Out Of Bounds Write in Gorilla Mux with Basic Auth

An Out Of Bounds Write occurs when code writes data past the allocated memory boundary, which can corrupt adjacent memory and lead to arbitrary code execution or crashes. When combining Gorilla Mux, a popular HTTP router for Go, with HTTP Basic Authentication, specific patterns can expose this class of vulnerability through unchecked input handling and misuse of request context.

Gorilla Mux provides route variables and request context utilities, but does not inherently validate the size or safety of data written into these variables. If a handler reads a user-supplied value (e.g., from headers, URL parameters, or a JSON body decoded into a fixed-size buffer) and writes it into a fixed-length byte array or slice without bounds checking, an Out Of Bounds Write can occur. This is especially risky when Basic Authentication is used, because the Authorization header is often parsed and its credentials are decoded and used directly in request handling.

Consider a scenario where a Basic Auth credential (e.g., a token or password) is extracted and copied into a fixed-size buffer. If the credential length exceeds the buffer, the write overflows. In Go, using a fixed-size array with copy or direct assignment can trigger this if length is not validated. For example, declaring var cred [16]byte and copying an arbitrarily long decoded password into it can corrupt the heap or stack. While Go’s runtime includes some bounds safety, using copy with a source that exceeds the destination length can still lead to out-of-bounds behavior when combined with unsafe patterns or Cgo interop.

Additionally, malicious actors can exploit this by sending an Authorization header with an unusually long Base64-encoded credential. If the server decodes this and writes it into a constrained structure (such as a session buffer or a custom authentication context), the overflow may overwrite control data, potentially altering execution flow. Even though Gorilla Mux itself is robust, the handler logic around authenticated requests must enforce strict size limits on user-controlled data to prevent this class of flaw.

To summarize, the vulnerability arises not from Gorilla Mux directly, but from insecure handling of Basic Auth-derived data in application code, where unchecked writes beyond allocated buffers occur. The router correctly routes the request, but the handler’s unsafe memory operations create the exploitable condition.

Remediation focuses on validating and bounding any data derived from the Basic Authentication header before use. Always treat credentials as untrusted input and enforce length and format constraints. Below are concrete code examples demonstrating secure patterns for Gorilla Mux handlers with Basic Auth.

First, use a standard library approach to parse Basic Auth safely and limit credential length. The following example shows a middleware that checks the Authorization header and rejects credentials longer than a safe threshold.

package main

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

    "github.com/gorilla/mux"
)

const maxCredentialLength = 1024

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, "Authorization header required", http.StatusUnauthorized)
            return
        }
        parts := strings.Split(auth, " ")
        if len(parts) != 2 || strings.ToLower(parts[0]) != "basic" {
            http.Error(w, "Invalid authorization header format", http.StatusUnauthorized)
            return
        }
        decoded, err := base64.StdEncoding.DecodeString(parts[1])
        if err != nil || len(decoded) > maxCredentialLength {
            http.Error(w, "Invalid credentials", http.StatusUnauthorized)
            return
        }
        // Safe to use decoded credentials within length bounds
        // Store minimal necessary data in context
        ctx := context.WithValue(r.Context(), "auth", string(decoded))
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

func handler(w http.ResponseWriter, r *http.Request) {
    // Handler logic
}

func main() {
    r := mux.NewRouter()
    r.Use(basicAuthMiddleware)
    r.HandleFunc("/secure", handler).Methods("GET")
    http.ListenAndServe(":8080", r)
}

Second, if you must copy credential segments into fixed-size buffers, validate lengths explicitly and use bounded copy operations. Avoid using fixed-size arrays for credentials; prefer dynamically sized slices. Here is an example of safe buffer handling:

func processCredential(cred string) error {
    if len(cred) == 0 || len(cred) > 256 {
        return fmt.Errorf("credential length out of allowed range")
    }
    // Use a dynamically sized buffer or string; avoid fixed arrays
    buffer := make([]byte, len(cred))
    copy(buffer, cred) // Safe because len(cred) is bounded
    // Further processing
    return nil
}

Finally, enforce transport security and avoid logging credentials. Combine these practices with regular security scanning using tools like middleBrick to detect potential misconfigurations in your API routes. The CLI tool can be invoked with middlebrick scan <url> to assess your endpoints, while the GitHub Action helps integrate checks into CI/CD pipelines to fail builds if risk thresholds are exceeded.