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CVE-2026-56740 high CVSS 7.5

Vulnerability in org.jline:jline-remote-telnet (CVE-2026-56740)

Summary

Vulnerability in org.jline:jline-remote-telnet (CVE-2026-56740). Risk of unauthorized operations or information disclosure. Exploitable via HashMap. Mitigation: upgrade to 3.30.14 or later.

Original description

The description as published by the source.

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:

// TelnetIO.java:1139-1178
boolean cont = true;
if (i == NE_VAR || i == NE_USERVAR) {
    do {
        switch (readNEVariableName(sbuf)) {
            case NE_VAR_OK:
                TelnetIO.this.connectionData.getEnvironment().put(str, sbuf.toString());
                // ← no per-connection count limit
                break;
            case NE_VAR_UNDEFINED:
                break; // cont remains true, loop continues
        }
    } while (cont);  // cont is never set to false; only exits via return
}

The variable accumulator map is a plain HashMap initialized with capacity 20 and
no maximum size:

// ConnectionData.java:98
environment = new HashMap(20);

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 occupie…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-56740 has been found in org.jline:jline-remote-telnet.
Attackers can target a specific entry point like HashMap over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade org.jline:jline-remote-telnet to 3.30.14 or later.
If unsure, ask your IT team or search "org.jline:jline-remote-telnet CVE-2026-56740" on the vendor's site.

CVE-2026-56740 (org.jline:jline-remote-telnet) — CWE-400 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: HashMap / ConnectionData / Map.Entry / OutOfMemoryError
Patched: 3.30.14 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-400) exists in org.jline:jline-remote-telnet. Attackers reach the vulnerable code path via HashMap without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of org.jline:jline-remote-telnet. Attack surface: HashMap / ConnectionData / Map.Entry / OutOfMemoryError.

🔥 Severity

Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 3.30.14.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    Audit SBOM/dependencies for affected components.

    資産台帳・SBOM・依存マニフェストで org.jline:jline-remote-telnet を使っているサービスとバージョンを洗い出す。

  2. 2
    Apply patch patch 
    Upgrade org.jline:jline-remote-telnet to 3.30.14

    ステージング環境で 3.30.14 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

maven org.jline:jline-remote-telnet
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.30.14"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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