Vulnerability report

Exploited in the wild High confidence Not in CISA KEV

CVE-2021-27850

Bypass of the fix for CVE-2019-0195

Apache Tapestry · Apache Tapestry 5.5.0

Watch CVE-2021-27850 Browse KEV feed
Severity
CVSS 9.8 · Critical
Confidence
High
Exploit status
Exploited in the wild
EPSS
93.6%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-27850 is an unauthenticated Bypass of the fix for CVE-2019-0195 affecting Apache Tapestry. A critical unauthenticated remote code execution vulnerability was found all recent versions of Apache Tapestry. The...

Is it exploited?

Yes. Previdian tracks this CVE as a known exploited vulnerability. Confidence is high. Also confirmed by third-party sources.

Who is affected?

Apache Tapestry apache tapestry 5.5.0.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

A critical unauthenticated remote code execution vulnerability was found all recent versions of Apache Tapestry

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 Software Foundation Affected
Apache Tapestry
Apache Tapestry 5.5.0 Apache Tapestry 5.7.0 Before Apache Tapestry 5.4.0* Before Apache Tapestry 5.6.0*
Published
15 Apr 2021
Exploitation Reported
28 Apr 2025
Attack vector
Remote
Complexity
Low
Privileges
None
User interaction
None

Tags

apache nuclei_scanner java

CVE References

Exploitation evidence

Why Previdian marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

The Shadowserver

Recorded 28 Apr 2025

A trusted third party reported exploitation.

Proof of concept available

GitHub

Recorded 26 Jun 2021

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

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Source Added
The Shadowserver First 2025-04-28 00:00 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
User-Agents
Callback hosts
0
0
0

Request targets and User-Agents available in Pro. Callback host details available in Enterprise.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

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

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.8 Critical

Potential damage if exploited. Separate from whether attackers are using it.

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

EPSS

93.6%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

Exposure of Sensitive Information to an Unauthorized Actor CWE-200 Deserialization of Untrusted Data CWE-502

All 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 High

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

Potential Proof of Concepts

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

Apache Tapestry - Remote Code Execution

nuclei · Created Unknown

Timeline

From disclosure to observed exploitation

  1. 14:20 UTC

    Public PoC available

    Public proof-of-concept code published

  2. 00:00 UTC

    Added to Previdian KEV Feed

    High-confidence, third-party attested exploitation

  3. 00:00 UTC

    Nuclei template available

    Scanner coverage available

  4. 07:40 UTC

    CVE published

    Vulnerability disclosed publicly

  5. 00:00 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-27850

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2021-27850",
  "confidence": "High",
  "cvss_score": 9.8,
  "cvss_estimated": false,
  "epss_score": 0.9356,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}