Request splitting via unvalidated HTTP/2 method
Published Aug 16, 2021 · Updated Aug 3, 2024
HTTP request splitting in Apache HTTP Server 2.4.17 through 2.4.48 allows remote attackers to poison caches via crafted HTTP/2 methods. The HTTP/2 request path constructs a request line from the unvalidated :method value and forwards it through mod_proxy without the HTTP/1 parser's method checks. Exploitation requires HTTP/2 and proxying to a backend that accepts the malformed request line, enabling access-rule bypass or cache poisoning.
Summary
What happened
HTTP request splitting in Apache HTTP Server 2.4.17 through 2.4.48 allows remote attackers to poison caches via crafted HTTP/2 methods. The HTTP/2 request path constructs a request line from the unvalidated :method value and forwards it through mod_proxy without the HTTP/1 parser's method checks. Exploitation requires HTTP/2 and proxying to a backend that accepts the malformed request line, enabling access-rule bypass or cache poisoning.
The record
- CVE
- CVE-2021-33193
- Published
- Aug 16, 2021
- Updated
- Aug 3, 2024
- Vendor
- The Apache Software Foundation
- Product
- Apache HTTP Server
- Classifications
- T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 16, 2021CVE publishedPublication date reported by the CVE source.
- Aug 3, 2024Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=Apache HTTP Server 2.4 2.4.17 to 2.4.48
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Apache HTTP/2 request-line injection demonstrationproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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