Stack buffer overflow in WebRequest cause

We started experiencing an intermittent issue about 3-4 weeks ago on our web servers. This issue happened 2-3 times with the same error appearing in the event viewer each time 0xc0000409.

Faulting application name: w3wp.exe, version: 10.0.14393.0, time stamp: 0x57899b8a
Faulting module name: clr.dll, version: 4.7.3130.0, time stamp: 0x5b175fed
Exception code: 0xc0000409
Fault offset: 0x0000000000302ffc
Faulting process id: 0x628
Faulting application start time: 0x01d42252ca1eb0c1
Faulting application path: c:\windows\system32\inetsrv\w3wp.exe
Faulting module path: C:\Windows\Microsoft.NET\Framework64\v4.0.30319\clr.dll
Report Id: 0b1db391-b366-412b-9e50-6041f8832288
Faulting package full name: 
Faulting package-relative application ID: 

At first I just googled this error code which quickly leads to the conclusion that its a STATUS_STACK_BUFFER_OVERRUN error. Supposedly this means that some unmanaged code is accessing memory that it shouldn't be resulting in this error. That information alone doesn't really help me figure out what the problem is so we enabled the crash dumps for the IIS process and managed to get full dump when the issue occurred today. After analyzing the crash dump that was created when the IIS process crashed, we found that the exception was apparently originating inside a System.Net.Sockets.Socket.Dispose(Boolean) call that is part of an HttpWebRequest being made to a 3rd party web service. There is nothing particularly interesting about this call and the response from the request is being disposed properly with a using block. I've included the top portion of the dumpstack call below as the full stack was too large to post in the question. My question is, what next steps could I take to determine the root cause of this error when its occurring inside a .NET library? It is really disconcerting to have an exception like this originating from a .NET library class like Socket, but I would like to rule out any possibility that it my applications usage of this library that is leading to the problem.

0:091> .excr
rax=0000000000000001 rbx=0000000000000001 rcx=0000000000000002
rdx=000002ace1ccafc0 rsi=0000000000000004 rdi=0000000000000004
rip=00007fffa3a22ffc rsp=000000cb44a3aa50 rbp=000000cb44a3ab09
r8=000002a8081966c0  r9=0000000000008000 r10=0000000000000000
r11=000000cb44a3aa80 r12=0000000000000000 r13=000002ab00000000
r14=000002ab5346b880 r15=000002ab5346bb01
iopl=0         nv up ei pl nz na pe nc
cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000202
clr!_report_gsfailure+0x1c:
00007fff`a3a22ffc cd29            int     29h
0:091> !dumpstack
OS Thread Id: 0xec (91)
Current frame: clr!_report_gsfailure+0x1c
Child-SP         RetAddr          Caller, Callee
000000cb44a3aa80 00007fffa3726cec clr!SafeHandle::Release+0xa4, calling clr!_security_check_cookie
000000cb44a3aac0 00007fffa3726e82 clr!SafeHandle::DangerousRelease+0xc2, calling clr!HelperMethodFrameRestoreState
000000cb44a3ab30 00007fffa372575a clr!FrameWithCookie<GCFrame>::FrameWithCookie<GCFrame>+0x42, calling clr!GetThread
000000cb44a3ab60 00007fffa38a3940 clr!SafeHandle::Dispose+0x30, calling clr!SafeHandle::Release
000000cb44a3abd0 00007fffa38a3a9b clr!SafeHandle::DisposeNative+0x9b, calling clr!SafeHandle::Dispose
000000cb44a3abf8 00007fffa3726e23 clr!SafeHandle::DangerousRelease+0x63, calling clr!LazyMachStateCaptureState
