Summary SctpSackChunk.ParseChunk reads the numGapAckBlocks and numDuplicateTSNs fields (each up to 65535) directly from an attacker-controlled SCTP SACK chunk and loops that many times reading 4 bytes per iteration, with no validation of the counts against the chunk length or the receive buffer. A single crafted SACK chunk from a negotiated WebRTC peer forces reads past the end of the 262144-byte receive buffer, raising IndexOutOfRangeException, which is not caught by the recoverable handler and terminates the dedicated SCTP receive thread — permanently killing the SCTP association and all data channels.

Root Cause src/SIPSorcery/net/SCTP/Chunks/SctpSackChunk.cs: - ushort numGapAckBlocks = NetConvert.ParseUInt16(buffer, startPosn + 8); (:141) - ushort numDuplicateTSNs = NetConvert.ParseUInt16(buffer, startPosn + 10); (:142) - gap-ack loop (:146) and duplicate-TSN loop (:154) index the buffer via NetConvert.ParseUInt16/32 (buffer[posn], no bounds check — sys/Net/NetConvert.cs:30,41). SctpPacket.ParseChunks (SctpPacket.cs:195-203) only validates chunkLength >= 4 and posn+chunkLength <= length; the counts inside the value are never checked. RTCSctpTransport.DoReceive calls SctpPacket.Parse(recvBuffer, 0, bytesRead) on a reused recvBuffer = new byte[262144].

Impact IndexOutOfRangeException is a SystemException, not ApplicationException, so the recoverable catch (ApplicationException) { … continue; } at RTCSctpTransport.cs:345 is skipped and control falls to the generic catch (Exception) { … break; } at :356. The break exits the receive loop, DoReceive returns, and the dedicated receiveThread = new Thread(DoReceive) (:173, started once) exits with no restart → the SCTP association and every data channel are permanently dead (denial of service).

Proof of Concept A negotiated WebRTC peer (post-DTLS) sends a checksum-valid SCTP packet: 12-byte common header + a SACK chunk (type 3) with chunkLength=16, numGapAckBlocks=0xFFFF, numDuplicateTSNs=0xFFFF. CRC32C is attacker-computable. The gap-ack loop reaches buffer[262144] on a 262144-byte array (valid indices 0..262143) → IndexOutOfRangeException.

Attack Chain 1. Entry: post-DTLS negotiated peer sends a checksum-valid SCTP packet with a SACK chunk (chunkLength=16, numGapAckBlocks=0xFFFF). Guard: VerifyChecksum (CRC32C). Bypass: CRC32C is computable by the sender. 2. Processing: DoReceive (RTCSctpTransport.cs:286) reads into reused recvBuffer (262144 bytes, :280) → SctpPacket.Parse(recvBuffer, 0, bytesRead) (:302) → ParseChunks → SACK dispatch (SctpChunk.Parse :340-341) → SctpSackChunk.ParseChunk. Guard: ParseChunks checks only chunkLength>=4 and posn+chunkLength<=length (SctpPacket.cs:195-203). Bypass: chunkLength=16 is well-formed; the counts are never validated. 3. Sink: gap-ack loop (SctpSackChunk.cs:146) calls NetConvert.ParseUInt16(buffer, reportPosn) with reportPosn starting at startPosn(16)+FIXEDPARAMETERS(12)=28, climbing +4 each iteration. Guard: none on the count. Bypass: NetConvert.ParseUInt16 (NetConvert.cs:30) indexes buffer[posn] unchecked. 4. Impact: at iteration 65529, reportPosn = 28 + 655294 = 262144 → buffer[262144] → IndexOutOfRangeException → generic catch at RTCSctpTransport.cs:356 → break → receive thread exits, no restart → association permanently dead.

Bypass Evidence - Unchecked counts at SctpSackChunk.cs:141-142; loops at :146,:154. - NetConvert.ParseUInt16 unchecked indexing (NetConvert.cs:30). - ParseChunks validates only chunkLength (SctpPacket.cs:195-203). - OOB math: 28 + 655354 = 262168 > 262144; buffer is 262144 (DEFAULTADVERTISEDRECEIVEWINDOW, SctpAssociation.cs:62). numGapAckBlocks alone suffices — the dup-TSN loop is not needed. - DoReceive catch split: recoverable catch(ApplicationException) at :345 (continue) vs generic catch(Exception) at :356 (break); receiveThread started once at :176.

Affected Versions nuget:SIPSorcery <= 10.0.13 (verified present on release tag v10.0.13 and HEAD da944543).

Dedup NOT a duplicate of GHSA-qmvg-569h-hqrh — that fix (fe5a1fa) touched only SctpPacket.cs (the chunk-cursor zero-length infinite loop, CWE-835). This is a distinct out-of-bounds read (CWE-125) in SctpSackChunk count loops, untouched by that fix.

Suggested Fix Validate startPosn + FIXEDPARAMETERSLENGTH + numGapAckBlocks4 + numDuplicateTSNs4 <= posn + chunkLen before the loops, and/or make NetConvert.Parse bounds-checked, and/or treat IndexOutOfRangeException/ArgumentException as recoverable in DoReceive.

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