cve/2021/CVE-2021-27850.md

### [CVE-2021-27850](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-27850)
![](https://img.shields.io/static/v1?label=Product&message=Apache%20Tapestry&color=blue)
![](https://img.shields.io/static/v1?label=Version&message=Apache%20Tapestry%205.4.5%20&color=brightgreen)
![](https://img.shields.io/static/v1?label=Version&message=Apache%20Tapestry%205.5.0%20&color=brightgreen)
![](https://img.shields.io/static/v1?label=Version&message=Apache%20Tapestry%205.6.2%20&color=brightgreen)
![](https://img.shields.io/static/v1?label=Version&message=Apache%20Tapestry%205.7.0%20&color=brightgreen)
![](https://img.shields.io/static/v1?label=Vulnerability&message=CWE-200%20Information%20Exposure&color=brightgreen)
![](https://img.shields.io/static/v1?label=Vulnerability&message=CWE-502%20Deserialization%20of%20Untrusted%20Data&color=brightgreen)

### 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.

### POC

#### Reference
No PoCs from references.

#### Github
- https://github.com/20142995/nuclei-templates
- https://github.com/20142995/sectool
- https://github.com/ARPSyndicate/cvemon
- https://github.com/ARPSyndicate/kenzer-templates
- https://github.com/HimmelAward/Goby_POC
- https://github.com/NaInSec/CVE-PoC-in-GitHub
- https://github.com/NyxAzrael/Goby_POC
- https://github.com/Ovi3/CVE_2021_27850_POC
- https://github.com/SYRTI/POC_to_review
- https://github.com/Threekiii/Awesome-POC
- https://github.com/WhooAmii/POC_to_review
- https://github.com/Whoopsunix/PPPVULNS
- https://github.com/Z0fhack/Goby_POC
- https://github.com/dorkerdevil/CVE-2021-27850_POC
- https://github.com/k0mi-tg/CVE-POC
- https://github.com/kahla-sec/CVE-2021-27850_POC
- https://github.com/manas3c/CVE-POC
- https://github.com/n0-traces/cve_monitor
- https://github.com/nomi-sec/PoC-in-GitHub
- https://github.com/novysodope/CVE-2021-27850
- https://github.com/rain3ways/AWS-Security-Engineering
- https://github.com/ranhn/Goby-Poc
- https://github.com/soosmile/POC
- https://github.com/trhacknon/Pocingit
- https://github.com/whoforget/CVE-POC
- https://github.com/youwizard/CVE-POC
- https://github.com/zecool/cve