000000cb44a3ac60 00007fff461a27f4 (MethodDesc 00007fff463e18a8 +0x124 DomainNeutralILStubClass.IL_STUB_PInvoke(System.Net.SafeCloseSocket, Int32)), calling (MethodDesc 00007fff9bd37630 +0 System.StubHelpers.StubHelpers.SafeHandleRelease(System.Runtime.InteropServices.SafeHandle))
000000cb44a3acd0 00007fff461a2799 (MethodDesc 00007fff463e18a8 +0xc9 DomainNeutralILStubClass.IL_STUB_PInvoke(System.Net.SafeCloseSocket, Int32)), calling (MethodDesc 00007fff463e18a8 +0xf5 DomainNeutralILStubClass.IL_STUB_PInvoke(System.Net.SafeCloseSocket, Int32))
000000cb44a3ad18 00007fffa38a3a6c clr!SafeHandle::DisposeNative+0x6c, calling clr!LazyMachStateCaptureState
000000cb44a3ad30 00007fff461a2770 (MethodDesc 00007fff463e18a8 +0xa0 DomainNeutralILStubClass.IL_STUB_PInvoke(System.Net.SafeCloseSocket, Int32))
000000cb44a3ad40 00007fff45f60fd7 (MethodDesc 00007fff45489f70 +0x197 System.Net.Sockets.Socket.Dispose(Boolean)), calling (MethodDesc 00007fff9be3a800 +0 System.Runtime.InteropServices.SafeHandle.Dispose())
000000cb44a3adc0 00007fff45f6059a (MethodDesc 00007fff45489f78 +0x1a System.Net.Sockets.Socket.Dispose())
000000cb44a3add0 00007fff9c1e2fc7 (MethodDesc 00007fff9be36be8 +0x57 System.Exception.Init()), calling clr!JIT_WriteBarrier
000000cb44a3adf0 00007fff461a2607 (MethodDesc 00007fff460c6bf0 +0x57 System.Net.Sockets.NetworkStream.Dispose(Boolean)), calling (MethodDesc 00007fff454897a0 +0 System.Net.Sockets.Socket.Close(Int32))
000000cb44a3ae00 00007fff9c1f9c30 (MethodDesc 00007fff9be2c058 +0x60 System.ObjectDisposedException..ctor(System.String)), calling clr!JIT_WriteBarrier
000000cb44a3ae10 00007fff461a258c (MethodDesc 00007fff460f2b58 +0x3c System.Net.Security.SecureChannel.Close())
000000cb44a3ae30 00007fff461a212f (MethodDesc 00007fff460f04f8 +0x12f System.Net.TlsStream.Dispose(Boolean)), calling (MethodDesc 00007fff460c6bf0 +0 System.Net.Sockets.NetworkStream.Dispose(Boolean))
000000cb44a3ae40 00007fff461a252c (MethodDesc 00007fff460f0ed8 +0x4c System.Net.Security.SslState.Close()), calling (MethodDesc 00007fff460f2b58 +0 System.Net.Security.SecureChannel.Close())
000000cb44a3ae80 00007fff461a2103 (MethodDesc 00007fff460f04f8 +0x103 System.Net.TlsStream.Dispose(Boolean)), calling (MethodDesc 00007fff460f04f8 +0x10c System.Net.TlsStream.Dispose(Boolean))
000000cb44a3aed0 00007fff9c23bbea (MethodDesc 00007fff9be40fa8 +0x1a System.IO.Stream.Close())
000000cb44a3af00 00007fff48fad9df (MethodDesc 00007fff460c57c8 +0xbf System.Net.Connection.HandleError(Boolean, Boolean, System.Net.WebExceptionStatus, System.Net.ConnectionReturnResult ByRef)), calling 00007fff45f0f0b0
000000cb44a3af20 00007fff45f66a48 (MethodDesc 00007fff460f0ce8 +0x18 System.Net.Security.SslState.get_IsAuthenticated()), calling (MethodDesc 00007fff460f2b18 +0 System.Net.Security.SecureChannel.get_IsValidContext())
000000cb44a3af60 00007fff46189188 (MethodDesc 00007fff460c5818 +0xb8 System.Net.Connection.ReadComplete(Int32, System.Net.WebExceptionStatus)), calling 00007fff49f53e20
000000cb44a3afe0 00007fff46185e38 (MethodDesc 00007fff460c5808 +0x178 System.Net.Connection.SyncRead(System.Net.HttpWebRequest, Boolean, Boolean)), calling 00007fff45f0a630
000000cb44a3b040 00007fff46185355 (MethodDesc 00007fff460c1420 +0xa5 System.Net.ConnectStream.ProcessWriteCallDone(System.Net.ConnectionReturnResult)), calling 00007fff45f0a390
000000cb44a3b080 00007fff46184bbf (MethodDesc 00007fff460c1410 +0xcf System.Net.ConnectStream.CallDone(System.Net.ConnectionReturnResult)), calling 00007fff45f09d70
000000cb44a3b0d0 00007fff46181302 (MethodDesc 00007fff460c1660 +0x442 System.Net.ConnectStream.CloseInternal(Boolean, Boolean)), calling 00007fff45f09bb0
