# What is CVE-2021-40438?

> CVE-2021-40438 is a critical-severity flaw (CVSS 9.0) in HTTP Server from Apache, published on 16 September 2021. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at over 99%. If you run an affected version, patch it first.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-40438

## At a glance

- **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.
- **Precursor Intelligence risk score:** 89 out of 100 (Critical); action: Emergency patch. Kept up to date as the evidence changes.
- **Severity:** Critical (CVSS 9.0 out of 10)
- **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited
- **Chance of use in the next 30 days:** over 99% (latest exploit-likelihood forecast)
- **Published:** 16 September 2021
- **Record last updated:** 27 October 2025
- **Main product:** HTTP Server from Apache
- **Affected products:** 39

## What is CVE-2021-40438? In plain terms

It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can read data, change data and take it offline.

### The official description

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.

### What kind of flaw is it?

- CWE-918: Server-Side Request Forgery (SSRF)

### How an attack on it works

- Where an attacker can be: Anywhere on the internet
- How hard it is to pull off: Hard: conditions must line up
- Login needed first: No login needed
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can read data, change data and take it offline

Severity vector: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H`

## Is CVE-2021-40438 being exploited? Yes

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at over 99%.

Exploit likelihood over the last 7 days: >99%, >99%, >99%, >99%, >99%, >99%, >99%.

### Timeline

- 16 September 2021: Published
- 27 October 2025: Record last updated

### How Precursor Intelligence ranks it

CVE-2021-40438 scores **89 out of 100 (Critical)**, with the action "Emergency patch".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2021-40438?

The published record lists 39 products. Versions are as published: "before 3.1" means every version up to, but not including, 3.1.

