HTTP request smuggling in Apache HTTP Server

Published: 2021-09-13 | Updated: 2021-09-15
Risk Medium
Patch available YES
Number of vulnerabilities 1
CVE-ID CVE-2021-33193
Exploitation vector Network
Public exploit N/A
Vulnerable software
Apache HTTP Server
Server applications / Web servers

Vendor Apache Foundation

Security Bulletin

This security bulletin contains information about 1 vulnerabilities.

Updated: 15.09.2021

Changed bulletin status to patched, added fixed version.

1) Inconsistent interpretation of HTTP requests

EUVDB-ID: #VU56474

Risk: Medium

CVSSv3.1: 6.3 [CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N/E:U/RL:O/RC:C]

CVE-ID: CVE-2021-33193

CWE-ID: CWE-444 - Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling')

Exploit availability: No


The vulnerability allows a remote attacker to preform HTTP/2 request smuggling attacks.

The vulnerability exists due to improper validation of HTTP/2 requests in mod_proxy in Apache HTTP Server. A remote attacker can send a specially crafted HTTP/2 request to the server and smuggle arbitrary HTTP headers.

Successful exploitation of vulnerability may allow an attacker to poison web server cache and perform phishing attacks.


Install update from vendor's website.

Vulnerable software versions

Apache HTTP Server: 2.4.17 - 2.4.48

CPE2.3 External links

Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

How the attacker can exploit this vulnerability?

The attacker would have to send a specially crafted request to the affected application in order to exploit this vulnerability.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.