000000cb44a3b0f0 00007fffa372b3bf clr!COMInterlocked::CompareExchangeObject+0x1f, calling clr!ErectWriteBarrier
000000cb44a3b140 00007fff46180e94 (MethodDesc 00007fff460c1618 +0x24 System.Net.ConnectStream.System.Net.ICloseEx.CloseEx(System.Net.CloseExState)), calling 00007fff45f08120
000000cb44a3b160 00007fffa38e01ec clr!GCInterface::CollectionCount+0x3c
000000cb44a3b170 00007fff461849d0 (MethodDesc 00007fff460456c8 +0x1e0 System.Net.HttpWebRequest.EndWriteHeaders_Part2()), calling 00007fff45f0ccb0
000000cb44a3b180 00007fff46184720 (MethodDesc 00007fff46045698 +0x60 System.Net.HttpWebRequest.FinishContinueWait()), calling (MethodDesc 00007fff46045698 +0x74 System.Net.HttpWebRequest.FinishContinueWait())
000000cb44a3b190 00007fff46184535 (MethodDesc 00007fff46045658 +0x15 System.Net.HttpWebRequest.ShouldWaitFor100Continue())
000000cb44a3b1c0 00007fff46184647 (MethodDesc 00007fff46045648 +0x67 System.Net.HttpWebRequest.EndWriteHeaders(Boolean)), calling 00007fff45f09750
000000cb44a3b1e0 00007fff4618441b (MethodDesc 00007fff460c56a8 +0x12b System.Net.Connection.CheckStartReceive(System.Net.HttpWebRequest)), calling (MethodDesc 00007fff4604fe90 +0 System.Net.LazyAsyncResult.ProtectedInvokeCallback(System.Object, IntPtr))
000000cb44a3b200 00007fff46184595 (MethodDesc 00007fff46045520 +0x15 System.Net.HttpWebRequest.WriteHeadersCallback(System.Net.WebExceptionStatus, System.Net.ConnectStream, Boolean)), calling 00007fff45f093d0
000000cb44a3b230 00007fff45f66634 (MethodDesc 00007fff460c1590 +0x2e4 System.Net.ConnectStream.WriteHeaders(Boolean)), calling 00007fff45f09210
000000cb44a3b260 00007fff45f65bd1 (MethodDesc 00007fff46045788 +0x651 System.Net.HttpWebRequest.SerializeHeaders()), calling (MethodDesc 00007fff460c39f0 +0 System.Net.WebHeaderCollection+HeaderEncoding.GetBytes(System.String, Int32, Int32, Byte[], Int32))
000000cb44a3b2c0 00007fff45f651ac (MethodDesc 00007fff46045638 +0x7c System.Net.HttpWebRequest.EndSubmitRequest()), calling 00007fff45f07450
000000cb44a3b300 00007fff45f61f7b (MethodDesc 00007fff460c5648 +0x29b System.Net.Connection.CompleteConnection(Boolean, System.Net.HttpWebRequest)), calling 00007fff45f06b90
000000cb44a3b328 00007fffa38a8a81 clr!JIT_NewCrossContext_Portable+0x75, calling clr!LazyMachStateCaptureState
000000cb44a3b3a0 00007fff45f61e58 (MethodDesc 00007fff460c5648 +0x178 System.Net.Connection.CompleteConnection(Boolean, System.Net.HttpWebRequest)), calling (MethodDesc 00007fff460c5648 +0x22b System.Net.Connection.CompleteConnection(Boolean, System.Net.HttpWebRequest))
000000cb44a3b3c0 00007fff4417dc33 (MethodDesc 00007fff443e87a8 +0x13 System.Net.Logging.get_On()), calling clr!JIT_GetSharedNonGCStaticBase_InlineGetAppDomain
000000cb44a3b460 00007fff45f50dd9 (MethodDesc 00007fff460c5628 +0x179 System.Net.Connection.CompleteStartConnection(Boolean, System.Net.HttpWebRequest)), calling 00007fff45f062d0
000000cb44a3b4c0 00007fff45f50afd (MethodDesc 00007fff460c5608 +0x5d System.Net.Connection.CompleteStartRequest(Boolean, System.Net.HttpWebRequest, System.Net.TriState)), calling 00007fff45f04e80
000000cb44a3b4e0 00007fff4417dc33 (MethodDesc 00007fff443e87a8 +0x13 System.Net.Logging.get_On()), calling clr!JIT_GetSharedNonGCStaticBase_InlineGetAppDomain
000000cb44a3b510 00007fff45f4fadf (MethodDesc 00007fff460c55c8 +0x45f System.Net.Connection.SubmitRequest(System.Net.HttpWebRequest, Boolean)), calling 00007fff45f04cc0
000000cb44a3b590 00007fff45f4b65c (MethodDesc 00007fff46049360 +0x9c System.Net.ServicePoint.SubmitRequest(System.Net.HttpWebRequest, System.String)), calling 00007fff45f04160
