GHSA-2r2c-cx56-8933 JLine3 Telnet server: Unauthenticated Remote DoS via Unbounded Telnet NAWS Terminal Geometry

• Maven org.jline:jline-remote-telnet: >= 4.1.0, < 4.2.1 → fixed in 4.2.1 • Maven org.jline:jline-remote-telnet: >= 4.0.0, < 4.0.16 → fixed in 4.0.16 • Maven org.jline:jline-remote-telnet: < 3.30.14 → fixed in 3.30.14

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()`: ```java // 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.