- [HTTP Server](https://www.precursorintelligence.com/product/apache/http_server) by [Apache](https://www.precursorintelligence.com/vendors/apache) (Application): up to 2.4.48
- [Enterprise Linux Server Aus](https://www.precursorintelligence.com/product/redhat/enterprise_linux_server_aus) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.2, 7.3, 7.4, 7.6, 7.7, 8.2, 8.4, 8.6
- [Enterprise Linux Server for Power Little Endian Update Services for Sap Solutions](https://www.precursorintelligence.com/product/redhat/enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.6, 7.7, 8.1, 8.2, 8.4, 8.6, 8.8
- [Enterprise Linux Server Tus](https://www.precursorintelligence.com/product/redhat/enterprise_linux_server_tus) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.6, 7.7, 8.2, 8.4, 8.6, 8.8
- [Enterprise Linux Eus](https://www.precursorintelligence.com/product/redhat/enterprise_linux_eus) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.1, 8.2, 8.4, 8.6, 8.8
- [Enterprise Linux for Power Little Endian Eus](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_power_little_endian_eus) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.1, 8.2, 8.4, 8.6, 8.8
- [Enterprise Linux Update Services for Sap Solutions](https://www.precursorintelligence.com/product/redhat/enterprise_linux_update_services_for_sap_solutions) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.1, 8.2, 8.4, 8.6, 8.8
- [Debian Linux](https://www.precursorintelligence.com/product/debian/debian_linux) by [Debian](https://www.precursorintelligence.com/vendors/debian) (Operating system): 9.0, 10.0, 11.0
- [Enterprise Linux for Ibm Z Systems Eus](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_ibm_z_systems_eus) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.1, 8.4, 8.8
- [Instantis Enterprisetrack](https://www.precursorintelligence.com/product/oracle/instantis_enterprisetrack) by [Oracle](https://www.precursorintelligence.com/vendors/oracle) (Application): 17.1, 17.2, 17.3
- [Enterprise Linux for Arm 64 Eus](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_arm_64_eus) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.6, 8.8
- [Enterprise Linux for Ibm Z Systems](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_ibm_z_systems) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.0_s390x, 8.0
- [Enterprise Linux for Power Little Endian](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_power_little_endian) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.0, 8.0
- [Enterprise Linux Server Update Services for Sap Solutions](https://www.precursorintelligence.com/product/redhat/enterprise_linux_server_update_services_for_sap_solutions) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.6, 7.7
- [F5os](https://www.precursorintelligence.com/product/f5/f5os) by [F5](https://www.precursorintelligence.com/vendors/f5) (Operating system): 1.1.0 to 1.1.4, 1.2.0 to 1.2.1
- [Fedora](https://www.precursorintelligence.com/product/fedoraproject/fedora) by [Fedoraproject](https://www.precursorintelligence.com/vendors/fedoraproject) (Operating system): 34, 35
- [HTTP Server](https://www.precursorintelligence.com/product/oracle/http_server) by [Oracle](https://www.precursorintelligence.com/vendors/oracle) (Application): 12.2.1.3.0, 12.2.1.4.0
- [Sinema Remote Connect Server](https://www.precursorintelligence.com/product/siemens/sinema_remote_connect_server) by [Siemens](https://www.precursorintelligence.com/vendors/siemens) (Application): before 3.1, 3.2
- [Brocade Fabric Operating System Firmware](https://www.precursorintelligence.com/product/broadcom/brocade_fabric_operating_system_firmware) by [Broadcom](https://www.precursorintelligence.com/vendors/broadcom) (Operating system): version not specified
- [Cloud Backup](https://www.precursorintelligence.com/product/netapp/cloud_backup) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [Clustered Data Ontap](https://www.precursorintelligence.com/product/netapp/clustered_data_ontap) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [Enterprise Linux](https://www.precursorintelligence.com/product/redhat/enterprise_linux) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.0
- [Enterprise Linux for Arm 64](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_arm_64) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.0
- [Enterprise Linux for Ibm Z Systems Eus S390x](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_ibm_z_systems_eus_s390x) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 8.2
- [Enterprise Linux for Power Big Endian](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_power_big_endian) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.0
- [Enterprise Linux for Scientific Computing](https://www.precursorintelligence.com/product/redhat/enterprise_linux_for_scientific_computing) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.0
- [Enterprise Linux Server](https://www.precursorintelligence.com/product/redhat/enterprise_linux_server) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.0
- [Enterprise Linux Workstation](https://www.precursorintelligence.com/product/redhat/enterprise_linux_workstation) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Operating system): 7.0
- [Enterprise Manager Ops Center](https://www.precursorintelligence.com/product/oracle/enterprise_manager_ops_center) by [Oracle](https://www.precursorintelligence.com/vendors/oracle) (Application): 12.4.0.0
- [Jboss Core Services](https://www.precursorintelligence.com/product/redhat/jboss_core_services) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Application): 1.0
- [Rocky Linux](https://www.precursorintelligence.com/product/resf/rocky_linux) by [Resf](https://www.precursorintelligence.com/vendors/resf) (Operating system): 8.0
- [Ruggedcom Nms](https://www.precursorintelligence.com/product/siemens/ruggedcom_nms) by [Siemens](https://www.precursorintelligence.com/vendors/siemens) (Application): all versions
- [Secure Global Desktop](https://www.precursorintelligence.com/product/oracle/secure_global_desktop) by [Oracle](https://www.precursorintelligence.com/vendors/oracle) (Application): 5.6
- [Sinec Nms](https://www.precursorintelligence.com/product/siemens/sinec_nms) by [Siemens](https://www.precursorintelligence.com/vendors/siemens) (Application): before 1.0.3
- [Sinema Server](https://www.precursorintelligence.com/product/siemens/sinema_server) by [Siemens](https://www.precursorintelligence.com/vendors/siemens) (Application): 14.0
- [Software Collections](https://www.precursorintelligence.com/product/redhat/software_collections) by [Red Hat](https://www.precursorintelligence.com/vendors/redhat) (Application): 1.0
- [Storagegrid](https://www.precursorintelligence.com/product/netapp/storagegrid) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [Tenable.sc](https://www.precursorintelligence.com/product/tenable/tenable.sc) by [Tenable](https://www.precursorintelligence.com/vendors/tenable) (Application): up to 5.19.1
- [Zfs Storage Appliance Kit](https://www.precursorintelligence.com/product/oracle/zfs_storage_appliance_kit) by [Oracle](https://www.precursorintelligence.com/vendors/oracle) (Application): 8.8

## How do I fix CVE-2021-40438?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisories and patch notes first in the list.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance.

