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Date: Wed, 14 Apr 2021 19:08:10 -0300
From: "Thiago H. de Paula Figueiredo" <thiagohp@...il.com>
To: oss-security@...ts.openwall.com
Subject: CVE-2021-27850: Apache Tapestry: Bypass of the fix for CVE-2019-0195
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 5.6.2, 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
Credit:
Apache Tapestry would like to thank Johannes Moritz for finding and
notifying this vulnerability
--
Thiago
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