CVE-2026-56740 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in org.jline:jline-remote-telnet. O3 Security confirms whether CVE-2026-56740 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
JLine: Unauthenticated Remote Memory Exhaustion via Unbounded Telnet NEW-ENVIRON Variables
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-56740.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-56740 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 366,526 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
org.jline:jline-remote-telnetReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.jline:jline-remote-telnet | all versions | 4.2.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.jline:jline-remote-telnet. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.jline:jline-remote-telnet to 4.2.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-56740 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-56740 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-56740. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-56740 in your dependencies?
O3 detects CVE-2026-56740 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.