How severe is CVE-2016-4000?

CVE-2016-4000 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2016-4000?

CVE-2016-4000 affects the following packages: org.python:jython-standalone (Maven), org.python:jython (Maven). Ecosystems affected: Maven.

How do I fix CVE-2016-4000?

Update org.python:jython-standalone to 2.7.1b3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2016-4000 is resolved across your whole dependency graph.

How do I detect CVE-2016-4000 in my Maven dependencies?

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

How do I mitigate CVE-2016-4000 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-2016-4000 actively exploited in the wild?

No public exploit code has been indexed for CVE-2016-4000 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2016-4000?

CVE-2016-4000 has an EPSS (Exploit Prediction Scoring System) score of 6.2%, placing it in the 94th 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. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2016-4000?

CVE-2016-4000 is classified as Deserialization of Untrusted Data (CWE-502). This weakness type 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.

When was CVE-2016-4000 published, and has it been updated?

CVE-2016-4000 was published on July 6, 2017 and was last updated on March 15, 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-2016-4000 — jython-standalone

CRITICALFix: jython/jython@ee4200f

CVE-2016-4000 is a critical-severity (CVSS 9.8) Deserialization of Untrusted Data vulnerability in org.python:jython-standalone. A fix is available for org.python:jython-standalone — see the affected versions and patch details below.

Deserialization of Untrusted Data in Jython

Also known asGHSA-6r7r-jj8h-pq6v

Published

Jul 6, 2017

Updated

Mar 15, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 4, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

6.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs94th percentile — riskier than 94% of all scored CVEsHighest risk

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

☕org.python:jython-standalone☕org.python:jython

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

Jython before 2.7.1rc1 allows attackers to execute arbitrary code via a crafted serialized PyFunction object.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.python:jython-standalone`	all versions	2.7.1b3`org.python:jython-standalone:2.7.1b3`
☕Maven	`org.python:jython`	all versions	2.7.1b3`org.python:jython:2.7.1b3`

Affected Products

2 products · 2 configurations

debian linuxdebian

1 version

8.0

Application

jythonjython_project

1 version

2.7.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.python:jython-standalone, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.python:jython-standalone to 2.7.1b3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2016-4000 is resolved across your whole dependency graph.
Workarounds
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.

Frequently Asked Questions

Jython before 2.7.1rc1 allows attackers to execute arbitrary code via a crafted serialized PyFunction object.

O3 Security · Impact-Aware SCA

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