CVE-2021-40438 | mod_proxy SSRF

Exp

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.

Published: 2021-09-16 Last update: 2025-10-27 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-40438 is rated Critical Active Threat (96/100): CVSS Critical severity, with high exploitation likelihood (EPSS 94.43%, 100th percentile). CISA KEV confirms active exploitation (added 2021-12-01) affecting Apache / Apache. a weakness (CWE-918) Unauthenticated remote administrative access may be possible. The CISA remediation deadline has passed—treat as an emergency patch priority.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

CISA KEV Record for CVE-2021-40438

Name: Apache HTTP Server-Side Request Forgery (SSRF) · CISA KEV detail

Exploit added: 2021-12-01

Action due: 2021-12-15

Required action: Apply updates per vendor instructions.

Exploit prediction scoring system (EPSS) score for CVE-2021-40438

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-01-24 94.47% 94.43% -0.03%
2 2026-01-21 94.43% 94.47% +0.03%
3 2025-11-21 — 94.43% —

Full EPSS history (34 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-40438

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.0 3.1 CRITICAL
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 2.2 6.0 [email protected]
9.0 3.1 CRITICAL
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 2.2 6.0 134c704f-9b21-4f2e-91b3-4a467353bcc0
6.8 2.0 MEDIUM
AV:N/AC:M/Au:N/C:P/I:P/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:M)
Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
 8.6 6.4 [email protected]

Weakness enumeration for CVE-2021-40438

OS Trackers for CVE-2021-40438

vendor priority summary link
alpine — CVE-2021-40438: 1 source package rows (apache2); 9 state rows across 9 repos (3.11-main, 3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 9, open 0. https://security.alpinelinux.org/vuln/CVE-2021-40438
debian not yet assigned CVE-2021-40438 not yet assigned priority: Debian including 1 source packages (apache2), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-40438
gentoo high CVE-2021-40438: 1 GLSA(s) (202208-20), 2 atom(s) (app-admin/apache-tools, www-servers/apache); latest impact high. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-40438
redhat high — https://access.redhat.com/security/cve/CVE-2021-40438
suse high CVE-2021-40438 severity important: SUSE including 293 source package names (amazon/suse-sles-15-sp1-chost-byos-v20210304-hvm-ssd-x86_64, amazon/suse-sles-15-sp1-chost-byos-v20220127-hvm-ssd-x86_64, …), 503 product×package rows across 61 product lines (HPE Helion OpenStack 8, Image SLES15-SP3-EC2-HVM, … (61 product lines)): Fixed 267, Known Affected 231, Known Not Affected 5. https://www.suse.com/security/cve/CVE-2021-40438/
ubuntu medium CVE-2021-40438 medium priority: Ubuntu including 1 source packages (apache2), 8 status rows across 8 suites (bionic, focal, hirsute, impish, jammy, trusty, upstream, xenial): released 7, not-affected 1. https://ubuntu.com/security/CVE-2021-40438

