flawopen.com/ssrf/C-cpp

● CWE-918 · 高危
安全研究 · FlawOpen

漏洞深度剖析：Server-Side Request Forgery in C/C++

How libcurl requests allow internal network pivots in C/C++, and how to enforce IP address validation with CURLOPT_RESOLVE.

💡 通俗易懂的原理解析 (ELI5)

想象一下，你派办公室助理去公共快递站取包裹，却故意给他留了总经理办公室带锁保险箱的内部地址。因为助理佩戴着公司内部通行工牌，保安直接放行，他便打开保险箱将公司核心机密交给了你。

核心概念与专有名词

Web Application Security
技术概念 (Web Application Security)：Core architecture component affected by CWE-918.
CWE-918
技术概念 (CWE-918)：Standard Common Weakness Enumeration classification for ssrf-c-cpp.
Defense-in-Depth
技术概念 (Defense-in-Depth)：Multi-layered engineering verification and runtime boundary isolation.

攻击执行流程分解

Step 1

接收不可信远程 URL

应用程序接受用户提交的外部 URL 用于拉取头像、Webhook 推送或生成网页预览。

Step 2

指向内部专用网络与元数据服务

攻击者输入指向内网回环地址或云厂商元数据接口（如 http://169.254.169.254/latest/meta-data/）的地址。

Step 3

未受限制的内部网络套接字请求

服务器内部 HTTP 客户端直接从受信任的私有 VPC 发起网络请求，未做 IP 白名单与私有地址段校验。

Step 4

云身份凭证与敏感数据外泄

内部元数据服务信任来自同主机的请求，回传临时 IAM 访问凭据、Kubernetes Token 或管理后台内容。

源代码对比：漏洞与安全实现

✕ 存在漏洞的实现
# VULNERABLE: Naive requests.get to user-provided URL
import requests
from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/fetch-avatar')
def fetch_avatar():
    url = request.args.get('url')
    # Attacker passes: http://169.254.169.254/latest/meta-data/iam/security-credentials/
    # Server queries AWS metadata and returns cloud credentials!
    resp = requests.get(url, timeout=5)
    return resp.content, 200, {'Content-Type': resp.headers.get('Content-Type', 'image/png')}
✓ 加固后的安全修复
# HARDENED: Resolve DNS, parse IP address, deny private/link-local CIDRs
import socket
import ipaddress
import urllib.parse
import requests
from flask import Flask, request, abort

app = Flask(__name__)

BLOCKED_NETWORKS = [
    ipaddress.ip_network('127.0.0.0/8'),      # Loopback
    ipaddress.ip_network('10.0.0.0/8'),       # RFC 1918 Private
    ipaddress.ip_network('172.16.0.0/12'),    # RFC 1918 Private
    ipaddress.ip_network('192.168.0.0/16'),   # RFC 1918 Private
    ipaddress.ip_network('169.254.0.0/16'),   # Link-local / Cloud IMDS
    ipaddress.ip_network('::1/128'),          # IPv6 Loopback
    ipaddress.ip_network('fc00::/7'),         # IPv6 Private ULA
]

def is_safe_ip(ip_str):
    ip = ipaddress.ip_address(ip_str)
    return not any(ip in net for net in BLOCKED_NETWORKS)

@app.route('/fetch-avatar')
def fetch_avatar():
    url = request.args.get('url', '')
    parsed = urllib.parse.urlparse(url)
    
    if parsed.scheme not in ('http', 'https') or not parsed.hostname:
        abort(400, "Invalid URL scheme")
        
    # 1. Resolve all DNS IPs for hostname
    try:
        addr_info = socket.getaddrinfo(parsed.hostname, parsed.port or (443 if parsed.scheme == 'https' else 80))
        resolved_ips = [ai[4][0] for ai in addr_info]
    except socket.gaierror:
        abort(400, "Unable to resolve host")
        
    # 2. Verify all resolved IPs are public
    for ip in resolved_ips:
        if not is_safe_ip(ip):
            abort(403, "Access to internal IP address is prohibited")
            
    # 3. Fetch with redirects disabled to prevent DNS rebinding or redirect bypass
    resp = requests.get(url, timeout=3, allow_redirects=False)
    return resp.content, 200, {'Content-Type': resp.headers.get('Content-Type', 'image/png')}

工程与系统安全加固清单

References