GHSA-47QP-HQVX-6R3F
Vulnerability from github – Published: 2026-06-18 13:07 – Updated: 2026-07-20 21:21Summary
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<String, String>(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 occupies approximately 2 KB of heap
(String headers + Map.Entry + backing char arrays). On a JVM with a 512 MB heap,
approximately 250,000 unique entries trigger an OutOfMemoryError.
Network cost: using sequential 1-byte names (e.g., \x01, \x02, ...) and 1-byte
values, each variable pair requires roughly 13 protocol bytes. Sending 250,000 pairs
requires only ~3.25 MB of network traffic — feasible in seconds over any reasonable
network connection.
No authentication is required. NEW-ENVIRON negotiation occurs before login.
Affected source files:
- remote-telnet/src/main/java/org/jline/builtins/telnet/TelnetIO.java lines 1127-1180
- remote-telnet/src/main/java/org/jline/builtins/telnet/ConnectionData.java line 98
PoC
Connect to the JLine3 Telnet server and, after completing WILL/DO option negotiation, send a NEW-ENVIRON SEND subneg followed by a single large IS subneg containing thousands of unique variable pairs before the final IAC SE.
Protocol structure (no authentication required): 1. Standard Telnet option negotiation (IAC DO NEW-ENVIRON, IAC WILL NEW-ENVIRON) 2. Server sends IAC SB NEW-ENVIRON SEND IAC SE 3. Client responds with: IAC SB NEW-ENVIRON IS [NE_VAR 0x01 NE_VALUE 0x01] ← variable pair 1 [NE_VAR 0x02 NE_VALUE 0x01] ← variable pair 2 ... repeated N times ... IAC SE ← only sent after N pairs
Each iteration adds one entry to the per-connection environment map. The connection thread blocks reading from the socket while accumulating pairs, so the attacker controls the timing of the OOM.
Reproduction environment:
- JLine3 built from current master on x86_64 Linux, OpenJDK 25.0.2
- remote-telnet module started with its default configuration
- Confirmed by source-code analysis; loop exit condition and missing count guard
verified by inspection of readNEVariables() and ConnectionData constructor
Impact
Type: Denial of Service (heap memory exhaustion / OutOfMemoryError)
Who is affected: Any application embedding the JLine3 remote-telnet module and
exposing its Telnet server. No credentials are required. A single connection can exhaust
the entire JVM heap, crashing the host process or triggering JVM out-of-memory
handling that impacts all users sharing that JVM instance.
Credits
This issue was identified by Michał Majchrowicz and Marcin Wyczechowski, members of the AFINE Team.
{
"affected": [
{
"package": {
"ecosystem": "Maven",
"name": "org.jline:jline-remote-telnet"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "4.2.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-56740"
],
"database_specific": {
"cwe_ids": [
"CWE-400"
],
"github_reviewed": true,
"github_reviewed_at": "2026-06-18T13:07:25Z",
"nvd_published_at": "2026-07-17T22:17:57Z",
"severity": "HIGH"
},
"details": "### Summary\n\nThe JLine3 Telnet server (`remote-telnet` module) does not limit the number of\nenvironment variables a client may inject via the Telnet NEW-ENVIRON option. An\nunauthenticated attacker can flood the server with a large number of unique\nvariable pairs before sending the terminating IAC SE byte, exhausting JVM heap\nmemory and causing an OutOfMemoryError (denial of service). Approximately 3\u20134 MB of\nnetwork traffic is sufficient to consume a 512 MB JVM heap.\n\n### Details\n\n`TelnetIO.readNEVariables()` (TelnetIO.java:1127-1180) processes incoming NEW-ENVIRON\nvariable pairs in a loop and stores each pair in a `HashMap` held by `ConnectionData`:\n\n```java\n// TelnetIO.java:1139-1178\nboolean cont = true;\nif (i == NE_VAR || i == NE_USERVAR) {\n do {\n switch (readNEVariableName(sbuf)) {\n case NE_VAR_OK:\n TelnetIO.this.connectionData.getEnvironment().put(str, sbuf.toString());\n // \u2190 no per-connection count limit\n break;\n case NE_VAR_UNDEFINED:\n break; // cont remains true, loop continues\n }\n } while (cont); // cont is never set to false; only exits via return\n}\n```\n\nThe variable accumulator map is a plain `HashMap` initialized with capacity 20 and\n**no maximum size**:\n\n```java\n// ConnectionData.java:98\nenvironment = new HashMap\u003cString, String\u003e(20);\n```\n\nPer-variable limits exist (name: max 50 chars, value: max 1000 chars), but there is no\ncap on the *count* of variables. Each map entry occupies approximately 2 KB of heap\n(String headers + `Map.Entry` + backing char arrays). On a JVM with a 512 MB heap,\napproximately 250,000 unique entries trigger an `OutOfMemoryError`.