Affected software / configurations for CVE-2021-40438

Vendor Product Version Raw CPE
resf rocky_linux 8.0 cpe:2.3:o:resf:rocky_linux:8.0:*:*:*:*:*:*:*
redhat enterprise_linux 8.0 cpe:2.3:o:redhat:enterprise_linux:8.0:*:*:*:*:*:*:*
redhat enterprise_linux_eus 8.1 cpe:2.3:o:redhat:enterprise_linux_eus:8.1:*:*:*:*:*:*:*
redhat enterprise_linux_eus 8.2 cpe:2.3:o:redhat:enterprise_linux_eus:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_eus 8.4 cpe:2.3:o:redhat:enterprise_linux_eus:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_eus 8.6 cpe:2.3:o:redhat:enterprise_linux_eus:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_eus 8.8 cpe:2.3:o:redhat:enterprise_linux_eus:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_for_arm_64 8.0 cpe:2.3:o:redhat:enterprise_linux_for_arm_64:8.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_arm_64_eus 8.6 cpe:2.3:o:redhat:enterprise_linux_for_arm_64_eus:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_for_arm_64_eus 8.8 cpe:2.3:o:redhat:enterprise_linux_for_arm_64_eus:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems 7.0_s390x cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems:7.0_s390x:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems 8.0 cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems:8.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems_eus 8.1 cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems_eus:8.1:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems_eus 8.4 cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems_eus:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems_eus 8.8 cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems_eus:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems_eus_s390x 8.2 cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems_eus_s390x:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_big_endian 7.0 cpe:2.3:o:redhat:enterprise_linux_for_power_big_endian:7.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian 7.0 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian:7.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian 8.0 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian:8.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian_eus 8.1 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian_eus:8.1:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian_eus 8.2 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian_eus:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian_eus 8.4 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian_eus:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian_eus 8.6 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian_eus:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian_eus 8.8 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian_eus:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_for_scientific_computing 7.0 cpe:2.3:o:redhat:enterprise_linux_for_scientific_computing:7.0:*:*:*:*:*:*:*
redhat enterprise_linux_server 7.0 cpe:2.3:o:redhat:enterprise_linux_server:7.0:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 7.2 cpe:2.3:o:redhat:enterprise_linux_server_aus:7.2:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 7.3 cpe:2.3:o:redhat:enterprise_linux_server_aus:7.3:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 7.4 cpe:2.3:o:redhat:enterprise_linux_server_aus:7.4:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 7.6 cpe:2.3:o:redhat:enterprise_linux_server_aus:7.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 7.7 cpe:2.3:o:redhat:enterprise_linux_server_aus:7.7:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 8.2 cpe:2.3:o:redhat:enterprise_linux_server_aus:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 8.4 cpe:2.3:o:redhat:enterprise_linux_server_aus:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_server_aus 8.6 cpe:2.3:o:redhat:enterprise_linux_server_aus:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 7.6 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:7.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 7.7 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:7.7:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 8.1 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:8.1:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 8.2 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 8.4 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 8.6 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions 8.8 cpe:2.3:o:redhat:enterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_server_tus 7.6 cpe:2.3:o:redhat:enterprise_linux_server_tus:7.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_tus 7.7 cpe:2.3:o:redhat:enterprise_linux_server_tus:7.7:*:*:*:*:*:*:*
redhat enterprise_linux_server_tus 8.2 cpe:2.3:o:redhat:enterprise_linux_server_tus:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_server_tus 8.4 cpe:2.3:o:redhat:enterprise_linux_server_tus:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_server_tus 8.6 cpe:2.3:o:redhat:enterprise_linux_server_tus:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_tus 8.8 cpe:2.3:o:redhat:enterprise_linux_server_tus:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_server_update_services_for_sap_solutions 7.6 cpe:2.3:o:redhat:enterprise_linux_server_update_services_for_sap_solutions:7.6:*:*:*:*:*:*:*
redhat enterprise_linux_server_update_services_for_sap_solutions 7.7 cpe:2.3:o:redhat:enterprise_linux_server_update_services_for_sap_solutions:7.7:*:*:*:*:*:*:*
redhat enterprise_linux_update_services_for_sap_solutions 8.1 cpe:2.3:o:redhat:enterprise_linux_update_services_for_sap_solutions:8.1:*:*:*:*:*:*:*
redhat enterprise_linux_update_services_for_sap_solutions 8.2 cpe:2.3:o:redhat:enterprise_linux_update_services_for_sap_solutions:8.2:*:*:*:*:*:*:*
redhat enterprise_linux_update_services_for_sap_solutions 8.4 cpe:2.3:o:redhat:enterprise_linux_update_services_for_sap_solutions:8.4:*:*:*:*:*:*:*
redhat enterprise_linux_update_services_for_sap_solutions 8.6 cpe:2.3:o:redhat:enterprise_linux_update_services_for_sap_solutions:8.6:*:*:*:*:*:*:*
redhat enterprise_linux_update_services_for_sap_solutions 8.8 cpe:2.3:o:redhat:enterprise_linux_update_services_for_sap_solutions:8.8:*:*:*:*:*:*:*
redhat enterprise_linux_workstation 7.0 cpe:2.3:o:redhat:enterprise_linux_workstation:7.0:*:*:*:*:*:*:*
redhat jboss_core_services 1.0 cpe:2.3:a:redhat:jboss_core_services:1.0:*:*:*:*:*:*:*
redhat software_collections 1.0 cpe:2.3:a:redhat:software_collections:1.0:*:*:*:*:*:*:*
apache http_server <= 2.4.48 cpe:2.3:a:apache:http_server:*:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
fedoraproject fedora 35 cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
debian debian_linux 11.0 cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:*
netapp cloud_backup — cpe:2.3:a:netapp:cloud_backup:-:*:*:*:*:*:*:*
netapp clustered_data_ontap — cpe:2.3:a:netapp:clustered_data_ontap:-:*:*:*:*:*:*:*
netapp storagegrid — cpe:2.3:a:netapp:storagegrid:-:*:*:*:*:*:*:*
broadcom brocade_fabric_operating_system_firmware — cpe:2.3:o:broadcom:brocade_fabric_operating_system_firmware:-:*:*:*:*:*:*:*
f5 f5os >= 1.1.0, <= 1.1.4 cpe:2.3:o:f5:f5os:*:*:*:*:*:*:*:*
f5 f5os >= 1.2.0, <= 1.2.1 cpe:2.3:o:f5:f5os:*:*:*:*:*:*:*:*
oracle enterprise_manager_ops_center 12.4.0.0 cpe:2.3:a:oracle:enterprise_manager_ops_center:12.4.0.0:*:*:*:*:*:*:*
oracle http_server 12.2.1.3.0 cpe:2.3:a:oracle:http_server:12.2.1.3.0:*:*:*:*:*:*:*
oracle http_server 12.2.1.4.0 cpe:2.3:a:oracle:http_server:12.2.1.4.0:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.1 cpe:2.3:a:oracle:instantis_enterprisetrack:17.1:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.2 cpe:2.3:a:oracle:instantis_enterprisetrack:17.2:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.3 cpe:2.3:a:oracle:instantis_enterprisetrack:17.3:*:*:*:*:*:*:*
oracle secure_global_desktop 5.6 cpe:2.3:a:oracle:secure_global_desktop:5.6:*:*:*:*:*:*:*
oracle zfs_storage_appliance_kit 8.8 cpe:2.3:a:oracle:zfs_storage_appliance_kit:8.8:*:*:*:*:*:*:*
siemens ruggedcom_nms — cpe:2.3:a:siemens:ruggedcom_nms:*:*:*:*:*:*:*:*
siemens sinec_nms < 1.0.3 cpe:2.3:a:siemens:sinec_nms:*:*:*:*:*:*:*:*
siemens sinema_remote_connect_server < 3.1 cpe:2.3:a:siemens:sinema_remote_connect_server:*:*:*:*:*:*:*:*

References for CVE-2021-40438

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