Stack Overflow with Basic Auth
How Stack Overflow Manifests in Basic Auth
Basic Auth relies on the Authorization header whose value is Basic <base64(username:password)>. When a service decodes this header, it often extracts the username and password strings and copies them into fixed‑size buffers for further processing (e.g., looking up credentials in a database or comparing against a hard‑coded list). If the code uses unsafe copying functions such as strcpy, gets, or memcpy without checking the length of the decoded data, an attacker can supply an excessively long username or password that overflows the buffer.
Consider a typical C implementation that allocates a 64‑byte buffer for the username:
#include <string.h>
#include <stdlib.h>
void handle_basic_auth(const char *auth_header) {
char user[64];
char pass[64];
/* Assume header already stripped of "Basic " */
char *decoded = base64_decode(auth_header); /* returns malloc’ed string */
char *colon = strchr(decoded, ':');
if (!colon) { free(decoded); return; }
*colon = '\0';
strcpy(user, decoded); /* <-- vulnerable */
strcpy(pass, colon + 1); /* <-- vulnerable */
free(decoded);
/* … use user/pass … */
}
If the attacker supplies a base64 string that decodes to more than 63 bytes before the colon, strcpy writes past the end of user (or pass), corrupting the stack. This can lead to arbitrary code execution, crash, or privilege escalation—classic stack‑overflow behavior. Similar issues appear in higher‑level languages when developers manually manage buffers (e.g., using System.Array.Copy in C# without length checks) or when they rely on native libraries that perform unsafe copies internally.
Because Basic Auth is often implemented in lightweight middleware or API gateways, the vulnerable code path is exercised on every request that contains an Authorization header, making the flaw easy to trigger remotely without authentication.
Basic Auth‑Specific Detection
middleBrick’s unauthenticated black‑box scan includes an Input Validation check that deliberately sends oversized values in the Authorization header to see whether the target crashes, returns an error, or behaves unexpectedly. The scanner does not need any credentials; it simply probes the endpoint with a series of increasingly long base64 strings.
Example using the middleBrick CLI:
# Install the CLI (npm)
npm i -g middlebrick
# Scan an API endpoint
middlebrick scan https://api.example.com/orders
The command returns a JSON report. A finding related to a stack‑overflow risk will appear under the Input Validation category, with severity high and a description such as:
{
"findings": [
{
"id": "INP-0042",
"title": "Possible buffer overflow in Basic Auth header processing",
"severity": "high",
"description": "The endpoint accepted a base64‑encoded username longer than 100 bytes without rejecting the request, indicating a lack of length validation before copying into internal buffers.",
"remediation": "Validate the length of the decoded username and password before copying them into fixed‑size buffers. Use language‑safe string functions or limit the header size at the gateway.",
"references": ["CWE-121: Stack-based Buffer Overflow", "OWASP API Security Top 10 2023 – A1: Broken Object Level Authorization"]
}
],
"score": 42,
"grade": "F"
}
The same check runs automatically in the GitHub Action (added as a step in your CI/CD pipeline) and can be configured to fail the build if the score drops below a threshold you set. Results are also visible in the middleBrick Dashboard**, where you can track the score over time and see historical trends for each API.
For developers using AI coding assistants, the MCP Server** integration lets you invoke a scan directly from the IDE (e.g., Claude or Cursor) and receive the same findings without leaving your editor.
Basic Auth‑Specific Remediation
The fix is to ensure that any data extracted from the Authorization header is length‑checked before it is copied into a fixed‑size buffer or used in a way that could cause overflow. Below are language‑specific examples that demonstrate safe handling.
C (safe version)
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#define MAX_CRED 64
void handle_basic_auth_safe(const char *auth_header) {
char user[MAX_CRED];
char pass[MAX_CRED];
char *decoded = base64_decode(auth_header);
if (!decoded) { return; }
char *colon = strchr(decoded, ':');
if (!colon) { free(decoded); return; }
*colon = '\0';
/* Copy with explicit length limit */
strncpy(user, decoded, MAX_CRED - 1);
user[MAX_CRED - 1] = '\0';
strncpy(pass, colon + 1, MAX_CRED - 1);
pass[MAX_CRED - 1] = '\0';
free(decoded);
/* Now user and pass are guaranteed NUL‑terminated and within bounds */
/* … continue with authentication … */
}
Node.js
const basicAuth = require('basic-auth');
function basicAuthMiddleware(req, res, next) {
const credentials = basicAuth(req);
if (!credentials) {
res.statusCode = 401;
res.setHeader('WWW-Authenticate', 'Basic realm="Secure Area"');
return res.end('Access denied');
}
// Length check before using the values
const MAX_LEN = 60;
if (credentials.name.length > MAX_LEN || credentials.pass.length > MAX_LEN) {
return res.statusCode = 400, res.end('Credentials too long');
}
// Proceed with normal validation (e.g., DB lookup)
next();
}
Python (Flask)
from flask import request, Response
import base64
MAX_LEN = 60
def check_auth(auth_header):
if not auth_header or not auth_header.startswith('Basic '):
return False
encoded = auth_header.split(' ', 1)[1]
try:
decoded = base64.b64decode(encoded).decode('utf-8')
except Exception:
return False
if ':' not in decoded:
return False
username, password = decoded.split(':', 1)
if len(username) > MAX_LEN or len(password) > MAX_LEN:
return False
# Validate username/password against store
return check_credentials(username, password)
@app.before_request
def authenticate():
auth = request.headers.get('Authorization')
if not check_auth(auth):
return Response('Access denied', 401, {'WWW-Authenticate': 'Basic realm="Secure Area"'})
Go
package main
import (
"encoding/base64"
"net/http"
"strings"
)
const maxLen = 60
func basicAuthMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
auth := r.Header.Get("Authorization")
if !strings.HasPrefix(auth, "Basic ") {
http.Error(w, "Unauthorized", http.StatusUnauthorized)
w.Header().Set("WWW-Authenticate", `Basic realm="Restricted"`)
return
}
encoded := strings.TrimPrefix(auth, "Basic ")
decoded, err := base64.StdEncoding.DecodeString(encoded)
if err != nil {
http.Error(w, "Bad request", http.StatusBadRequest)
return
}
parts := strings.SplitN(string(decoded), ":", 2)
if len(parts) != 2 {
http.Error(w, "Bad request", http.StatusBadRequest)
return
}
user, pass := parts[0], parts[1]
if len(user) > maxLen || len(pass) > maxLen {
http.Error(w, "Credentials too long", http.StatusBadRequest)
return
}
// Proceed with normal auth
next.ServeHTTP(w, r)
})
}
After applying these fixes, rescan the endpoint with middleBrick (via CLI, GitHub Action, Dashboard, or MCP Server). The Input Validation finding should disappear, and the security score will improve.