What’s Vulnerable › Vulnerabilities

CVE-2026-56740: JLine: Unauthenticated Remote Memory Exhaustion via Unbounded Telnet NEW-ENVIRON Variables

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.9% 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 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 terminating IAC SE byte and exhaust JVM heap memory with an OutOfMemoryError. 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/0389f0ee6d0375901b602671ad5dafd4d1d4ee09
• https://github.com/jline/jline3/commit/4ee3a73849ffb9a85ec748e4e8cd8f6d81f84f40
• https://github.com/jline/jline3/commit/934f09e6128cee33c2b13d42b6e859c1ee2d194b
• https://github.com/jline/jline3/pull/2000
• https://github.com/jline/jline3/pull/2001
• 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/releases/tag/jline-3.30.14
• https://github.com/jline/jline3/security/advisories/GHSA-47qp-hqvx-6r3f
• https://github.com/jline/jline3/security/advisories/GHSA-47qp-hqvx-6r3f

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