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Fencer Vulns
high 7.5

CVE-2021-33193 in the critical feed json

Request splitting via HTTP/2 method injection and mod_proxy

published 2021-08-16 5 y ago modified 2026-09-08 2 mo ago also BIT-apache-2021-33193 , EUVD-2021-19907

cvss
7.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
46.2%
EPSS 46.2% chance of exploitation in 30 days, percentile 98%
public exploits
0
none known

Summary

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.

Patch / fix

  • bitnami apache 2.4.49
  • apache http_server 2.4.49
  • oracle secure_backup 18.1.0.1.0
  • apache2 Debian 14, Debian 13, Debian 12 2.4.48-4

debian advisories DEBIAN-CVE-2021-33193

  • apache2 Ubuntu 18.04 LTS 2.4.29-1ubuntu4.17
  • apache2 Ubuntu 20.04 LTS 2.4.41-4ubuntu3.5

ubuntu advisories UBUNTU-CVE-2021-33193

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • httpd24-httpd RHEL 3 SOFTWARE_COLLECTIONS 2.4.34-23.el7.5
  • httpd24-mod_ssl RHEL 3 SOFTWARE_COLLECTIONS 1:2.4.34-23.el7.5
  • httpd24-mod_ldap RHEL 3 SOFTWARE_COLLECTIONS 2.4.34-23.el7.5
  • httpd24-httpd-tools RHEL 3 SOFTWARE_COLLECTIONS 2.4.34-23.el7.5
  • httpd24-mod_session RHEL 3 SOFTWARE_COLLECTIONS 2.4.34-23.el7.5
  • httpd24-httpd-manual RHEL 3 SOFTWARE_COLLECTIONS 2.4.34-23.el7.5
  • httpd24-mod_proxy_html RHEL 3 SOFTWARE_COLLECTIONS 1:2.4.34-23.el7.5
  • httpd RHEL 8 2.4.37-47.module+el8.6.0+14529+083145da.1

red hat advisories RHSA-2022:6753RHSA-2022:1915RHSA-2022:7143

  • apache2 SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 2.4.33-3.55.1
  • apache2 SLE Module for Server Applications 15 SP3, SLE Module for Server Applications 15 SP2, SLE Module for Package Hub 15 SP3 +2 more 2.4.43-3.25.1
  • apache2 SLES for SAP 12 SP5, SLES 12 SP5, SLE Software Development Kit 12 SP5 2.4.23-29.77.2

suse advisories SUSE-SU-2021:3335-1SUSE-SU-2021:2954-1SUSE-SU-2021:2918-1

  • apache2 openSUSE Leap 15.2 2.4.43-lp152.2.18.1
  • apache2 openSUSE Leap 15.3 2.4.43-3.25.1

opensuse advisories openSUSE-SU-2021:1234-1openSUSE-SU-2021:2954-1

  • httpd AlmaLinux 8 2.4.37-47.module_el8.6.0+2935+fb177b09.2
  • httpd AlmaLinux 8 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
  • mod_md AlmaLinux 8 1:2.0.8-8.module_el8.6.0+2872+fe0ff7aa
  • mod_ssl AlmaLinux 8 1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
  • mod_ssl AlmaLinux 8 1:2.4.37-47.module_el8.6.0+2935+fb177b09.2
  • mod_ldap AlmaLinux 8 2.4.37-47.module_el8.6.0+2935+fb177b09.2
  • mod_ldap AlmaLinux 8 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
  • mod_http2 AlmaLinux 8 1.15.7-5.module_el8.6.0+2872+fe0ff7aa

almalinux advisories ALSA-2022:1915

  • httpd Rocky Linux 8 2.4.37-47.module+el8.6.0+823+f143cee1.1
  • mod_md Rocky Linux 8 1:2.0.8-8.module+el8.5.0+695+1fa8055e
  • mod_md Rocky Linux 8 1:2.0.8-8.module+el8.4.0+553+7a69454b
  • mod_http2 Rocky Linux 8 1.15.7-5.module+el8.6.0+823+f143cee1

rocky linux advisories RLSA-2022:1915

  • apache2 Alpine 3.24, Alpine 3.23, Alpine 3.22 +11 more 2.4.49-r0

alpine advisories ALPINE-CVE-2021-33193

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • httpd CBL-Mariner 2.0 2.4.52-1

