Http-Desync-Guardian - Analyze HTTP Requests To Minimize Risks Of HTTP Desync Attacks (Precursor For HTTP Request Smuggling/Splitting)


HTTP/1.1 went through a long evolution since 1991 to 2014:

This means there is a variety of servers and clients, which might have different views on request boundaries, creating opportunities for desynchronization attacks (a.k.a. HTTP Desync).

It might seem simple to follow the latest RFC recommendations. However, for large scale systems that have been there for a while, it may come with unacceptable availability impact.

http_desync_guardian library is designed to analyze HTTP requests to prevent HTTP Desync attacks, balancing security and availability. It classifies requests into different categories and provides recommendations on how each tier should be handled.

It can be used either for raw HTTP request headers or already parsed by an HTTP engine. Consumers may configure logging and metrics collection. Logging is rate limited and all user data is obfuscated.

If you think you might have found a security impacting issue, please follow our Security Notification Process.


  • Uniformity across services is key. This means request classification, logging, and metrics must happen under the hood and with minimally available settings (e.g., such as log file destination).
  • Focus on reviewability. The test suite must require no knowledge about the library/programming languages but only about HTTP protocol. So it's easy to review, contribute, and re-use.
  • Security is efficient when it's easy for users. Our goal is to make integration of the library as simple as possible.
  • Ultralight. The overhead must be minimal and impose no tangible tax on request handling (see benchmarks).

Supported HTTP versions

The main focus of this library is HTTP/1.1. See tests for all covered cases. Predecessors of HTTP/1.1 don't support connection re-use which limits opportunities for HTTP Desync, however some proxies may upgrade such requests to HTTP/1.1 and re-use backend connections, which may allow to craft malicious HTTP/1.0 requests. That's why they are analyzed using the same criteria as HTTP/1.1. For other protocol versions have the following exceptions:

  • HTTP/0.9 requests are never considered Compliant, but are classified as Acceptable. If any of Content-Length/Transfer-Encoding is present then it's Ambiguous.
  • HTTP/1.0 - the presence of Transfer-Encoding makes a request Ambiguous.
  • HTTP/2+ is out of scope. But if your proxy downgrades HTTP/2 to HTTP/1.1, make sure the outgoing request is analyzed.

See documentation to learn more.

Usage from C

This library is designed to be primarily used from HTTP engines written in C/C++.

  1. Install cbindgen: cargo install --force cbindgen
  2. Generate the header file:
    • Run cbindgen --output http_desync_guardian.h --lang c for C.
    • Run cbindgen --output http_desync_guardian.h --lang c++ for C++.
  3. Run cargo build --release. The binaries are in ./target/release/libhttp_desync_guardian.* files.

Learn more: generic and Nginx examples.

#include "http_desync_guardian.h"

* http_engine_request_t - already parsed by the HTTP engine
static int check_request(http_engine_request_t *req) {
http_desync_guardian_request_t guardian_request = construct_http_desync_guardian_from(req);
http_desync_guardian_verdict_t verdict = {0};

http_desync_guardian_analyze_request(&guardian_request, &verdict);

switch (verdict.tier) {
// The request is good. green light
// Reject, if mode == STRICTEST
// Otherwise, OK
// The request is ambiguous.
// Reject, if mode == STRICTEST
// Otherwise send it, but don't reuse both FE/BE connections.
// Send 400 and close the FE connection.
// unreachable code

Usage from Rust

See benchmarks as an example of usage from Rust.

Security issue notifications

If you discover a potential security issue in http_desync_guardian we ask that you notify AWS Security via our vulnerability reporting page. Please do not create a public github issue.


See CONTRIBUTING for more information.

Http-Desync-Guardian - Analyze HTTP Requests To Minimize Risks Of HTTP Desync Attacks (Precursor For HTTP Request Smuggling/Splitting) Http-Desync-Guardian - Analyze HTTP Requests To Minimize Risks Of HTTP Desync Attacks (Precursor For HTTP Request Smuggling/Splitting) Reviewed by Zion3R on 5:30 PM Rating: 5