### References

- [oracle.com](https://www.oracle.com/security-alerts/cpuapr2022.html) (Patch, Third-party advisory)
- [oracle.com](https://www.oracle.com/security-alerts/cpujan2022.html) (Patch, Third-party advisory)
- [httpd.apache.org](https://httpd.apache.org/security/vulnerabilities_24.html) (Release notes, Vendor advisory)
- [cert-portal.siemens.com](https://cert-portal.siemens.com/productcert/pdf/ssa-685781.pdf) (Third-party advisory)
- [lists.apache.org](https://lists.apache.org/thread.html/r210807d0bb55f4aa6fbe1512be6bcc4dacd64e84940429fba329967a%40%3Cusers.httpd.apache.org%3E) (Mailing list)
- [lists.apache.org](https://lists.apache.org/thread.html/r2eb200ac1340f69aa22af61ab34780c531d110437910cb9c0ece3b37%40%3Cbugs.httpd.apache.org%3E) (Mailing list)
- [lists.apache.org](https://lists.apache.org/thread.html/r3925e167d5eb1c75def3750c155d753064e1d34a143028bb32910432%40%3Cusers.httpd.apache.org%3E) (Mailing list)
- [lists.apache.org](https://lists.apache.org/thread.html/r61fdbfc26ab170f4e6492ef3bd5197c20b862ce156e9d5a54d4b899c%40%3Cusers.httpd.apache.org%3E) (Mailing list)
- [lists.apache.org](https://lists.apache.org/thread.html/r82838efc5fa6fc4c73986399c9b71573589f78b31846aff5bd9b1697%40%3Cusers.httpd.apache.org%3E) (Mailing list)
- [lists.apache.org](https://lists.apache.org/thread.html/r82c077663f9759c7df5a6656f925b3ee4f55fcd33c889ba7cd687029%40%3Cusers.httpd.apache.org%3E) (Mailing list)
- [lists.apache.org](https://lists.apache.org/thread.html/rf6954e60b1c8e480678ce3d02f61b8a788997785652e9557a3265c00%40%3Cusers.httpd.apache.org%3E) (Mailing list)
- [lists.debian.org](https://lists.debian.org/debian-lts-announce/2021/10/msg00001.html) (Mailing list, Third-party advisory)
- [lists.fedoraproject.org](https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/) (Release notes)
- [lists.fedoraproject.org](https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/) (Release notes)
- [security.gentoo.org](https://security.gentoo.org/glsa/202208-20) (Third-party advisory)
- [security.netapp.com](https://security.netapp.com/advisory/ntap-20211008-0004/) (Third-party advisory)
- [tools.cisco.com](https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-httpd-2.4.49-VWL69sWQ) (Broken link, Third-party advisory)
- [debian.org](https://www.debian.org/security/2021/dsa-4982) (Mailing list, Third-party advisory)
- [tenable.com](https://www.tenable.com/security/tns-2021-17) (Third-party advisory)
- [cisa.gov](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40438) (Government resource)

## Questions

### What is CVE-2021-40438?

CVE-2021-40438 is a critical-severity flaw in HTTP Server from Apache, published on 16 September 2021. It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can read data, change data and take it offline.

### How serious is CVE-2021-40438?

It scores 9.0 out of 10 on the CVSS severity scale, which is critical. Our risk score for CVE-2021-40438 is 89 out of 100, which is Critical. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.

### Is CVE-2021-40438 being exploited?

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at over 99%.

### Which versions does CVE-2021-40438 affect?

For HTTP Server from Apache: up to 2.4.48. The record lists 39 products in all, each with its versions, under "Who is affected" on this page.

### How do I fix CVE-2021-40438?

Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in.

## Does CVE-2021-40438 affect you?

Precursor Intelligence keeps checking for you:

- **Tell us what you run.** Pick your software from a list, or let us work it out from your website.
- **We match it to every flaw.** Over 400,000 published flaws, checked against your list.
- **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it.
- **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools.

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