azure linux advisories AZL-6483

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

  • apache Arch Linux 2.4.49-1

arch linux advisories AVG-2289

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-33193 updated cvss 7.5

httpd: Request splitting via HTTP/2 method injection and mod_proxy

  • Red Hat Enterprise Linux 6 httpd, mod_ssl, httpd-tools, httpd-manual Red Hat Enterprise Linux 7 httpd, mod_ssl, mod_ldap, httpd-tools, mod_session, httpd-manual, +1 more
    affected: workaround, no fix planned “This flaw can be mitigated by disabling HTTP/2. More information available at: https://httpd.apache.org/docs/2.4/mod/mod_http2.html”
  • Red Hat Enterprise Linux AppStream (v. 8) mod_md 2.0.8-8.module+el8.3.0+6814+67d1e611, httpd 2.4.37-47.module+el8.6.0+14529+083145da.1, mod_http2 1.15.7-5.module+el8.6.0+13996+01710940, mod_ssl 2.4.37-47.module+el8.6.0+14529+083145da.1, mod_ldap 2.4.37-47.module+el8.6.0+14529+083145da.1, httpd-tools 2.4.37-47.module+el8.6.0+14529+083145da.1, +4 more
    fixed: vendor fix “This flaw can be mitigated by disabling HTTP/2. More information available at: https://httpd.apache.org/docs/2.4/mod/mod_http2.html” RHSA-2022:1915
  • Red Hat JBoss Core Services
    fixed: vendor fix “This flaw can be mitigated by disabling HTTP/2. More information available at: https://httpd.apache.org/docs/2.4/mod/mod_http2.html” RHSA-2022:7144
  • Red Hat JBoss Core Services on RHEL 7 Server jbcs-httpd24-httpd 2.4.51-28.el7jbcs, jbcs-httpd24-mod_ssl 2.4.51-28.el7jbcs, jbcs-httpd24-mod_ldap 2.4.51-28.el7jbcs, jbcs-httpd24-httpd-tools 2.4.51-28.el7jbcs, jbcs-httpd24-mod_session 2.4.51-28.el7jbcs, jbcs-httpd24-httpd-manual 2.4.51-28.el7jbcs, +2 more Red Hat JBoss Core Services on RHEL 8 jbcs-httpd24-httpd 2.4.51-28.el8jbcs, jbcs-httpd24-mod_ssl 2.4.51-28.el8jbcs, jbcs-httpd24-mod_ldap 2.4.51-28.el8jbcs, jbcs-httpd24-httpd-tools 2.4.51-28.el8jbcs, jbcs-httpd24-mod_session 2.4.51-28.el8jbcs, jbcs-httpd24-httpd-manual 2.4.51-28.el8jbcs, +2 more
    fixed: vendor fix “This flaw can be mitigated by disabling HTTP/2. More information available at: https://httpd.apache.org/docs/2.4/mod/mod_http2.html” RHSA-2022:7143
  • Red Hat Software Collections for RHEL Workstation(v. 7) httpd24-httpd 2.4.34-23.el7.5, httpd24-mod_ssl 2.4.34-23.el7.5, httpd24-mod_ldap 2.4.34-23.el7.5, httpd24-httpd-tools 2.4.34-23.el7.5, httpd24-mod_session 2.4.34-23.el7.5, httpd24-httpd-manual 2.4.34-23.el7.5, +1 more Red Hat Software Collections for RHEL(v. 7) httpd24-httpd 2.4.34-23.el7.5, httpd24-mod_ssl 2.4.34-23.el7.5, httpd24-mod_ldap 2.4.34-23.el7.5, httpd24-httpd-tools 2.4.34-23.el7.5, httpd24-mod_session 2.4.34-23.el7.5, httpd24-httpd-manual 2.4.34-23.el7.5, +1 more
    fixed: vendor fix “This flaw can be mitigated by disabling HTTP/2. More information available at: https://httpd.apache.org/docs/2.4/mod/mod_http2.html” RHSA-2022:6753
  • Red Hat Enterprise Linux 9 httpd, mod_lua, mod_ssl, mod_ldap, httpd-core, httpd-tools, +4 more Red Hat JBoss Core Services on RHEL 7 Server jbcs-httpd24-apr 1.7.0-6.el7jbcs, jbcs-httpd24-curl 7.83.1-6.el7jbcs, jbcs-httpd24-brotli 1.0.9-2.el7jbcs, jbcs-httpd24-jansson 2.14-1.el7jbcs, jbcs-httpd24-mod_md 2.4.0-15.el7jbcs, jbcs-httpd24-libcurl 7.83.1-6.el7jbcs, +19 more Red Hat JBoss Core Services on RHEL 8 jbcs-httpd24-apr 1.7.0-6.el8jbcs, jbcs-httpd24-curl 7.83.1-6.el8jbcs, jbcs-httpd24-brotli 1.0.9-2.el8jbcs, jbcs-httpd24-jansson 2.14-1.el8jbcs, jbcs-httpd24-mod_md 2.4.0-15.el8jbcs, jbcs-httpd24-libcurl 7.83.1-6.el8jbcs, +19 more Red Hat JBoss Enterprise Application Platform 6 httpd22 httpd22 Red Hat JBoss Enterprise Web Server 2 httpd22 httpd22
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

46.18% chance of exploitation in the next 30 days, higher than 98% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • portswigger.net https://portswigger.net/research/http2 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:P/A:N
OSV 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected packages

ecosystem package affected fixed in
bitnami apache
>=2.4.17, <2.4.49
2.4.49

Affected products

  • Apache Software Foundation Apache HTTP Server

Classification

Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.