CVE-2016-4000: Jython before 2.7.1rc1 allows attackers to execute arbitrary code via a crafted serialized PyFunction object.

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CRITICAL 9.8REMOTEWATCHLISTDOWNGRADEMITIGATE

CVE-2016-4000 is a critical-severity flaw (CVSS 9.8) affecting jython, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Critical severity
EPSS top 6%: very likely to be exploited
Remotely exploitable, no login, no user action
On your watchlist

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	6.2% (top 6%)
CISA exploited list	No
Weakness	CWE-502
Published	2017-07-06
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · jython project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2017-07-06 16:29 UTCPublished CVE record published
2026-06-17 00:46 UTCUpdated Record last modified

jython project jython DOWNGRADE

Affected: 2.7.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.7.0

1. Check installed version

apt-cache policy jython || rpm -q jython || jython --version

2. No fixed release: DOWNGRADE to a release before 2.7.0 (Debian / Ubuntu)

apt-cache madison jython sudo apt install jython=<VERSION_BEFORE_2.7.0> sudo apt-mark hold jython

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list jython sudo dnf downgrade jython-<VERSION_BEFORE_2.7.0> sudo dnf versionlock add jython

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

debian debian linux MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2016-4000?

jython: downgrade or apply the listed mitigation; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2016-4000 being exploited?

CVE-2016-4000 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2016-4000 affect?

CVE-2016-4000 affects jython, debian linux. Severity is critical (CVSS 9.8). Jython before 2.7.1rc1 allows attackers to execute arbitrary code via a crafted serialized PyFunction object.

How urgent is CVE-2016-4000?

Priority 68/100. Critical severity; EPSS top 6%: very likely to be exploited; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

Related CVEs

CVE-2013-2027 MEDIUM 4.6 MITIGATE DOWNGRADE Jython 2.2.1 uses the current umask to set the privileges of the class cache files, which allows local users t

References

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