\n\nNetwork cost: using sequential 1-byte names (e.g., `\\x01`, `\\x02`, ...) and 1-byte\nvalues, each variable pair requires roughly 13 protocol bytes. Sending 250,000 pairs\nrequires only ~3.25 MB of network traffic \u2014 feasible in seconds over any reasonable\nnetwork connection.\n\nNo authentication is required. NEW-ENVIRON negotiation occurs before login.\n\nAffected source files:\n- `remote-telnet/src/main/java/org/jline/builtins/telnet/TelnetIO.java` lines 1127-1180\n- `remote-telnet/src/main/java/org/jline/builtins/telnet/ConnectionData.java` line 98\n\n### PoC\n\nConnect to the JLine3 Telnet server and, after completing WILL/DO option negotiation,\nsend a NEW-ENVIRON SEND subneg followed by a single large IS subneg containing\nthousands of unique variable pairs before the final IAC SE.\n\nProtocol structure (no authentication required):\n1. Standard Telnet option negotiation (IAC DO NEW-ENVIRON, IAC WILL NEW-ENVIRON)\n2. Server sends IAC SB NEW-ENVIRON SEND IAC SE\n3. Client responds with: IAC SB NEW-ENVIRON IS\n [NE_VAR 0x01 NE_VALUE 0x01] \u2190 variable pair 1\n [NE_VAR 0x02 NE_VALUE 0x01] \u2190 variable pair 2\n ... repeated N times ...\n IAC SE \u2190 only sent after N pairs\n\nEach iteration adds one entry to the per-connection environment map. The connection\nthread blocks reading from the socket while accumulating pairs, so the attacker\ncontrols the timing of the OOM.\n\nReproduction environment:\n- JLine3 built from current master on x86_64 Linux, OpenJDK 25.0.2\n- `remote-telnet` module started with its default configuration\n- Confirmed by source-code analysis; loop exit condition and missing count guard\n verified by inspection of `readNEVariables()` and `ConnectionData` constructor\n\n### Impact\n\n**Type**: Denial of Service (heap memory exhaustion / OutOfMemoryError)\n**Who is affected**: Any application embedding the JLine3 `remote-telnet` module and\nexposing its Telnet server. No credentials are required. A single connection can exhaust\nthe entire JVM heap, crashing the host process or triggering JVM out-of-memory\nhandling that impacts all users sharing that JVM instance.\n\n### Credits \n\nThis issue was identified by Micha\u0142 Majchrowicz and Marcin Wyczechowski, members of the AFINE Team.",
"id": "GHSA-47qp-hqvx-6r3f",
"modified": "2026-07-20T21:21:41Z",
"published": "2026-06-18T13:07:25Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/jline/jline3/security/advisories/GHSA-47qp-hqvx-6r3f"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-56740"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/pull/2000"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/pull/2001"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/commit/0389f0ee6d0375901b602671ad5dafd4d1d4ee09"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/commit/4ee3a73849ffb9a85ec748e4e8cd8f6d81f84f40"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/commit/934f09e6128cee33c2b13d42b6e859c1ee2d194b"
},
{
"type": "PACKAGE",
"url": "https://github.com/jline/jline3"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/releases/tag/4.0.16"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/releases/tag/4.2.1"
},
{
"type": "WEB",
"url": "https://github.com/jline/jline3/releases/tag/jline-3.30.14"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "JLine3 Telnet server: Unauthenticated Remote Memory Exhaustion via Unbounded Telnet NEW-ENVIRON Variables"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.