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9.8
CVSS
Critical

CVE-2021-27850

PUBLISHED

Bypass of the fix for CVE-2019-0195

Exploited in the wild PoC available Remote Low complexity No user interaction
Vendor
Apache Software Foundation
Product
Apache Tapestry
Published
Apr 15, 2021
EPSS
—

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 java nuclei_scanner

CVSS scores

CVSS v3.1 9.8 Critical

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS v2.0 10.0

AV:N/AC:L/Au:N/C:C/I:C/A:C

Exploitation status

Exploited in the wild

Recorded 2025-04-28 00:00:00 UTC · Source

Proof of concept available

Recorded 2021-06-26 14:16:00 UTC · Source

References

Known exploited vulnerability sources

Catalogues that list this CVE as a known exploited vulnerability.

Source Added
The Shadowserver (via CIRCL) Apr 28, 2025

Scanner integrations

Scanner Reference Detected
Nuclei https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-27850.yaml Apr 25, 2025

Potential proof of concepts

These PoCs are unverified and could contain malware. Use at your own risk.

dorkerdevil/CVE-2021-27850_POC

github · Created 2021-06-26 14:16:00 UTC · 3 stars

A Proof of concept for CVE-2021-27850 affecting Apache Tapestry and leading to unauthencticated remote code execution.

kahla-sec/CVE-2021-27850_POC

github · Created 2021-06-25 13:55:41 UTC · 5 stars

A Proof of concept for CVE-2021-27850 affecting Apache Tapestry and leading to unauthencticated remote code execution.

Timeline

  • CVE ID Reserved

  • CVE Published to Public

  • Proof of Concept Exploit Available

  • Detected by Nuclei

  • Added to KEVIntel