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Date: Wed, 14 Apr 2021 19:08:10 -0300
From: "Thiago H. de Paula Figueiredo" <>
Subject: CVE-2021-27850: Apache Tapestry: Bypass of the fix for CVE-2019-0195


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.


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


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`.


Unfortunately, the blacklist solution can simply be bypassed by

appending a `/` at the end of the URL:


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 5.6.2, upgrade to 5.6.2 or later.

* For Apache Tapestry 5.7.0, upgrade to 5.7.1 or later.

This issue is being tracked as TAP5-2663


Apache Tapestry would like to thank Johannes Moritz for finding and
notifying this vulnerability

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