flawopen.com/ssrf/Kotlin

● CWE-918 · 高危
安全研究 · FlawOpen

漏洞深度剖析：Server-Side Request Forgery in Kotlin

How HTTP clients in Kotlin/Ktor allow SSRF, and how to enforce IP boundaries.

💡 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-kotlin.
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 java.net.URL openStream()
import java.net.URL;
import java.io.InputStream;
import javax.servlet.http.*;

public class ProxyServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
        String targetUrl = req.getParameter("url");
        // Attacker input: http://169.254.169.254/latest/meta-data/
        try {
            URL url = new URL(targetUrl);
            InputStream in = url.openStream();
            in.transferTo(resp.getOutputStream());
        } catch (Exception e) {
            resp.setStatus(500);
        }
    }
}
✓ 加固后的安全修复
// HARDENED: Resolve all IPs, assert non-private, and disable redirects
import java.net.*;
import java.net.http.*;
import java.time.Duration;
import javax.servlet.http.*;

public class ProxyServlet extends HttpServlet {
    private final HttpClient client = HttpClient.newBuilder()
        .connectTimeout(Duration.ofSeconds(3))
        .followRedirects(HttpClient.Redirect.NEVER)
        .build();

    private boolean isSafeAddress(InetAddress addr) {
        return !addr.isLoopbackAddress() &&
               !addr.isSiteLocalAddress() &&
               !addr.isLinkLocalAddress() &&
               !addr.isAnyLocalAddress();
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
        String targetUrl = req.getParameter("url");
        try {
            URI uri = URI.create(targetUrl);
            if (!uri.getScheme().equalsIgnoreCase("http") && !uri.getScheme().equalsIgnoreCase("https")) {
                resp.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid scheme");
                return;
            }

            // 1. Resolve and validate all IP addresses
            InetAddress[] addresses = InetAddress.getAllByName(uri.getHost());
            for (InetAddress addr : addresses) {
                if (!isSafeAddress(addr)) {
                    resp.sendError(HttpServletResponse.SC_FORBIDDEN, "Access to internal IP prohibited");
                    return;
                }
            }

            // 2. Fetch using hardened client
            HttpRequest request = HttpRequest.newBuilder(uri)
                .timeout(Duration.ofSeconds(4))
                .GET()
                .build();

            HttpResponse<byte[]> response = client.send(request, HttpResponse.BodyHandlers.ofByteArray());
            resp.getOutputStream().write(response.body());

        } catch (Exception e) {
            resp.setStatus(HttpServletResponse.SC_FORBIDDEN);
        }
    }
}

工程与系统安全加固清单

References