How severe is CVE-2021-27850?

CVE-2021-27850 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-27850?

CVE-2021-27850 affects the following packages: org.apache.tapestry:tapestry-core (Maven), org.apache.tapestry:tapestry-core (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-27850?

Update org.apache.tapestry:tapestry-core to 5.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-27850 is resolved across your whole dependency graph.

How do I detect CVE-2021-27850 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.apache.tapestry:tapestry-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-27850 if there is no patch (or I can't update yet)?

Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Is CVE-2021-27850 actively exploited in the wild?

Yes. There are 9 known exploit references for CVE-2021-27850, including 7 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-27850?

CVE-2021-27850 has an EPSS (Exploit Prediction Scoring System) score of 93.6%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

What type of vulnerability is CVE-2021-27850?

CVE-2021-27850 is classified as Information Exposure (CWE-200), Deserialization of Untrusted Data (CWE-502). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

Is there exploit tooling available for CVE-2021-27850?

A Nuclei detection template exists for CVE-2021-27850, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-27850 published, and has it been updated?

CVE-2021-27850 was published on April 15, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

CRITICAL severity

CVE-2021-27850 — tapestry-core

CRITICAL

CVE-2021-27850 is a critical-severity (CVSS 9.8) Information Exposure vulnerability in org.apache.tapestry:tapestry-core. 9 public exploit references exist, so weaponization risk is real. A fix is available for org.apache.tapestry:tapestry-core — see the affected versions and patch details below.

Remote code execution in Apache Tapestry

Also known asGHSA-mj8x-cpr8-x39h

Published

Apr 15, 2021

Updated

Oct 20, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

9 known

Exploitation data as of Oct 23, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

93.6%probability of exploitation in next 30 days

Very High Risk+0.09%

Lower risk than most CVEs100th percentile — riskier than 100% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-27850 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).

Where this sits among everything scored

Of 394,803 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected

☕org.apache.tapestry:tapestry-core☕org.apache.tapestry:tapestry-core

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

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.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.apache.tapestry:tapestry-core`	≥ 5.4.0&&< 5.6.3	5.6.3`org.apache.tapestry:tapestry-core:5.6.3`
☕Maven	`org.apache.tapestry:tapestry-core`	≥ 5.7.0&&< 5.7.1	5.7.1`org.apache.tapestry:tapestry-core:5.7.1`

Affected Products

1 product · 2 configurations

Application

tapestryapache

≥ 5.7.0 && < 5.7.1

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.