CVE-2021-27850
Bypass of the fix for CVE-2019-0195
- Published
- Apr 15, 2021
- Updated
- Aug 3, 2024
- Assigning CNA
- apache
- Evidence observed
- Aug 8, 2026
Primary CVSS
nvd · CVSS 3.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HHigh · next 30 days
- Percentile
- 99.8%
- Model date
- Oct 17, 2026
EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.
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.
Sources
modules/auxiliary/gather/cve_2021_27850_apache_tapestry_hmac_key.rb
Metasploit Framework modulesDeclared conditions
- verification: check_declared($.auxiliary_gather/cve_2021_27850_apache_tapestry_hmac_key.check)
http/cves/2021/CVE-2021-27850.yaml
Nuclei TemplatesDeclared conditions
- network: http($.protocol_keys)
- verification: status,word($.matchers[*].type)
- CVE-2021-27850_POCExploit
A Proof of concept for CVE-2021-27850 affecting Apache Tapestry and leading to unauthencticated remote code execution.
A Proof of concept for CVE-2021-27850 affecting Apache Tapestry and leading to unauthencticated remote code execution.
CVE-2021-27850 ysoserial
- CVE_2021_27850_POCExploit
Apache Tapestry CVE-2021-27850 PoC
Responsible use
Use vulnerability information only on systems you own or are authorized to test. Kitploit links to public research metadata and does not store exploit code or malicious payloads.