Http Request Smuggling in Chi

HTTP Request Smuggling is a critical HTTP/1.1 protocol vulnerability that occurs when multiple HTTP requests are parsed differently by front-end and back-end servers. In Chi, this manifests through several specific attack vectors related to how Chi handles HTTP request parsing and forwarding.

The primary attack surface in Chi involves the chi.Router middleware chain. When Chi receives HTTP requests, it processes headers and body content before passing them to backend handlers. The vulnerability emerges when Chi's parsing logic differs from downstream servers like nginx, Apache, or other reverse proxies.

Consider this Chi-specific scenario: A request with a malformed Content-Length header can cause Chi to parse the body differently than a backend server. For example:

POST /api/resource HTTP/1.1
Host: example.com
Content-Length: 6
Content-Type: application/x-www-form-urlencoded

data=123456
GET /api/secret HTTP/1.1
Host: example.com
Foo: Bar
Content-Length: 0

In this case, Chi might interpret the first Content-Length: 6 as valid and parse only data=123, leaving 456 as the start of a new request. However, a backend server might interpret the same content differently, potentially treating 456 as the beginning of a new request that gets processed.

Chi's middleware functions create another attack surface. When multiple middleware components process requests, inconsistent handling of Transfer-Encoding headers can lead to smuggling. A malicious request might look like:

POST /upload HTTP/1.1
Host: example.com
Transfer-Encoding: chunked
Content-Type: application/json

3
abc
0

GET /admin HTTP/1.1
Host: example.com
Content-Length: 0

Here, Chi processes the chunked encoding but a backend might mishandle the chunked terminator, allowing the second request to be smuggled through.

The chi.URLParam and chi.RouteContext functions can also be exploited. When parameters are extracted from URLs, smuggling attacks can manipulate how these values are parsed and forwarded to backend services.

Detecting HTTP Request Smuggling in Chi applications requires both manual testing and automated scanning. middleBrick's black-box scanning approach is particularly effective for this vulnerability since it tests the actual HTTP surface without requiring access to source code.

middleBrick specifically tests for Chi-related smuggling patterns by sending requests with manipulated Content-Length and Transfer-Encoding headers. The scanner identifies discrepancies between how Chi parses requests versus how backend servers interpret them.

For manual detection in Chi applications, developers should test with requests that have:

• Mismatched Content-Length headers (e.g., Content-Length: 5 with 6 bytes of body data)
• Conflicting Content-Length and Transfer-Encoding headers
• Malformed chunked encoding with incorrect chunk sizes
• Multiple Content-Length headers

middleBrick's scanning includes these specific tests and provides detailed findings with severity levels. The scanner reports whether the application is vulnerable to request smuggling and provides evidence of successful exploitation attempts.

Additionally, middleBrick analyzes the Chi application's HTTP behavior by examining response patterns. If a smuggling attack causes different responses than expected, this indicates a vulnerability. The scanner also checks for Connection: close header inconsistencies that might reveal smuggling attempts.

For CI/CD integration, middleBrick's GitHub Action can be configured to fail builds if smuggling vulnerabilities are detected, ensuring that these issues are caught before deployment.

Remediating HTTP Request Smuggling in Chi applications requires a multi-layered approach. The primary defense is ensuring consistent HTTP parsing across all components in the request chain.

First, configure Chi to normalize request headers. Use the chi.CanonicalHeaderKey function to ensure header names are processed consistently:

router.Use(func(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Normalize headers to prevent smuggling
        r.Header = http.Header{
            http.CanonicalHeaderKey(