Summary

TurnServer.ReceiveUdpAsync places its generic catch (Exception) OUTSIDE the while receive loop, and Start() launches the loop fire-and-forget with no supervision or restart. A single pre-authentication UDP datagram whose STUN header first byte is in 0x80–0xFF causes STUNHeader.ParseSTUNHeader to throw ApplicationException, which unwinds past the loop and terminates it. The TURN UDP relay is then dead for ALL clients until the process is restarted.

Root Cause

src/SIPSorcery/net/TURN/TurnServer.cs:

• ReceiveUdpAsync (:555-577): the inner try (:562-567) wraps only _udpSocket.ReceiveAsync(); HandleUdpDatagram(result.Buffer, result.RemoteEndPoint) (:569) is inside the while body but OUTSIDE that inner try. The generic catch (Exception ex) (:573) is lexically OUTSIDE the while.
• Start() does _ = ReceiveUdpAsync(); (:381) — fire-and-forget, no restart.
• HandleUdpDatagram (:579) calls STUNMessage.ParseSTUNMessage(data, data.Length) (:600) for any non-ChannelData datagram; ParseSTUNMessage (STUNMessage.cs:94) has no try/catch.

Impact

ApplicationException propagates out of the while, is caught at :573, logged, and the method returns. _running remains true but nothing re-invokes ReceiveUdpAsync → TURN UDP relay permanently unavailable for all clients (whole-server DoS). Pre-authentication: STUN parsing precedes any TURN allocation/credential check.

Proof of Concept

Send one UDP datagram to the TURN port (default 3478) with first byte 0x80 (e.g. 80 00 00 00). 0x80 & 0xC0 = 0x80 ≠ 0x40 → not ChannelData → ParseSTUNMessage → ParseSTUNHeader executes if ((Array[startIndex] & 0xC0) != 0) throw new ApplicationException(...) (STUNHeader.cs:169-172); 0x80 & 0xC0 = 0x80 ≠ 0 → throws.

Attack Chain

1. Entry: one UDP datagram to the TURN port, first byte 0x80–0xFF. Guard: ChannelData branch requires (data[0] & 0xC0) == 0x40 (:583). Bypass: 0x80 & 0xC0 = 0x80 ≠ 0x40 → falls through to ParseSTUNMessage (:600).
2. Sink: STUNMessage.ParseSTUNMessage → STUNHeader.ParseSTUNHeader (STUNHeader.cs:169-172) throws ApplicationException. Guard: none before the throw; ParseSTUNMessage has no try/catch. Bypass: 0x80 & 0xC0 = 0x80 ≠ 0 → throws.
3. Impact: exception unwinds past the while into catch(Exception) at :573 → logged → method returns → loop exits. Guard: none — no restart (Start() :381 fire-and-forget). Bypass: N/A. TURN UDP relay dead for all clients until process restart.

Bypass Evidence

• Loop/catch structure: catch at TurnServer.cs:573 is outside the while at :559; HandleUdpDatagram at :569 is outside the inner try (:562-567).
• Unguarded ParseSTUNMessage at :600; throw at STUNHeader.cs:169-172.
• Fire-and-forget start at :381 with no restart in Start().
• TurnServerConfig.ListenAddress defaults to IPAddress.Loopback (:42), but a functioning TURN server must bind a routable address to serve clients, so real deployments are exposed. Non-default config narrows the vulnerable population, not the attack difficulty → AC:L.

Affected Versions

nuget:SIPSorcery <= 10.0.13 (TurnServer component present since 10.0.5; verified on release tag v10.0.13 and HEAD).

Dedup

NOT a duplicate of GHSA-28gm-jrmw-xx93 (CVE-2026-54632), which covers the client RTP/ICE socket (UdpReceiver/RTPChannel). TurnServer is a distinct shipped RFC 5766 server component with its own loop and fix location.

Suggested Fix

Wrap HandleUdpDatagram in a per-datagram try/log-and-continue INSIDE the while (matching the drop-and-continue intent of fix bdb76cb), and/or add loop supervision/restart.

Reported by zx (Jace) — GitHub: @manus-use

References

• GHSA-pfvm-w89x-94jw
• sipsorcery-org/sipsorcery@ccb0b5a