GHSA-2R2C-CX56-8933

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():

// TelnetIO.java:869-875
private void setTerminalGeometry(int columns, int rows) {
    if (columns < SMALLEST_BELIEVABLE_WIDTH) columns = DEFAULT_WIDTH;  // lower bound only
    if (rows    < SMALLEST_BELIEVABLE_HEIGHT) rows    = DEFAULT_HEIGHT;
    connectionData.setTerminalGeometry(columns, rows);
    connection.processConnectionEvent(
        new ConnectionEvent(connection, ConnectionEvent.Type.CONNECTION_TERMINAL_GEOMETRY_CHANGED));
}

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() (LineReaderImpl.java:937,953): loops size.getColumns()-1 = 65534
  iterations, building and writing a space-padding string across the network socket.
• columnSplitLength(terminal, size.getColumns(), ...): called multiple times,
  each processing all characters against the 65535-wide line width.

Because WINCH only fires on change, the attacker alternates between two large values
(e.g., 65535 and 65534) to trigger an unlimited stream of expensive render cycles.
No authentication is required; the NAWS option is negotiated before any login sequence.

Affected source files:

• remote-telnet/src/main/java/org/jline/builtins/telnet/TelnetIO.java lines 856-879
• remote-telnet/src/main/java/org/jline/builtins/telnet/Telnet.java lines 140-175
• reader/src/main/java/org/jline/reader/impl/LineReaderImpl.java lines 929-962, 1293-1313

PoC

Send the following two raw Telnet packets in a loop to a running JLine Telnet server.
No login or authentication is required.

Packet 1, NAWS 65535 × 65535:
FF FA 1F FF FF FF FF FF F0
(IAC SB NAWS 0xFF 0xFF 0xFF 0xFF IAC SE)

Packet 2, NAWS 65534 × 65534:
FF FA 1F FF FE FF FE FF F0
(IAC SB NAWS 0xFF 0xFE 0xFF 0xFE IAC SE)

Sending these alternately at ~10 packets/second is sufficient to peg one CPU core on
the server. The server remains in this state for as long as the connection is open.

Reproduction environment:

• JLine3 built from current master on x86_64 Linux, OpenJDK 25.0.2
• remote-telnet module started with its default Telnet server configuration
• Test confirmed by source-code analysis and tracing the call chain at runtime

Credits

This issue was identified by Michał Majchrowicz and Marcin Wyczechowski, members of the AFINE Team.

Type: Denial of Service (CPU exhaustion)
Who is affected: Any application that embeds the JLine3 remote-telnet module and
exposes its Telnet server on a network interface. The attacker requires no credentials.
A single connection making ~10 alternating NAWS packets per second fully occupies the
connection-handling thread and produces continuous I/O on the server's output stream.
Because connection threads are re-used for the life of the session, one attacker per
available connection slot can deny service to all users of that slot.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

GHSA-2R2C-CX56-8933 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.2.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is GHSA-2R2C-CX56-8933? GHSA-2R2C-CX56-8933 is a high-severity uncontrolled resource consumption vulnerability in org.jline:jline-remote-telnet (maven), affecting versions < 4.2.1. It is fixed in 4.2.1. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is GHSA-2R2C-CX56-8933? GHSA-2R2C-CX56-8933 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of org.jline:jline-remote-telnet are affected by GHSA-2R2C-CX56-8933? org.jline:jline-remote-telnet (maven) versions < 4.2.1 is affected.
4. Is there a fix for GHSA-2R2C-CX56-8933? Yes. GHSA-2R2C-CX56-8933 is fixed in 4.2.1. Upgrade to this version or later.
5. Is GHSA-2R2C-CX56-8933 exploitable, and should I be worried? Whether GHSA-2R2C-CX56-8933 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-2R2C-CX56-8933 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-2R2C-CX56-8933? Upgrade org.jline:jline-remote-telnet to 4.2.1 or later.