000000cb44a3b5f0 00007fff45f4901d (MethodDesc 00007fff460455f8 +0x33d System.Net.HttpWebRequest.SubmitRequest(System.Net.ServicePoint)), calling (MethodDesc 00007fff46049360 +0 System.Net.ServicePoint.SubmitRequest(System.Net.HttpWebRequest, System.String))
000000cb44a3b650 00007fff45f462db (MethodDesc 00007fff46045100 +0x50b System.Net.HttpWebRequest.GetResponse()), calling 00007fff45f02100
000000cb44a3b670 00007fff45f3855a (MethodDesc 00007fff4604c3a0 +0x4a System.Net.KnownHttpVerb.Parse(System.String)), calling clr!JIT_IsInstanceOfClass
000000cb44a3b680 00007fff45f381e0 (MethodDesc 00007fff460a9e58 +0x30 System.Net.ValidationHelper.IsInvalidHttpString(System.String)), calling 00007fffa38a9d00 (stub for System.String.IndexOfAny(Char[], Int32, Int32))
000000cb44a3b6f0 00007fff4989a8a3 (MethodDesc 00007fff46ba3328 +0xd3 Royal.Common.Eligibility.Mahler.MahlerUtils.SendRequest(System.String))
000000cb44a3b700 00007fff9c236921 (MethodDesc 00007fff9be36a60 +0x71 System.String.Concat(System.String, System.String, System.String)), calling (MethodDesc 00007fff9be363a0 +0 System.String.FillStringChecked(System.String, Int32, System.String))
000000cb44a3b720 00007fff9c21668c (MethodDesc 00007fff9be360d0 +0x2c System.String.IsNullOrWhiteSpace(System.String)), calling (MethodDesc 00007fff9be38cd0 +0 System.Char.IsWhiteSpaceLatin1(Char))
000000cb44a3b760 00007fff4989a611 (MethodDesc 00007fff46ba3318 +0x341 Royal.Common.Eligibility.Mahler.MahlerUtils.SendEligibilityRequest(System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<System.DateTime>, System.String, System.String, System.String, System.Nullable`1<System.DateTime>, System.String, System.String, System.String, System.String)), calling (MethodDesc 00007fff46ba3328 +0 Royal.Common.Eligibility.Mahler.MahlerUtils.SendRequest(System.String))
000000cb44a3b800 00007fff49898a94 (MethodDesc 00007fff46ba3308 +0x164 Royal.Common.Eligibility.Mahler.MahlerUtils.SendRequest(Royal.Common.Eligibility.EligibilityRequest, System.String)), calling (MethodDesc 00007fff46ba3318 +0 Royal.Common.Eligibility.Mahler.MahlerUtils.SendEligibilityRequest(System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<System.DateTime>, System.String, System.String, System.String, System.Nullable`1<System.DateTime>, System.String, System.String, System.String, System.String))
000000cb44a3b810 00007fff9c2b7bd1 (MethodDesc 00007fff9be44758 +0xe1 DomainNeutralILStubClass.IL_STUB_PInvoke(IntPtr, IntPtr, System.String, System.String, Int32, Int32, Boolean, Int64))
000000cb44a3b8f0 00007fff9c23034c (MethodDesc 00007fff9bf9dd80 +0x7c System.Globalization.CompareInfo.GetHashCodeOfString(System.String, System.Globalization.CompareOptions, Boolean, Int64)), calling 00007fff9c16e4f8 (stub for System.Globalization.CompareInfo.InternalGetGlobalizedHashCode(IntPtr, IntPtr, System.String, System.String, Int32, Int32, Boolean, Int64))
000000cb44a3b920 00007fffa38ab3f3 clr!COMNlsInfo::InternalFindNLSStringEx+0x26d, calling clr!NewApis::IndexOfString
000000cb44a3b970 00007fff9c2302be (MethodDesc 00007fff9bf9bfb8 +0x2e System.CultureAwareComparer.GetHashCode(System.String)), calling (MethodDesc 00007fff9bf9dd80 +0 System.Globalization.CompareInfo.GetHashCodeOfString(System.String, System.Globalization.CompareOptions, Boolean, Int64))
000000cb44a3b9a0 00007fffa3724d15 clr!JIT_InitPInvokeFrame+0x15, calling clr!GetThread
000000cb44a3b9b0 00007fff9c20d3b5 (MethodDesc 00007fff9be3f1f0 +0x15 System.Collections.Hashtable.InitHash(System.Object, Int32, UInt32 ByRef, UInt32 ByRef))
