Request Smuggling in Chi with Api Keys

Request smuggling occurs when an application processes HTTP requests differently in transit versus after they reach the backend, enabling an attacker to smuggle a request across security boundaries. When you use Api Keys for authorization in the Chi framework, the presence of custom headers can interact with header parsing inconsistencies between front-end proxies and the Chi application.

In Chi, Api Keys are commonly passed via headers such as X-API-Key. If a front-end proxy (or load balancer) and the Chi app parse headers differently—especially when one strips or reorders headers related to content-length or transfer-encoding—a request that appears authorized at the edge may be interpreted differently by the application router. This mismatch can allow a request intended for one user or endpoint to be associated with another, effectively bypassing key-based access controls.

Chi routes are typically defined with pattern matching and middleware. If middleware that inspects Api Keys is placed after body parsing or if the application defers authorization until after routing, an attacker may craft a request that appears valid at the proxy but is interpreted as belonging to a different route or user inside Chi. For example, an attacker might send a request with a valid Api Key but with an ambiguous Transfer-Encoding and Content-Length combination, causing the proxy to route one request while Chi interprets it as two separate requests, one of which may lack proper authorization checks.

This vulnerability is not inherent to Chi or to Api Keys themselves, but arises from the interaction between how keys are transmitted and how the infrastructure handles message framing. Because middleBrick scans the unauthenticated attack surface, it can detect signs of request smuggling—such as inconsistent responses to specially crafted requests—without requiring credentials, and surface the issue alongside relevant findings from the BFLA/Privilege Escalation and Authentication checks.

When you scan a Chi-based API with middleBrick, the tool runs parallel checks including input validation and rate limiting, which can highlight anomalies in how requests are processed. Findings may include indications that key-bearing requests are not consistently validated before routing, or that header parsing leaves room for request splitting. These results complement the broader OWASP API Top 10 mapping and compliance framework references provided in the report, helping you understand the risk context for your Chi service.

To reduce request smuggling risk when using Api Keys in Chi, ensure consistent header handling, strict parsing order, and early authorization checks. The following examples show how to implement these practices with syntactically correct Chi code.

1. Standardize header parsing and enforce Api Key presence early

Place middleware at the top of the pipeline to extract and validate the Api Key before any routing or body parsing occurs. This reduces the window for mismatched interpretations between proxy and application.

open System
open Microsoft.AspNetCore.Http
open FSharp.Control.Tasks

let apiKeyValidator (key: string) (next: HttpFunc -> HttpContext -> HttpFuncResult) : HttpFunc -> HttpContext -> HttpFuncResult =
    fun ctx next ->
        let headers = ctx.Request.Headers
        match headers.TryGetValue("X-API-Key") with
        | true, apiKey when apiKey = key -> next ctx
        | _ -> Results.status 401 "Unauthorized" >= ctx

let app = 
    chi {
        plug apiKeyValidator "your-secret-key"
        get "public" (fun _ _ -> task { return Results.text "public" })
        get "secure" (fun _ _ -> task { return Results.text "secure" })
    }

2. Avoid header manipulation after routing decisions

Do not modify or re-parse headers after routing has begun. Ensure that any header-based authorization logic is applied consistently and that content-length and transfer-encoding headers are not altered by middleware in a way that could create ambiguity.

// Good: inspect Api Key before any body or header mutation
let secureEndpoint (ctx: HttpContext) = 
    task {
        let ok = ctx.Request.Headers.TryGetValue("X-API-Key", &_) // safe read-only access
        if not ok then return Results.status 401 "Missing key"
        else return Results.json [|"ok", true|]
    }

// Route definition with early guard
let app2 = 
    chi {
        pipe_through (apiKeyValidator "your-secret-key") 
        post "submit" secureEndpoint
    }

3. Align proxy and application headers

Configure your front-end proxy to normalize headers—preserve casing, avoid dropping or reordering Content-Length and Transfer-Encoding, and ensure that the Api Key header is forwarded verbatim. In Chi, avoid logic that might conditionally strip or reinterpret these headers based on request path or method.

middleBrick can help verify that your endpoints resist smuggling attempts by running the 12 parallel security checks, including BFLA/Privilege Escalation and Input Validation. The resulting report maps findings to frameworks such as OWASP API Top 10 and provides remediation guidance tailored to Chi-based services.

For teams using the CLI, you can run middlebrick scan <url> to test your Chi API from the terminal and receive structured output with prioritized findings. Pro plan users gain continuous monitoring and CI/CD integration, including GitHub Action PR gates that can fail builds if risk scores exceed your chosen threshold.