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Fencer Intel
critical 9.8

CVE-2021-27850 in the critical feed json

Bypass of the fix for CVE-2019-0195

published 5y ago modified 4mo ago also GHSA-mj8x-cpr8-x39h , EUVD-2021-1359

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
93.6%
EPSS 93.6% chance of exploitation in 30 days, percentile 99.8%
public exploits
1
Metasploit, plus a Nuclei check

Summary

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.

Patch / fix

  • maven org.apache.tapestry:tapestry-core 5.6.3, 5.7.1
  • apache tapestry 5.6.2, 5.7.1

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Timeline

  1. First public exploit (Metasploit auxiliary/gather/cve_2021_27850_apache_tapestry_hmac_key)
  2. Published

Exploit prediction

93.56% chance of exploitation in the next 30 days, higher than 99.8% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

CVSS scores

assigned by version score vector
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 10.0 AV:N/AC:L/Au:N/C:C/I:C/A:C

Affected packages

ecosystem package affected fixed in
maven org.apache.tapestry:tapestry-core
>=5.4.0, <5.6.3
>=5.7.0, <5.7.1
5.6.3
5.7.1

Affected products

  • Apache Software Foundation Apache Tapestry

Classification

Weakness
  • CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
  • CWE-502 Deserialization of Untrusted Data
  • CWE-922 Insecure Storage of Sensitive Information

References

Sources

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