What’s Vulnerable › Vulnerabilities

CVE-2021-33193: Request splitting via HTTP/2 method injection and mod_proxy

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 46.2% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-08-16 · updated 2026-06-17

Description

A crafted method sent through HTTP/2 will bypass validation and be forwarded by mod_proxy, which can lead to request splitting or cache poisoning. This issue affects Apache HTTP Server 2.4.17 to 2.4.48.

Affected products

• Apache HTTP Server: from 2.4.17 before 2.4.49 — fixed in 2.4.49
• Debian Linux: 10.0
• Fedoraproject Fedora: 34, 35
• Oracle Corporation Oracle Secure Backup: before 18.1.0.1.0 — fixed in 18.1.0.1.0
• Oracle Corporation Oracle ZFS Storage Appliance Kit: 8.8
• Tenable.sc: through 5.19.1

How to fix

Upgrade to a fixed version: Apache HTTP Server 2.4.49; Oracle Corporation Oracle Secure Backup 18.1.0.1.0. Check the vendor advisory for mitigations.

References

• https://github.com/apache/httpd/commit/ecebcc035ccd8d0e2984fe41420d9e944f456b3c.patch
• https://lists.apache.org/thread.html/re4162adc051c1a0a79e7a24093f3776373e8733abaff57253fef341d%40%3Ccvs.httpd.apache.org%3E
• https://lists.apache.org/thread.html/ree7519d71415ecdd170ff1889cab552d71758d2ba2904a17ded21a70%40%3Ccvs.httpd.apache.org%3E
• https://lists.debian.org/debian-lts-announce/2023/03/msg00002.html
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DSM6UWQICBJ2TU727RENU3HBKEAFLT6T/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/EUVJVRJRBW5QVX4OY3NOHZDQ3B3YOTSG/
• https://portswigger.net/research/http2
• https://security.gentoo.org/glsa/202208-20
• https://security.netapp.com/advisory/ntap-20210917-0004/
• https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-httpd-2.4.49-VWL69sWQ
• https://www.oracle.com/security-alerts/cpuapr2022.html
• https://www.oracle.com/security-alerts/cpujan2022.html

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