Heap Overflow with Basic Auth

Heap overflow vulnerabilities in Basic Auth implementations typically arise from improper handling of the Authorization header's base64-encoded credentials. When a client sends an Authorization: Basic header, the server decodes the base64 payload into a username:password string. If the implementation uses fixed-size buffers or fails to validate input length, an attacker can craft oversized credentials to overflow memory buffers.

The most common Basic Auth heap overflow pattern occurs when servers use C-style string handling without bounds checking. Consider this vulnerable implementation:

char username[32];
char password[32];
char *creds = base64_decode(auth_header);
sscanf(creds, "%[^":"]:%[^":"]", username, password);

If the base64-decoded credentials exceed 31 characters (plus null terminator), the sscanf function will write beyond the allocated buffer boundaries. An attacker can exploit this by sending a base64-encoded string that decodes to hundreds of characters, potentially overwriting adjacent memory structures.

Another variant occurs in authentication middleware that concatenates credentials for logging or processing. A vulnerable pattern looks like:

char log_message[256];
snprintf(log_message, sizeof(log_message), "Auth attempt: %s:%s", username, password);

If either username or password exceeds the buffer space, the snprintf function may write beyond the log_message buffer, especially if the base64 decoding produces unexpectedly long strings.

Language-specific implementations also face heap overflow risks. In Node.js, using Buffer without proper length validation can lead to memory corruption:

const [user, pass] = Buffer.from(auth.split(' ')[1], 'base64').toString().split(':');
const combined = Buffer.allocUnsafe(64);
combined.write(`${user}:${pass}`); // No bounds checking

The allocUnsafe call creates a buffer without zeroing, and the subsequent write operation doesn't verify that the combined string fits within 64 bytes, creating a heap overflow condition.

Detecting heap overflow vulnerabilities in Basic Auth requires both static analysis and dynamic testing approaches. Static analysis tools can identify dangerous patterns like fixed-size buffers, unsafe string functions, and lack of input validation in authentication code paths.

Dynamic detection focuses on testing the Authorization header's boundaries. A comprehensive test involves sending progressively larger base64-encoded credentials to observe server behavior:

import requests
import base64
import time

def test_basic_auth_overflow(url, max_size=1024):
    for size in range(32, max_size, 32):
        creds = "A" * size + ":B" * size
        b64 = base64.b64encode(creds.encode()).decode()
        headers = {"Authorization": f"Basic {b64}"}
        try:
            response = requests.get(url, headers=headers)
            if response.status_code == 500:
                print(f"Potential overflow at size {size}")
            elif response.status_code == 200:
                print(f"Auth accepted at size {size}")
        except requests.exceptions.ConnectionError:
            print(f"Connection reset at size {size}")
            break

Heap overflow detection tools like middleBrick specifically test Basic Auth implementations by sending malformed base64 strings and monitoring for memory corruption indicators. The scanner tests for buffer overflows by sending credentials that decode to increasingly large strings, watching for server crashes, memory errors, or unusual response patterns.

middleBrick's approach includes testing the unauthenticated attack surface by sending Authorization headers with base64 payloads ranging from 64 to 4096 bytes, then analyzing server responses for signs of memory corruption. The tool also checks for common Basic Auth implementation patterns that are susceptible to heap overflow, such as:

The scanner also tests for authentication bypass conditions that might result from heap corruption, where memory overwrites could affect authentication logic or credential comparison operations.

Remediating heap overflow vulnerabilities in Basic Auth implementations requires systematic input validation and safe string handling practices. The most effective approach is to validate credential lengths before any processing occurs.

Here's a secure Basic Auth implementation in Node.js that prevents heap overflow:

const crypto = require('crypto');

function validateBasicAuthHeader(header) {
    if (!header || !header.startsWith('Basic ')) {
        return false;
    }
    
    try {
        const b64string = header.substring(6);
        
        // Maximum credential length (username:password)
        if (decoded.length > 512) {
            return false; // Reject oversized credentials
        }
        
        const [username, password] = decoded.split(':');
        if (!username || !password) {
            return false;
        }
        
        // Use constant-time comparison to prevent timing attacks
        return crypto.timingSafeEqual(
            Buffer.from(username + ':' + password),
            Buffer.from(process.env.VALID_USER + ':' + process.env.VALID_PASS)
        );
        
    } catch (error) {
        return false; // Invalid base64 or other decoding errors
    }
}

For C/C++ implementations, use safe string functions and dynamic allocation:

#include <string.h>
#include <stdlib.h>

int validate_basic_auth(const char *auth_header) {
    if (strncmp(auth_header, "Basic ", 6) != 0) {
        return 0;
    }
    
    // Decode base64 (using a safe library function)
    size_t decoded_len = base64_decoded_length(auth_header + 6);
    if (decoded_len > 1024) {
        return 0; // Reject oversized credentials
    }
    
    char *decoded = malloc(decoded_len + 1);
    if (!decoded) {
        return 0;
    }
    
    if (base64_decode(auth_header + 6, decoded, &decoded_len) != 0) {
        free(decoded);
        return 0;
    }
    decoded[decoded_len] = '\0';
    
    // Find colon separator safely
    char *colon = strchr(decoded, ':');
    if (!colon) {
        free(decoded);
        return 0;
    }
    
    // Use constant-time comparison
    int result = timing_safe_compare(decoded, VALID_CREDENTIALS, decoded_len);
    free(decoded);
    return result;
}

Additional remediation strategies include:

• Implement strict maximum lengths for username and password components (e.g., 64 characters each)
• Use safe string functions (strncpy, snprintf) with explicit bounds checking
• Validate base64 input before decoding to prevent malformed payloads
• Implement rate limiting to prevent brute-force attacks that might exploit memory corruption
• Use memory-safe languages or compile with stack protection (SSP, ASLR) enabled

For existing vulnerable systems, deploying a WAF rule that validates Authorization header length can provide immediate protection while implementing proper fixes:

# Nginx configuration example
location /api {
    if ($http_authorization ~* "^Basic\s+[A-Za-z0-9+/=]{1,1024}$") {
        # Only allow base64 strings up to 1024 characters
        auth_basic "Restricted";
        auth_basic_user_file /etc/nginx/.htpasswd;
    }
    # Reject other Authorization headers
    if ($http_authorization) {
        return 403;
    }
}