JLine3 Telnet server: Unauthenticated Remote Memory Exhaustion via Unbounded Telnet NEW-ENVIRON Variables

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-2026-56740: 0.59% estimated exploitation activity in the next 30 days; percentile 48.36 (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 limit the number of environment variables a client may inject via the Telnet NEW-ENVIRON option. An unauthenticated attacker can flood the server with a large number of unique variable pairs before sending the terminating IAC SE byte, exhausting JVM heap memory and causing an OutOfMemoryError (denial of service). Approximately 3–4 MB of network traffic is sufficient to consume a 512 MB JVM heap. Details TelnetIO.readNEVariables() (TelnetIO.java:1127-1180) processes incoming NEW-ENVIRON variable pairs in a loop and stores each pair in a HashMap held by ConnectionData: The variable accumulator map is a plain HashMap initialized with capacity 20 and no maximum size: Per-variable limits exist (name: max 50 chars, value: max 1000 chars), but there is no cap on the count of variables. Each map entry occupies approximately 2 KB of heap (String headers + Map.Entry + backing char arrays). On a JVM with a 512 MB heap,…

Published by the source

18 Jun 2026, 13:07 UTC

Last modified by the source

1 Sep 2026, 21:03 UTC

Source last checked

26 Oct 2026, 16:55 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-47qp-hqvx-6r3f

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# Patch Next advisory brief: CVE-2026-56740

Advisory: JLine3 Telnet server: Unauthenticated Remote Memory Exhaustion via Unbounded Telnet NEW-ENVIRON Variables
IDs: CVE-2026-56740, GHSA-47qp-hqvx-6r3f
Record: https://patchnext.com/advisories/cve-2026-56740 (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-47qp-hqvx-6r3f (GitHub-reviewed advisory GHSA-47qp-hqvx-6r3f lists affected ranges: Maven org.jline:jline-remote-telnet >=4.1.0 =4.0.0 =0 <3.30.14)

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