Security readout for executives and security teams
Plain-English summary
Apache HTTP Server can mishandle a specially crafted HTTP/2 request method when proxying traffic. In affected versions, this may let one request be interpreted as more than one request downstream, creating risk of cache poisoning or request confusion.
Executive priority
Treat this as high priority for internet-facing Apache reverse proxies, especially where shared caches or sensitive backend applications are involved. Lack of confirmed exploitation lowers urgency slightly, but the affected component is widely deployed.
Technical view
CVE-2021-33193 affects Apache HTTP Server 2.4.17 through 2.4.48. The issue involves HTTP/2 method handling and mod_proxy validation, where a crafted method can be forwarded and may cause request splitting or cache poisoning.
Likely exposure
Exposure is most likely on Apache HTTP Server deployments in the affected version range that accept HTTP/2 traffic and use mod_proxy to forward requests to backends or caches.
Exploitation context
The source bundle does not show CISA KEV listing or confirmed active exploitation. Public research discusses HTTP/2 request smuggling classes, and Apache plus downstream vendors published advisories or updates.
Researcher notes
The key condition is the combination of HTTP/2 request handling and mod_proxy forwarding. Evidence supports request splitting and cache poisoning impact, but the bundle does not provide CVSS, CWE, exploit prevalence, or detailed product-specific fix versions.
Mitigation direction
- Upgrade Apache HTTP Server through vendor-supported packages outside the affected range.
- Apply relevant distribution or appliance updates from Fedora, Cisco, Oracle, NetApp, or other vendors.
- Review Apache guidance before changing HTTP/2 or proxy behavior as a temporary control.
- Prioritize reverse proxies and systems fronting shared caches or critical applications.
Validation and detection
- Inventory Apache HTTP Server versions and flag 2.4.17 through 2.4.48.
- Identify hosts with HTTP/2 enabled and mod_proxy in use.
- Check package manager or vendor advisory status for fixed builds.
- Review proxy and cache logs for unexplained request parsing or cache anomalies.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CVE-2021-33193 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Unknown
- CVSS
- Not scored
- Known Exploited
- No
- Published
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS and timeline data
No CVSS vectors or timeline events were available in the normalized CVE source material.
Source materials
- CVE List V5 sourceCVE List V5
- https://portswigger.net/research/http2CVE reference
- https://github.com/apache/httpd/commit/ecebcc035ccd8d0e2984fe41420d9e944f456b3c.patchCVE reference
- [httpd-cvs] 20210916 [httpd-site] branch main updated: Revert "Add descriptions for CVE-2021-33193 CVE-2021-36160"CVE reference · mailing-list
- [httpd-cvs] 20210916 [httpd-site] branch main updated: Add descriptions for CVE-2021-33193 CVE-2021-36160CVE reference · mailing-list
- FEDORA-2021-5d2d4b6ac5CVE reference · vendor-advisory
- FEDORA-2021-f94985afcaCVE reference · vendor-advisory
- 20211124 Multiple Vulnerabilities in Apache HTTP Server Affecting Cisco Products: November 2021CVE reference · vendor-advisory
- https://www.oracle.com/security-alerts/cpujan2022.htmlCVE reference
- https://security.netapp.com/advisory/ntap-20210917-0004/CVE reference
- https://www.tenable.com/security/tns-2021-17CVE reference
- https://www.oracle.com/security-alerts/cpuapr2022.htmlCVE reference
- GLSA-202208-20CVE reference · vendor-advisory
- [debian-lts-announce] 20230303 [SECURITY] [DLA 3351-1] apache2 security updateCVE reference · mailing-list
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.