000000cb44a3b9d0 00007fff9c2b73e0 (MethodDesc 00007fff9be44608 +0x100 DomainNeutralILStubClass.IL_STUB_PInvoke(IntPtr, IntPtr, System.String, Int32, System.String, Int32, Int32, System.String, Int32))
000000cb44a3ba50 00007fff9c2b73e0 (MethodDesc 00007fff9be44608 +0x100 DomainNeutralILStubClass.IL_STUB_PInvoke(IntPtr, IntPtr, System.String, Int32, System.String, Int32, Int32, System.String, Int32))
000000cb44a3bc80 00007fff49898859 (MethodDesc 00007fff4a9aedc8 +0x49 Royal.Common.Eligibility.EligibilityUtils+<>c__DisplayClass71_1.<SendRequestInPayerDrivenOrder>b__1()), calling (MethodDesc 00007fff46ba3308 +0 Royal.Common.Eligibility.Mahler.MahlerUtils.SendRequest(Royal.Common.Eligibility.EligibilityRequest, System.String))
000000cb44a3bcc0 00007fff498978ff (MethodDesc 00007fff46ba0440 +0x38f Royal.Common.Eligibility.EligibilityUtils.SendRequestInPayerDrivenOrder(Royal.Common.Eligibility.EligibilityRequest, Royal.Data.Model.IMF_Eligibility_Payer, Boolean, Royal.Data.Model.IMF_Eligibility_Request, Boolean, Boolean, Royal.Data.Model.RoyalContext))
000000cb44a3bcd0 00007fff4624f599 (MethodDesc 00007fff45c865e0 +0x1a9 System.Data.SqlClient.SqlTransaction.Commit()), calling (MethodDesc 00007fff45c8c160 +0 Bid.ScopeLeave(IntPtr ByRef))
000000cb44a3bce0 00007fff4623edb5 (MethodDesc 00007fff45c87090 +0x35 System.Data.Linq.SqlClient.SqlConnectionManager.set_Transaction(System.Data.Common.DbTransaction)), calling clr!JIT_WriteBarrier
000000cb44a3bdd0 00007fff49894407 (MethodDesc 00007fff46ba0430 +0xd07 Royal.Common.Eligibility.EligibilityUtils.SendRequest(System.Guid, Int32, System.Nullable`1<Int64>, System.Nullable`1<Int64>, Royal.Common.Eligibility.EligibilityRequest, Royal.Data.Model.IMF_Eligibility_Payer)), calling (MethodDesc 00007fff46ba0440 +0 Royal.Common.Eligibility.EligibilityUtils.SendRequestInPayerDrivenOrder(Royal.Common.Eligibility.EligibilityRequest, Royal.Data.Model.IMF_Eligibility_Payer, Boolean, Royal.Data.Model.IMF_Eligibility_Request, Boolean, Boolean, Royal.Data.Model.RoyalContext))
000000cb44a3bf30 00007fff49891f84 (MethodDesc 00007fff46ba04b0 +0x5d4 Royal.Common.Eligibility.EligibilityUtils.GetSingleEligibility(Int32, System.Nullable`1<Int64>, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int64>, System.Collections.Generic.List`1<Royal.Data.Model.V_Appointment>, Int32, System.String, System.String, System.String, System.String, System.String, System.Collections.Generic.HashSet`1<System.String>, System.String, Boolean, System.String, System.Collections.Generic.List`1<System.String> ByRef, System.String, System.Guid, System.Nullable`1<Int64> ByRef, System.String)), calling (MethodDesc 00007fff46ba0430 +0 Royal.Common.Eligibility.EligibilityUtils.SendRequest(System.Guid, Int32, System.Nullable`1<Int64>, System.Nullable`1<Int64>, Royal.Common.Eligibility.EligibilityRequest, Royal.Data.Model.IMF_Eligibility_Payer))
000000cb44a3bfd0 00007fff9c233c3d (MethodDesc 00007fff9be30970 +0x6d System.MulticastDelegate.RemoveImpl(System.Delegate)), calling (MethodDesc 00007fff9be308f0 +0 System.MulticastDelegate.Equals(System.Object))
000000cb44a3bff0 00007fff9c233ba5 (MethodDesc 00007fff9bd382f8 +0x25 System.Delegate.Remove(System.Delegate, System.Delegate)), calling 00007fffa37cf5b0 (stub for System.Delegate.InternalEqualTypes(System.Object, System.Object))
000000cb44a3c030 00007fff45c38cfd (MethodDesc 00007fff44915df8 +0x3d System.ComponentModel.EventHandlerList.RemoveHandler(System.Object, System.Delegate)), calling clr!JIT_WriteBarrier
