What’s Vulnerable › Vulnerabilities

CVE-2026-56741: JLine: Unauthenticated Remote DoS via Unbounded Telnet NAWS Terminal Geometry

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-07-17 · updated 2026-10-24
• Weakness: CWE-400

Description

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 apply an upper bound to terminal dimensions received via the Telnet NAWS option, and TelnetIO.handleNAWS() in TelnetIO.java:856-879 reads client-supplied width and height as 16-bit unsigned integers and passes values such as 65535x65535 to setTerminalGeometry(), allowing an unauthenticated remote attacker to repeatedly alternate values and trigger continuous expensive rendering work that causes CPU exhaustion and denial of service. This issue is fixed in versions 3.30.14, 4.0.16, and 4.2.1.

Affected products

• jline3: before 3.30.14; from 4.0.0 before 4.0.16; from 4.1.0 before 4.2.1 — fixed in 3.30.14, 4.0.16, 4.2.1

How to fix

Upgrade to a fixed version: jline3 3.30.14, 4.0.16, 4.2.1. Check the vendor advisory for mitigations.

References

• https://github.com/jline/jline3/commit/3ea9cad8699714dc072fade29d36be0d1e23d708
• https://github.com/jline/jline3/commit/733eb353dca7b0ea0252e724445b6defa29c393e
• https://github.com/jline/jline3/commit/86b7ba7801988aadb1a67555629522a71d603bd3
• https://github.com/jline/jline3/pull/2000
• https://github.com/jline/jline3/releases/tag/4.0.16
• https://github.com/jline/jline3/releases/tag/4.2.1
• https://github.com/jline/jline3/security/advisories/GHSA-2r2c-cx56-8933
• https://github.com/jline/jline3/security/advisories/GHSA-2r2c-cx56-8933

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