CVE-2026-56740 jline CVE debrief

Who should care

Developers and administrators using JLine library versions prior to 3.30.14, 4.0.16, or 4.2.1 should be aware of this vulnerability and take immediate action to update to a patched version.

Technical summary

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. The TelnetIO.readNEVariables() function in TelnetIO.java stores each variable pair in a HashMap held by ConnectionData. This allows an unauthenticated attacker to flood unique variable pairs before the terminating IAC SE byte, potentially causing an OutOfMemoryError and exhausting JVM heap memory.

Defensive priority

High priority should be given to updating JLine library versions to 3.30.14, 4.0.16, or 4.2.1. Additionally, monitoring for unusual Telnet activity and implementing compensating controls, such as limiting Telnet connections, may help mitigate potential attacks.

Recommended defensive actions

• Update JLine library to version 3.30.14, 4.0.16, or 4.2.1
• Monitor for unusual Telnet activity
• Implement compensating controls to limit Telnet connections
• Perform inventory checks to identify potentially affected systems
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-07-17T22:17:57.153Z and has not been modified since then. The NVD entry is currently 7.5 HIGH. Limited information is available about the specific affected scope, and further investigation is required to determine the full impact.

Official resources