000000cb44a3c040 00007fff9c1ddf3e (MethodDesc 00007fff9be3b258 +0xe System.Collections.Generic.EqualityComparer`1[[System.__Canon, mscorlib]].get_Default()), calling clr!JIT_GetGenericsGCStaticBase
000000cb44a3c060 00007fff45c38c0c (MethodDesc 00007fff45c870e8 +0x2c System.Data.Linq.SqlClient.SqlConnectionManager.RemoveInfoMessageHandler()), calling 00007fff441caa40
000000cb44a3c090 00007fff45c37885 (MethodDesc 00007fff45c870a8 +0x35 System.Data.Linq.SqlClient.SqlConnectionManager.CloseConnection()), calling (MethodDesc 00007fff45c870e8 +0 System.Data.Linq.SqlClient.SqlConnectionManager.RemoveInfoMessageHandler())
000000cb44a3c170 00007fff4989135c (MethodDesc 00007fff46ba04d0 +0x11c Royal.Common.Eligibility.EligibilityUtils.WrapEligibility(Int32, System.Nullable`1<Int64>, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int64>, System.Collections.Generic.List`1<Royal.Data.Model.V_Appointment>, Int32, System.String, System.String, System.String, System.String, System.String, System.Collections.Generic.HashSet`1<System.String>, System.String, Boolean, System.String, System.Collections.Generic.List`1<System.String> ByRef, System.String, System.Guid, System.Nullable`1<Int64> ByRef, System.String, System.Collections.Generic.List`1<System.Tuple`2<System.String,Int64>>, Royal.Common.Eligibility.EligibilityResponse ByRef, System.String)), calling (MethodDesc 00007fff46ba04b0 +0 Royal.Common.Eligibility.EligibilityUtils.GetSingleEligibility(Int32, System.Nullable`1<Int64>, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int64>, System.Collections.Generic.List`1<Royal.Data.Model.V_Appointment>, Int32, System.String, System.String, System.String, System.String, System.String, System.Collections.Generic.HashSet`1<System.String>, System.String, Boolean, System.String, System.Collections.Generic.List`1<System.String> ByRef, System.String, System.Guid, System.Nullable`1<Int64> ByRef, System.String))
000000cb44a3c270 00007fff4989031f (MethodDesc 00007fff46ba04c0 +0x64f Royal.Common.Eligibility.EligibilityUtils.GetEligibility(System.String, Int64, Int64, Int32, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Boolean, System.String, Int32, System.Collections.Generic.List`1<System.String> ByRef, Royal.Common.Eligibility.EligibilityResponse ByRef, Royal.Common.Eligibility.EligibilityResponse ByRef, System.String, System.Nullable`1<Int64> ByRef, System.Nullable`1<Int64> ByRef, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Royal.Common.Eligibility.EligibilityResponse ByRef, System.Nullable`1<Int64> ByRef, System.String)), calling (MethodDesc 00007fff46ba04d0 +0 Royal.Common.Eligibility.EligibilityUtils.WrapEligibility(Int32, System.Nullable`1<Int64>, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int64>, System.Collections.Generic.List`1<Royal.Data.Model.V_Appointment>, Int32, System.String, System.String, System.String, System.String, System.String, System.Collections.Generic.HashSet`1<System.String>, System.String, Boolean, System.String, System.Collections.Generic.List`1<System.String> ByRef, System.String, System.Guid, System.Nullable`1<Int64> ByRef, System.String, System.Collections.Generic.List`1<System.Tuple`2<System.String,Int64>>, Royal.Common.Eligibility.EligibilityResponse ByRef, System.String))
