Overview
HTTP web proxies and web accelerators that support HTTP/2 for an HTTP/1.1 backend webserver are vulnerable to HTTP Request Smuggling.
Description
The affected systems allow invalid characters such as carriage return and newline characters in HTTP/2 headers. When an attacker passes these invalid contents to a vulnerable system, the forwarded HTTP/1 request includes the unintended malicious data. This is commonly known as HTTP Request Splitting. In the case of HTTP web proxies, this vulnerability can lead to HTTP Request smuggling, which enables an attacker to access protected internal sites.
Impact
An attacker can send a crafted HTTP/2 request with malicious content to bypass network security measures, thereby reaching internal protected servers and accessing sensitive data.
Solution
Apply updates
Install vendor-provided patches and updates to ensure malicious HTTP/2 content is blocked or rejected as described in RFC 7540 (Section 8.1.2.6) and RFC 7540 (Section 10.3). Both "request" and "response" should be inspected by the web proxy and rejected in accordance with Stream Error Handling as described in RFC 7450 (Section 5.4.2).
Inspect and block anomalous HTTP/2 traffic
If HTTP/2 is not supported, block the protocol on the web proxies to avoid abuse of HTTP/2 protocol. Where HTTP/2 is supported, enforce strict rules for HTTP header checks to ensure malicious headers are normalized or rejected.
Checks of this type include:
* HTTP Headers with invalid Header name or value
* HTTP Headers with invalid or no content-length
* Unsupported or invalid HTTP methods
Test and verify your web proxy
Scan your public web server proxy with OWASP recommended tests to ensure your web servers are not vulnerable to abuse via HTTP response splitting.
Acknowledgements
Thanks to the reporter James Kettle of PortSwigger for the information about this vulnerability.
This document was written by Timur Snoke.
References
- https://portswigger.net/research/request-smuggling
- https://datatracker.ietf.org/doc/html/rfc7540
- https://portswigger.net/research/http2
- https://owasp.org/www-project-web-security-testing-guide/latest/4-Web_Application_Security_Testing/07-Input_Validation_Testing/15-Testing_for_HTTP_Splitting_Smuggling
Other Information
| Date Public: | 2021-08-06 |
| Date First Published: | 2021-08-06 |
| Date Last Updated: | 2021-08-12 11:44 UTC |
| Document Revision: | 3 |