0x00 前言

SSRF 形成的原因大都是由于服務(wù)端提供了從其他服務(wù)器應(yīng)用獲取數(shù)據(jù)的功能且沒(méi)有對(duì)目標(biāo)地址做過(guò)濾與限制。比如從指定 URL 地址獲取網(wǎng)頁(yè)文本內(nèi)容，加載指定地址的圖片，下載等等。這里主要介紹java中URLConnection（）和openStream（）兩個(gè)方法產(chǎn)生SSRF的原理和修復(fù)方法

0x01 URLConnection

    @RequestMapping(value = "/urlConnection/vuln", method = {RequestMethod.POST, RequestMethod.GET})
    public String URLConnectionVuln(String url) {
        return HttpUtils.URLConnection(url);
    }

這里調(diào)用的是HttpUtils.URLConnection(url)

    public static String URLConnection(String url) {
        try {
            URL u = new URL(url);
            URLConnection urlConnection = u.openConnection();
            BufferedReader in = new BufferedReader(new InputStreamReader(urlConnection.getInputStream())); //send request
            // BufferedReader in = new BufferedReader(new InputStreamReader(u.openConnection().getInputStream()));
            String inputLine;
            StringBuilder html = new StringBuilder();

            while ((inputLine = in.readLine()) != null) {
                html.append(inputLine);
            }
            in.close();
            return html.toString();
        } catch (Exception e) {
            logger.error(e.getMessage());
            return e.getMessage();
        }
    }

跟進(jìn)URLConnection方法，而URLConnection里又調(diào)用了URL.openConnection()來(lái)發(fā)起請(qǐng)求, 這個(gè)請(qǐng)求可以直接執(zhí)行url協(xié)議（偽協(xié)議）
漏洞利用:
使用file協(xié)議讀文件

使用http協(xié)議訪問(wèn)百度

修復(fù)方法:
這里先是對(duì)url調(diào)用了SecurityUtil.isHttp()來(lái)進(jìn)行檢查

    @GetMapping("/urlConnection/sec")
    public String URLConnectionSec(String url) {

        // Decline not http/https protocol
        if (!SecurityUtil.isHttp(url)) {
            return "[-] SSRF check failed";
        }

        try {
            SecurityUtil.startSSRFHook();
            return HttpUtils.URLConnection(url);
        } catch (SSRFException | IOException e) {
            return e.getMessage();
        } finally {
            SecurityUtil.stopSSRFHook();
        }

    }

SecurityUtil.isHttp()比較簡(jiǎn)單，就是判斷url是否是以http://或https://開頭

    public static boolean isHttp(String url) {
        return url.startsWith("http://") || url.startsWith("https://");
    }

單純的ban掉其他協(xié)議顯然是不夠的，還不能夠防止對(duì)內(nèi)網(wǎng)進(jìn)行探測(cè)，于是在獲取url內(nèi)容之前，開啟了一個(gè)hook來(lái)對(duì)用戶行為進(jìn)行監(jiān)聽，SecurityUtil.startSSRFHook()，就有效防止了ssrf攻擊

0x02 openStream

openStream（）方法的實(shí)現(xiàn)也是調(diào)用了openConnection生成一個(gè)URLConnection對(duì)象，然后再通過(guò)這個(gè)對(duì)象調(diào)用的getInputStream（）方法的

    @GetMapping("/openStream")
    public void openStream(@RequestParam String url, HttpServletResponse response) throws IOException {
        InputStream inputStream = null;
        OutputStream outputStream = null;
        try {
            String downLoadImgFileName = WebUtils.getNameWithoutExtension(url) + "." + WebUtils.getFileExtension(url);
            // download
            response.setHeader("content-disposition", "attachment;fileName=" + downLoadImgFileName);

            URL u = new URL(url);
            int length;
            byte[] bytes = new byte[1024];
            inputStream = u.openStream(); // send request
            outputStream = response.getOutputStream();
            while ((length = inputStream.read(bytes)) > 0) {
                outputStream.write(bytes, 0, length);
            }

        } catch (Exception e) {
            logger.error(e.toString());
        } finally {
            if (inputStream != null) {
                inputStream.close();
            }
            if (outputStream != null) {
                outputStream.close();
            }
        }
    }

通過(guò)WebUtils.getNameWithoutExtension(url) + "." + WebUtils.getFileExtension(url)來(lái)獲取下載文件名,然后執(zhí)行inputStream = u.openStream();來(lái)看一下openStream()，也是調(diào)用了openConnection()，也會(huì)根據(jù)傳入的協(xié)議的不同來(lái)進(jìn)行處理

    public final InputStream openStream() throws java.io.IOException {
        return openConnection().getInputStream();
    }

由此可以得知，openStream（）方法同樣也可以進(jìn)行ssrf來(lái)探測(cè)內(nèi)網(wǎng)以及文件下載，修復(fù)方案同上

0x03 總結(jié)

關(guān)鍵詞：
URLConnection、openConnection、openStream
漏洞利用：
關(guān)于SSRF漏洞利用相關(guān)可以看這篇文章，總結(jié)的很詳細(xì)！
從一文中了解SSRF的各種繞過(guò)姿勢(shì)及攻擊思路