CVE-2021-40438: Apache HTTP Server-Side Request Forgery (SSRF)

Q: How do I fix CVE-2021-40438?

rocky linux: mitigate or apply the listed mitigation; enterprise linux: mitigate or apply the listed mitigation; enterprise linux eus: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-40438 being exploited?

Yes. CVE-2021-40438 is on the CISA Known Exploited Vulnerabilities list, so treat it as patch-now.

Q: What does CVE-2021-40438 affect?

CVE-2021-40438 affects rocky linux, enterprise linux, enterprise linux eus. Severity is critical (CVSS 9.0). A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.

Q: How urgent is CVE-2021-40438?

Priority 100/100. Critical severity; EPSS top 1%: very likely to be exploited; Exploited in the wild (CISA KEV); Used in ransomware campaigns; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action; On your watchlist

100

Patch now

CRITICAL 9.0EXPLOITEDREMOTEWATCHLISTMITIGATEMITIGATEMITIGATE

CVE-2021-40438 is a critical-severity flaw (CVSS 9.0) affecting rocky linux, enterprise linux, enterprise linux eus, reported as exploited in the wild. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.

Why this priority

Critical severity
EPSS top 1%: very likely to be exploited
Exploited in the wild (CISA KEV)
Used in ransomware campaigns
Public exploit reference
CISA SSVC: automatable with total technical impact
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.0 (Critical)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	100.0% (top 1%)
CISA exploited list	Yes, fix due 2021-12-15, used in ransomware
Weakness	CWE-918
Published	2021-09-16
Last modified	2026-08-06

Publisher (CNA): Apache Software Foundation · Vendor: redhat · resf

Last updated 2026-08-06 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-09-16 15:15 UTCPublished CVE record published
2021-12-01Exploited Added to the CISA Known Exploited Vulnerabilities list
2021-12-15Deadline CISA due date for US federal agencies to patch
2026-08-06 05:16 UTCUpdated Record last modified

resf rocky linux MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

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.

redhat enterprise linux MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

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.

redhat enterprise linux eus MITIGATE

Affected: 8.1 · 8.2 · 8.4 · 8.6 · 8.8

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.1

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.

redhat enterprise linux for arm 64 MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

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.

redhat enterprise linux for arm 64 eus MITIGATE

Affected: 8.6 · 8.8

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.6

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.

redhat enterprise linux for ibm z systems MITIGATE

Affected: 7.0_s390x · 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 7.0_s390x

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.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · bitcoin-core-selinux · FIXED · 0.3-2.fc46 · FEDORA-2026-51fc806272 · source

sudo dnf upgrade --advisory=FEDORA-2026-51fc806272

Fedora ELN · guestfs-tools · FIXED · 1.57.5-1.eln159 · FEDORA-2026-9577c5ec16 · source

sudo dnf upgrade --advisory=FEDORA-2026-9577c5ec16

Ubuntu 20.04 · apache2 · FIXED · 2.4.41-4ubuntu3.6

sudo apt update && sudo apt install --only-upgrade apache2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.41-4ubuntu3.6" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 22.04 · apache2 · FIXED · 2.4.48-3.1ubuntu2

sudo apt update && sudo apt install --only-upgrade apache2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.48-3.1ubuntu2" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · apache2 · FIXED · 2.4.49-1

sudo apt update && sudo apt install --only-upgrade apache2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.49-1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · apache2 · FIXED · 2.4.49-1

sudo apt update && sudo apt install --only-upgrade apache2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' apache2)" ge "2.4.49-1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 7 · httpd · FIXED · httpd-2.4.6-97.el7_9.1

sudo dnf upgrade --advisory=RHSA-2021:3856 rpm -q httpd

RHEL 7.2 Advanced Update Support · httpd · FIXED · httpd-2.4.6-40.el7_2.7

sudo dnf upgrade --advisory=RHSA-2021:3856 rpm -q httpd

RHEL 7.3 Advanced Update Support · httpd · FIXED · httpd-2.4.6-45.el7_3.6

sudo dnf upgrade --advisory=RHSA-2021:3856 rpm -q httpd

RHEL 7.4 Advanced Update Support · httpd · FIXED · httpd-2.4.6-67.el7_4.7

sudo dnf upgrade --advisory=RHSA-2021:3856 rpm -q httpd

RHEL 7.6 Advanced Update Support · httpd · FIXED · httpd-2.4.6-89.el7_6.2

sudo dnf upgrade --advisory=RHSA-2021:3856 rpm -q httpd

Alpine 3.20 · apache2 · FIXED · apache2-2.4.49-r0

apk add --upgrade apache2 apk info -v apache2

Alpine 3.21 · apache2 · FIXED · apache2-2.4.49-r0

apk add --upgrade apache2 apk info -v apache2

Alpine 3.22 · apache2 · FIXED · apache2-2.4.49-r0

apk add --upgrade apache2 apk info -v apache2

Alpine 3.23 · apache2 · FIXED · apache2-2.4.49-r0

apk add --upgrade apache2 apk info -v apache2

Amazon Linux 2 · see advisory · FIXED · ALAS2-2021-1716 · source

sudo yum update --advisory=ALAS2-2021-1716

Amazon Linux 1 · see advisory · FIXED · ALAS-2021-1543 · source

sudo yum update --advisory=ALAS-2021-1543

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: Kashkovsky/CVE-2021-40438 (18 stars)
GitHub PoC: sixpacksecurity/CVE-2021-40438 (13 stars)
GitHub PoC: sergiovks/CVE-2021-40438-Apache-2.4.48-SSRF-exploit (11 stars)
Write-up: www.oracle.com/security-alerts/cpuapr2022.html
Write-up: www.oracle.com/security-alerts/cpujan2022.html
GitHub PoC: BabyTeam1024/CVE-2021-40438
GitHub PoC: Cappricio-Securities/CVE-2021-40438
Nuclei scan template: Apache <= 2.4.48 Mod_Proxy - Server-Side Request Forgery

Verify at the official source NVD · CVE Program · CISA KEV catalog · 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.

Quick answers

How do I fix CVE-2021-40438?

rocky linux: mitigate or apply the listed mitigation; enterprise linux: mitigate or apply the listed mitigation; enterprise linux eus: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-40438 being exploited?

Yes. CVE-2021-40438 is on the CISA Known Exploited Vulnerabilities list, so treat it as patch-now.

What does CVE-2021-40438 affect?

CVE-2021-40438 affects rocky linux, enterprise linux, enterprise linux eus. Severity is critical (CVSS 9.0). A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.

How urgent is CVE-2021-40438?

Priority 100/100. Critical severity; EPSS top 1%: very likely to be exploited; Exploited in the wild (CISA KEV); Used in ransomware campaigns; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Apache HTTP Server vulnerabilities

References

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