CVE-2026-56740: JLine is a Java library for handling console input

Q: How do I fix CVE-2026-56740?

jline: upgrade to 3.30.14, 4.0.16, 4.2.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-56740 being exploited?

CVE-2026-56740 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-56740 affect?

CVE-2026-56740 affects jline. Severity is high (CVSS 7.5). JLine is a Java library for handling console input. Prior to 3.30.14, 4.0.16, and 4.2.1, the JLine3 Telnet server remote-telnet module does not limit the number of environment variables a client may inject via the Telnet NEW-ENVIRON option,

Q: How urgent is CVE-2026-56740?

Priority 51/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

This week

HIGH 7.5REMOTEUPGRADE

CVE-2026-56740 is a high-severity flaw (CVSS 7.5) affecting jline. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

JLine is a Java library for handling console input. Prior to 3.30.14, 4.0.16, and 4.2.1, the JLine3 Telnet server remote-telnet module does not limit the number of environment variables a client may inject via the Telnet NEW-ENVIRON option, and TelnetIO.readNEVariables() in TelnetIO.java:1127-1180 stores each variable pair in a HashMap held by ConnectionData, allowing an unauthenticated attacker to flood unique variable pairs before the terminati

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.9% (top 41%)
CISA exploited list	No
Weakness	CWE-400
Published	2026-07-17
Last modified	2026-10-24

Publisher (CNA): GitHub, Inc. · Vendor: jline

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-17 22:17 UTCPublished CVE record published
2026-10-24 20:10 UTCUpdated Record last modified

jline jline UPGRADE

Affected: < 3.30.14 · >= 4.0.0 and < 4.0.16 · >= 4.1.0 and < 4.2.1

Fixed in: 3.30.14, 4.0.16, 4.2.1

Safe to downgrade to: a release before 4.0.0

1. Check installed version

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

2. UPGRADE to 3.30.14 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade jline

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade jline

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 4.0.0 (Debian / Ubuntu)

apt-cache madison jline sudo apt install jline=<VERSION_BEFORE_4.0.0> sudo apt-mark hold jline

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list jline sudo dnf downgrade jline-<VERSION_BEFORE_4.0.0> sudo dnf versionlock add jline

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.

Exact fixes by distribution

Maven · org.jline:jline-remote-telnet · FIXED · 4.2.1 · vulnerable: >= 4.1.0, < 4.2.1 · source

Set org.jline:jline-remote-telnet to 4.2.1 or later in pom.xml or build.gradle, then rebuild and redeploy.

Fedora 46 · conda · FIXED · 26.9.4-2.fc46 · FEDORA-2026-09931756f1 · source

sudo dnf upgrade --advisory=FEDORA-2026-09931756f1

Debian 12 · jline3 · NO FIX YET

Debian 13 · jline3 · NO FIX YET

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Write-up: github.com/jline/jline3/security/advisories/GHSA-47qp-hqvx-6r3f

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-2026-56740?

jline: upgrade to 3.30.14, 4.0.16, 4.2.1. Exact commands for each OS are on this page.

Is CVE-2026-56740 being exploited?

CVE-2026-56740 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-2026-56740 affect?

CVE-2026-56740 affects jline. Severity is high (CVSS 7.5). JLine is a Java library for handling console input. Prior to 3.30.14, 4.0.16, and 4.2.1, the JLine3 Telnet server remote-telnet module does not limit the number of environment variables a client may inject via the Telnet NEW-ENVIRON option,

How urgent is CVE-2026-56740?

Priority 51/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Java vulnerabilities

Related CVEs

CVE-2026-56741 HIGH 7.5 UPGRADE JLine is a Java library for handling console input

References

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