-enterprisetrack || instantis-enterprisetrack --version
2. No fixed release: DOWNGRADE to a release before 17.1 (Debian / Ubuntu)
apt-cache madison instantis-enterprisetrack
sudo apt install instantis-enterprisetrack=<VERSION_BEFORE_17.1>
sudo apt-mark hold instantis-enterprisetrack
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list instantis-enterprisetrack
sudo dnf downgrade instantis-enterprisetrack-<VERSION_BEFORE_17.1>
sudo dnf versionlock add instantis-enterprisetrack
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
oracle secure backup UPGRADE
Affected: < 18.1.0.1.0
Fixed in: 18.1.0.1.0
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy secure-backup || rpm -q secure-backup || secure-backup --version
2. UPGRADE to 18.1.0.1.0 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade secure-backup
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade secure-backup
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
Exact fixes by distribution
A distro patch exists: prefer upgrading over downgrading.
Fedora 46 · librepo · FIXED · 1.21.3-1.fc46 · FEDORA-2026-c502136ca7 · source
sudo dnf upgrade --advisory=FEDORA-2026-c502136ca7
Ubuntu 20.04 · apache2 · FIXED · 2.4.41-4ubuntu3.3
sudo apt update && sudo apt install --only-upgrade apache2
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.41-4ubuntu3.3" && echo PATCHED || echo STILL-VULNERABLE
Ubuntu 22.04 · apache2 · FIXED · 2.4.46-4ubuntu2
sudo apt update && sudo apt install --only-upgrade apache2
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.46-4ubuntu2" && echo PATCHED || echo STILL-VULNERABLE
Debian 12 · apache2 · FIXED · 2.4.46-6
sudo apt update && sudo apt install --only-upgrade apache2
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.46-6" && echo PATCHED || echo STILL-VULNERABLE
Debian 13 · apache2 · FIXED · 2.4.46-6
sudo apt update && sudo apt install --only-upgrade apache2
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.46-6" && echo PATCHED || echo STILL-VULNERABLE
RHEL 7 · httpd · FIXED · httpd-2.4.6-97.el7_9.4
sudo dnf upgrade --advisory=RHSA-2022:0143
rpm -q httpd
RHEL 6 · httpd · NOT AFFECTED
RHEL 9 · httpd · NOT AFFECTED
Alpine 3.20 · apache2 · FIXED · apache2-2.4.48-r0
apk add --upgrade apache2
apk info -v apache2
Alpine 3.21 · apache2 · FIXED · apache2-2.4.48-r0
apk add --upgrade apache2
apk info -v apache2
Alpine 3.22 · apache2 · FIXED · apache2-2.4.48-r0
apk add --upgrade apache2
apk info -v apache2
Alpine 3.23 · apache2 · FIXED · apache2-2.4.48-r0
apk add --upgrade apache2
apk info -v apache2
sudo yum update --advisory=ALAS2-2021-1674
sudo yum update --advisory=ALAS-2021-1514
Public exploits and detection
Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.
- Write-up: www.oracle.com/security-alerts/cpujan2022.html
- Write-up: www.oracle.com/security-alerts/cpuoct2021.html
- GitHub PoC: fkm75P8YjLkb/CVE-2021-26691
Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2021-26691?
http server: upgrade to newer than 2.4.46; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.
Is CVE-2021-26691 being exploited?
CVE-2021-26691 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2021-26691 affect?
CVE-2021-26691 affects http server, debian linux, fedora. Severity is critical (CVSS 9.8). 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
How urgent is CVE-2021-26691?
Priority 69/100. Critical severity; EPSS top 1%: very likely to be exploited; Remotely exploitable, no login, no user action; On your watchlist
Software pages
Apache HTTP Server vulnerabilities · Debian vulnerabilities
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