Heap Overflow in Chi with Basic Auth

A heap overflow in a Chi application that uses HTTP Basic Authentication can occur when user-controlled data (e.g., credentials or request payloads) is copied into fixed-size buffers on the heap without proper bounds checking. Chi is a lightweight HTTP router for Go, and when developers use it to handle authenticated routes, they may parse headers and bodies into structures or buffers whose sizes are determined at runtime based on header values or input length. If the size is derived from attacker-controlled input (such as the Base64-encoded credentials in the Authorization header) and then used to allocate or copy data into a fixed-capacity heap buffer, an oversized input can overflow the buffer, corrupting adjacent memory.

Basic Authentication encodes a username and password in Base64 and places them in the Authorization header. In Chi, you might extract this header to validate credentials. Because the encoded string can be arbitrarily long, failing to validate its length before using functions like copy or append on byte slices can lead to writes beyond allocated capacity. This is especially risky when the decoded credentials are stored in buffers or reused across requests in a pooled object. An attacker can craft a long header to trigger a heap overflow, potentially leading to arbitrary code execution or denial of service. The combination of Chi’s flexible routing and Basic Auth’s header-based credentials increases the attack surface if input validation is incomplete.

During a black-box scan, middleBrick tests unauthenticated endpoints and analyzes OpenAPI specs where available, checking for indicators of unsafe handling of headers and authentication data. For endpoints using Basic Auth, it examines whether the API validates header length and whether server-side code appears to process credentials in a way that could exceed buffer limits. Although middleBrick does not fix the issue, its findings highlight risky patterns and provide remediation guidance, such as enforcing strict length checks and avoiding fixed-size heap buffers for user-controlled data.

To mitigate heap overflow risks in Chi when using Basic Authentication, validate and bound all inputs derived from headers before using them in buffer or slice operations. Always enforce maximum lengths for the Authorization header and decoded credentials, and avoid copying raw header values directly into fixed-size structures. Prefer high-level operations that check bounds, and handle credentials using secure types that do not rely on manual memory management.

Below are example code snippets demonstrating secure handling of Basic Auth in Chi. These examples enforce length limits and use safe copying to reduce the risk of overflow.

// Secure Basic Auth handling in Chi
package main

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

    "github.com/go-chi/chi/v5"
)

const maxAuthHeaderLen = 1024
const maxCredentialsLen = 512

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
        }
        if len(auth) > maxAuthHeaderLen {
            http.Error(w, "authorization header too long", http.StatusRequestHeaderFieldsTooLarge)
            return
        }
        if !strings.HasPrefix(auth, "Basic ") {
            http.Error(w, "invalid authorization header format", http.StatusBadRequest)
            return
        }
        encoded := strings.TrimPrefix(auth, "Basic ")
        if len(encoded) > maxCredentialsLen {
            http.Error(w, "credentials too long", http.StatusRequestHeaderFieldsTooLarge)
            return
        }
        decoded, err := base64.StdEncoding.DecodeString(encoded)
        if err != nil {
            http.Error(w, "invalid credentials encoding", http.StatusBadRequest)
            return
        }
        if len(decoded) > maxCredentialsLen {
            http.Error(w, "decoded credentials too long", http.StatusRequestHeaderFieldsTooLarge)
            return
        }
        // Validate username:password format safely
        parts := strings.SplitN(string(decoded), ":", 2)
        if len(parts) != 2 || parts[0] == "" || parts[1] == "" {
            http.Error(w, "invalid credentials", http.StatusUnauthorized)
            return
        }
        // At this point, credentials are safely bounded and can be used
        ctx := context.WithValue(r.Context(), "authUser", parts[0])
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

func main() {
    r := chi.NewRouter()
    r.Use(basicAuthMiddleware)
    r.Get("/secure", func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, "secure endpoint accessed\n")
    })
    http.ListenAndServe(":8080", r)
}

In this example, the middleware checks the length of the Authorization header and the decoded credentials before any copying or parsing. By avoiding direct use of unbounded header values and using safe split operations, you reduce the risk of heap overflow. For production, pair this with secure credential storage and transport-layer protections.