JLine3 Telnet server: Unauthenticated Remote DoS via Unbounded Telnet NAWS Terminal Geometry
- When
- Where
- Global (internet)
- Category
- cyber_advisory · maven
### 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.
Sources
- GitHub Advisory Database ↗ · first seen 2026-06-18 13:07 UTC
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Correlated events
Computed by the Defaxon correlation engine — linked by shared actors, co-location, and temporal proximity. Scored hypotheses, never causal claims.