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    CVE-2021-33193

    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.

    Published:Aug 5, 2021
    Last Modified:May 1, 2025
    EPS:Aug 16, 2021
    EPSS Score:0.01298
    CVSS Score:7.5

    Affected Products

    Vendor
    Apache
    Product
    Http Server
    Vendor
    Debian
    Product
    Debian Linux
    Vendor
    Fedoraproject
    Product
    Fedora
    Vendor
    Oracle
    Product
    Secure Backup
    Vendor
    Oracle
    Product
    Zfs Storage Appliance Kit
    Vendor
    Redhat
    Product
    Enterprise Linux
    Vendor
    Redhat
    Product
    Jboss Core Services
    Vendor
    Redhat
    Product
    Rhel Software Collections
    Vendor
    Tenable
    Product
    Tenable.sc

    Exploits

    https://portswigger.net/research/http2https://portswigger.net/research/http2

    Common Weakness Enumeration

    CWE-476 NULL Pointer Dereference

    Common Attack Pattern Enumeration and Classification (CAPEC)

    No CAPEC recorded yet

    Related CVEs

    CVE-2026-48913 (CVSS 7.3) — Use After Free vulnerability in Apache HTTP Server module mod_http2 when file handles are already exhausted. …
    CVE-2026-42536 (CVSS 7.5) — Heap-based Buffer Overflow vulnerability in Apache HTTP Server with mod_xml2enc, xml2StartParse, and untrusted…
    CVE-2026-44185 (CVSS 7.3) — Buffer Over-read vulnerability in Apache HTTP Server via outbound OCSP requests to an attacker controlled OCSP…
    CVE-2026-34355 (CVSS 7.5) — A buffer overflow in mod_proxy_html in Apache HTTP Server 2.4.67 and earlier allows an attack by an untrusted …
    CVE-2026-44631 (CVSS 9.8) — Buffer Underwrite vulnerability in Apache HTTP Server on crafted regular expressions in the configuration. Th…
    CVE-2026-44119 (CVSS 5.5) — Improper Privilege Management vulnerability in Apache HTTP Server 2.4.67 and earlier allows local .htaccess au…
    CVE-2026-43951 (CVSS 6.5) — Out-of-bounds Read vulnerability in Apache HTTP Server with mod_headers and mod_mime and multiple response lan…
    CVE-2026-42535 (CVSS 9.1) — A path handling issue in mod_dav_fs in Apache 2.4.67 and earlier allows a WebDAV content author to directly ma…
    CVE-2026-34356 (CVSS 7.5) — Heap-based Buffer Overflow vulnerability in Apache HTTP Server with malicious backend servers and ProxyPassRev…
    CVE-2026-44186 (CVSS 7.3) — Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in the mod_proxy_ftp module in Apache HTT…

    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
    https://www.tenable.com/security/tns-2021-17
    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
    https://www.tenable.com/security/tns-2021-17

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High