Http Request Smuggling in Gin with Basic Auth

Http Request Smuggling occurs when an HTTP request is interpreted differently by different components in a request path (e.g., a client, a proxy/load balancer, and the application). In Gin, this can arise when requests containing Basic Auth are handled with inconsistent parsing or forwarding logic, especially when headers like Authorization and Content-Length are treated differently by middleware or upstream services.

Gin does not parse the Authorization header itself; it is available via c.Request.Header.Get("Authorization"). If middleware or custom handlers split, modify, or forward requests without normalizing header formatting or ensuring consistent Content-Length handling, an attacker can craft requests where the proxy and Gin interpret the request boundaries differently. For example, an attacker may send a request with two Content-Length headers or use chunked encoding combined with Basic Auth credentials to cause a request to be partially buffered by the proxy and partially processed by Gin, leading to request splitting or injection.

Basic Auth introduces a structured credential header that may be retained or duplicated when requests are forwarded. If Gin routes or proxies requests to another service while preserving the original Authorization header but altering body parsing rules, the boundary between requests can be blurred. This is particularly risky when the upstream service uses a different HTTP library or configuration, as the proxy might normalize headers (e.g., lowercasing) while Gin preserves the original case, enabling a smuggling attack.

An example of a problematic Gin handler that forwards requests without normalizing headers might look like this:

func ForwardHandler(c *gin.Context) {
    req, _ := http.NewRequest(c.Request.Method, "https://upstream.example.com/"+c.Request.URL.Path, c.Request.Body)
    req.Header = c.Request.Header.Clone()
    client := &http.Client{}
    resp, _ := client.Do(req)
    defer resp.Body.Close()
    c.Status(resp.StatusCode)
}

In this pattern, if the client sends a smuggling-crafted request with ambiguous Content-Length and Authorization headers, the forwarded request may be interpreted differently by the upstream server. Attackers can exploit this to bypass authentication or inject requests into the backend pipeline.

To detect such issues, middleBrick scans unauthenticated endpoints and checks for inconsistencies in how headers and body lengths are handled across the request path. This includes testing how the API behaves when Authorization is combined with ambiguous body framing.

Remediation focuses on ensuring consistent header handling, avoiding header duplication or mutation, and strictly validating message framing before processing or forwarding requests.

• Do not forward requests with copied headers without normalization. Always explicitly set required headers and remove or canonicalize Authorization when it should not be passed through.
• Enforce a single source of truth for Content-Length. Do not rely on the client-provided value when proxies or load balancers may alter body encoding.
• Use middleware to validate and sanitize headers before routing or proxying. Reject requests with duplicate or conflicting framing headers.

Below is a secure Gin handler example that extracts Basic Auth credentials, validates them, and does not forward raw headers to upstream services:

func SecureHandler(c *gin.Context) {
    auth := c.Request.Header.Get("Authorization")
    if auth == "" {
        c.AbortWithStatusJSON(401, gin.H{"error": "authorization header required"})
        return
    }
    // Basic Auth format: Basic base64(username:password)
    const prefix = "Basic "
    if !strings.HasPrefix(auth, prefix) {
        c.AbortWithStatusJSON(400, gin.H{"error": "invalid authorization header format"})
        return
    }
    encoded := strings.TrimPrefix(auth, prefix)
    decoded, err := base64.StdEncoding.DecodeString(encoded)
    if err != nil {
        c.AbortWithStatusJSON(400, gin.H{"error": "invalid base64"})
        return
    }
    parts := strings.SplitN(string(decoded), ":", 2)
    if len(parts) != 2 {
        c.AbortWithStatusJSON(400, gin.H{"error": "invalid credentials format"})
        return
    }
    username, password := parts[0], parts[1]
    if !isValidUser(username, password) {
        c.AbortWithStatusJSON(401, gin.H{"error": "invalid credentials"})
        return
    }
    // At this point, authentication is successful and no Authorization header is forwarded.
    c.Set("user", username)
    c.Next()
}

func isValidUser(username, password string) bool {
    // Replace with secure user store lookup
    return username == "admin" && password == "correcthorsebatterystaple"
}

If you must forward requests, sanitize the headers explicitly:

func ForwardSanitized(c *gin.Context) {
    // Remove potential duplicate or smuggling-prone headers
    c.Request.Header.Del("X-Forwarded-For")
    c.Request.Header.Del("X-Forwarded-Host")
    c.Request.Header.Del("X-Forwarded-Proto")

    // Build new request with canonical headers
    req, _ := http.NewRequest(c.Request.Method, "https://upstream.example.com/"+c.Request.URL.Path, io.LimitReader(c.Request.Body, 10<<20))
    req.Header.Set("Authorization", "Basic "+c.Request.Header.Get("Authorization")) // explicitly set if needed
    req.Header.Set("Content-Type", "application/json")
    // Ensure Content-Length is set correctly by the transport
    c.Status(200)
}

These practices reduce the risk of request smuggling by ensuring consistent header interpretation and avoiding reliance on potentially ambiguous client input.