Out Of Bounds Write in Chi with Basic Auth

An Out Of Bounds Write occurs when a program writes data before or after the intended buffer, which can corrupt stack or heap structures and lead to arbitrary code execution or crashes. In the Chi framework, when Basic Auth is used for route-level or middleware authentication, developers often bind user input directly into buffers or fixed-size structures to parse credentials or enforce per-route access without additional validation. If input length or offsets are not rigorously bounded, an attacker can supply a username or password that exceeds expected sizes, causing writes outside allocated memory.

Chi routes with Basic Auth typically rely on extracting credentials from the Authorization header and validating them against a user store. Because the header value is untrusted, an overly permissive parser that copies the header into fixed-length buffers or uses unchecked slice operations can be exploited. For example, a handler that decodes Base64 credentials and copies them into a stack-allocated array without length checks may experience an Out Of Bounds Write when the encoded payload is oversized. This can alter return addresses or function pointers, leading to security bypasses or denial of service. The combination of Chi’s pattern-based routing and Basic Auth’s header-based credential flow increases risk if input validation is delegated to the developer rather than enforced by the framework.

During a middleBrick scan, unchecked header handling and missing boundary checks in Chi routes with Basic Auth are flagged under Input Validation and BFLA/Privilege Escalation categories. The scanner submits crafted Authorization headers with oversized or malformed credentials to detect anomalous behavior such as crashes or unexpected responses. Findings include missing length validation on Basic Auth credentials, use of fixed-size buffers, and missing bounds checks before writing to slices or maps. Remediation guidance emphasizes validating header size before processing, using dynamic types with explicit length checks, and avoiding direct copying of untrusted input into fixed memory regions.

Secure handling of Basic Auth in Chi requires explicit validation of credential length and format before any buffer operations. Always treat the Authorization header as untrusted and enforce maximum lengths for usernames and passwords. Use Go’s standard library for Base64 decoding and avoid manual byte manipulation where possible. Below are concrete, safe patterns for Chi routes that require Basic Auth.

Example 1: Safe Basic Auth extraction with length validation

import (
    "encoding/base64"
    "net/http"
    "strings"
)

func basicAuthMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        auth := r.Header.Get("Authorization")
        if auth == "" {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        const prefix = "Basic "
        if !strings.HasPrefix(auth, prefix) {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        payload := auth[len(prefix):]
        // Enforce max length before decoding to prevent oversized input
        if len(payload) > 1024 {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        decoded, err := base64.StdEncoding.DecodeString(payload)
        if err != nil {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        // Ensure format "username:password" and validate parts
        parts := strings.SplitN(string(decoded), ":", 2)
        if len(parts) != 2 {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        username, password := parts[0], parts[1]
        // Apply further checks, e.g., max username/password length
        if len(username) > 128 || len(password) > 128 {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        // Proceed with authentication logic
        if !validateCredentials(username, password) {
            http.Error(w, "Forbidden", http.StatusForbidden)
            return
        }
        next.ServeHTTP(w, r)
    })
}

func validateCredentials(username, password string) bool {
    // Replace with secure lookup and constant-time comparison
    return username == "admin" && password == "s3cr3t"
}

Example 2: Chi route using validated Basic Auth

import (
    "net/http"
    "github.com/go-chi/chi/v5"
)

func adminRoute() http.Handler {
    r := chi.NewRouter()
    r.Use(basicAuthMiddleware)
    r.Get("/admin", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Admin area"))
    })
    return r
}

These examples demonstrate how to integrate Basic Auth safely within Chi by bounding input sizes, avoiding unchecked writes into fixed buffers, and validating credentials before use. middleBrick’s scans can verify that such controls are present by checking for header length validation and safe decoding patterns.