JLine3 Telnet server: Unauthenticated Remote DoS via Unbounded Telnet NAWS Terminal Geometry

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-56741: 0.60% estimated exploitation activity in the next 30 days; percentile 48.51 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The JLine3 Telnet server (remote-telnet module) does not apply an upper bound to terminal dimensions received via the Telnet NAWS (Negotiate About Window Size) option. An unauthenticated remote attacker can send a NAWS subnegotiation advertising a 65535×65535 terminal and repeatedly alternate values to trigger continuous, expensive rendering work on the server, causing CPU exhaustion and denial of service. Details TelnetIO.handleNAWS() (TelnetIO.java:856-879) reads the client-supplied width and height as 16-bit unsigned integers and passes them to setTerminalGeometry(): Only a lower bound is enforced (minimum 20 columns / 6 rows). Values up to 65535 are accepted and stored. The geometry change event propagates to Telnet.java:153-158 where it calls: terminal.setSize(new Size(65535, 65535)); terminal.raise(Signal.WINCH); The WINCH signal triggers LineReaderImpl.handleSignal() → redisplay(). Inside redisplay(), multiple paths iterate up to size.getColumns() times: - freshLine()…

Published by the source

18 Jun 2026, 13:07 UTC

Last modified by the source

1 Sep 2026, 21:02 UTC

Source last checked

26 Oct 2026, 16:55 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-2r2c-cx56-8933

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# Patch Next advisory brief: CVE-2026-56741

Advisory: JLine3 Telnet server: Unauthenticated Remote DoS via Unbounded Telnet NAWS Terminal Geometry
IDs: CVE-2026-56741, GHSA-2r2c-cx56-8933
Record: https://patchnext.com/advisories/cve-2026-56741 (revision 1, source checked 26 Oct 2026, 16:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.jline:jline-remote-telnet 4.2.1, 4.0.16, 3.30.14. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.jline:jline-remote-telnet: affected >= 4.1.0, < 4.2.1; >= 4.0.0, < 4.0.16; < 3.30.14; fixed in 4.2.1, 3.30.14, 4.0.16

## Suggested inspection (read-only)
- Maven org.jline:jline-remote-telnet: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.jline:jline-remote-telnet.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2r2c-cx56-8933 (GitHub-reviewed advisory GHSA-2r2c-cx56-8933 lists affected ranges: Maven org.jline:jline-remote-telnet >=4.1.0 =4.0.0 =0 <3.30.14)

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