000000cb44a3c7e0 00007fff4988fca0 (MethodDesc 00007fff46ba04a0 +0x210 Royal.Common.Eligibility.EligibilityUtils.GetEligibility(System.String, Int64, Int64, Int32, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Boolean, System.String, Int32, System.Collections.Generic.List`1<System.String> ByRef, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, System.String)), calling (MethodDesc 00007fff46ba04c0 +0 Royal.Common.Eligibility.EligibilityUtils.GetEligibility(System.String, Int64, Int64, Int32, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Boolean, System.String, Int32, System.Collections.Generic.List`1<System.String> ByRef, Royal.Common.Eligibility.EligibilityResponse ByRef, Royal.Common.Eligibility.EligibilityResponse ByRef, System.String, System.Nullable`1<Int64> ByRef, System.Nullable`1<Int64> ByRef, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Royal.Common.Eligibility.EligibilityResponse ByRef, System.Nullable`1<Int64> ByRef, System.String))
000000cb44a3c990 00007fff4988f973 (MethodDesc 00007fff477a9ec8 +0x543 Products.RoyalAppointments.EligibilityCheck(System.String, Int64, Int64, Int32, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Boolean, System.String, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, System.String)), calling (MethodDesc 00007fff46ba04a0 +0 Royal.Common.Eligibility.EligibilityUtils.GetEligibility(System.String, Int64, Int64, Int32, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, Boolean, System.String, Int32, System.Collections.Generic.List`1<System.String> ByRef, System.String, System.Nullable`1<Int32>, System.String, System.String, System.String, System.String, System.String, Boolean, System.String))
000000cb44a3c9b0 00007fffa37c1a4d clr!CustomAttributeParser::GetNonNullString+0xe, calling clr!CustomAttributeParser::GetString
000000cb44a3cb50 00007fffa3726bc3 clr!CallDescrWorkerInternal+0x83
000000cb44a3cc80 00007fffa3726a80 clr!CallDescrWorkerWithHandler+0x4e, calling clr!CallDescrWorkerInternal
000000cb44a3cca0 00007fff9c1ddf1b (MethodDesc 00007fff9be3a8d8 +0x5b System.Collections.Generic.Dictionary`2[[System.__Canon, mscorlib],[System.__Canon, mscorlib]]..ctor(Int32, System.Collections.Generic.IEqualityComparer`1<System.__Canon>)), calling clr!JIT_WriteBarrier
000000cb44a3ccb0 00007fffa3814006 clr!MetaSig::GetElemSize+0x3c, calling clr!TypeHandle::GetSize
000000cb44a3ccc0 00007fffa377bfe6 clr!CallDescrWorkerReflectionWrapper+0x1a, calling clr!CallDescrWorkerWithHandler
000000cb44a3cd10 00007fffa377c49f clr!RuntimeMethodHandle::InvokeMethod+0x45f, calling clr!CallDescrWorkerReflectionWrapper
000000cb44a3cd30 00007fffa37278c9 clr!ArgIteratorTemplate<ArgIteratorBase>::GetNextOffset+0x51, calling clr!ArgIteratorBase::IsVarArg
000000cb44a3cd60 00007fffa372737d clr!MethodDescCallSite::CallTargetWorker+0x108, calling clr!_security_check_cookie

The code that is making the WebRequest call is shown below. It should be noted however that this exception does not occur every time, the process could run for a day and execute this code thousands of times before we see the exception.

        var wreq = (HttpWebRequest) WebRequest.Create(url);
        wreq.Method = WebRequestMethods.Http.Get;
        wreq.Timeout = 30000;
        byte[] content;
        using (var wres = (HttpWebResponse) wreq.GetResponse())
        {
            using (var rs = wres.GetResponseStream())
            {
                using (var ms = new MemoryStream())
                {
                    int count;
                    var buf = new byte[4096];
                    while ((count = rs.Read(buf, 0, buf.Length)) > 0)
                        ms.Write(buf, 0, count);
                    content = ms.ToArray();
                }
            }
        }