JLine3 Telnet server: Unauthenticated Remote DoS via Unbounded Telnet NAWS Terminal Geometry

Summary

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

Impact

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.

Credits

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