diff --git a/exec/pack/Messages.properties b/exec/pack/Messages.properties
index 52ad940aa..45cff8b10 100644
--- a/exec/pack/Messages.properties
+++ b/exec/pack/Messages.properties
@@ -1029,6 +1029,7 @@ VALUESET_SUPPLEMENT_MISSING_one = Required supplement not found: {1}
 VALUESET_SUPPLEMENT_MISSING_other = Required supplements not found: {1}
 VALUESET_TOO_COSTLY = The value set ''{0}'' expansion has too many codes to display ({1})
 VALUESET_TOO_COSTLY_COUNT = The value set ''{0}'' expansion has {2} codes, which is too many to display ({1})
+CODESYSTEM_TOO_COSTLY_TIME = The operation ''{1}'' took too long to process (>{0}sec) (at {2})
 VALUESET_TOO_COSTLY_TIME = The value set ''{0}'' {2} took too long to process (>{1}sec)
 REQUEST_TOO_COSTLY_TIME = Request took too long to process (>{0}sec)
 TX_SERVER_NOT_READY = The full data set is only {0}% loaded ({1} of {2}) for searching - repeat this query in a few minutes (max ~15min)');
diff --git a/library/fhir/fhir_tx.pas b/library/fhir/fhir_tx.pas
index 8b72110c1..8a3e62338 100644
--- a/library/fhir/fhir_tx.pas
+++ b/library/fhir/fhir_tx.pas
@@ -38,7 +38,7 @@   TTerminologyOperationContext = class (TTxOperationContext)
     property reqId : String read FId;
     property TimeTracker : TFslTimeTracker read FTimeTracker write SetTimeTracker;
     function copy : TTerminologyOperationContext;
-    function deadCheck(var time : integer) : boolean;
+    function deadCheck(var time : integer) : boolean; override;
     procedure seeContext(vurl : String);
     procedure clearContexts;
 
diff --git a/library/ftx/fhir_codesystem_service.pas b/library/ftx/fhir_codesystem_service.pas
index ea8353356..84432d181 100644
--- a/library/ftx/fhir_codesystem_service.pas
+++ b/library/ftx/fhir_codesystem_service.pas
@@ -34,7 +34,7 @@ interface
 
 uses
   SysUtils, Classes, Generics.Defaults, Generics.Collections,  
-  fsl_base, fsl_utilities, fsl_collections, fsl_http, fsl_lang, fsl_versions, fsl_fpc, fsl_logging, fsl_regex, fsl_i18n,
+  fsl_base, fsl_utilities, fsl_collections, fsl_http, fsl_lang, fsl_versions, fsl_fpc, fsl_logging, fsl_regex, fsl_i18n, fsl_threads,
   fhir_objects, fhir_factory, fhir_common, fhir_cdshooks,  fhir_utilities, fhir_features, fhir_uris,
   ftx_service;
 
@@ -203,7 +203,7 @@   TFhirCodeSystemProvider = class (TCodeSystemProvider)
     function doLocate(list : TFhirCodeSystemConceptListW; code : String; altOpt : TAlternateCodeOptions) : TFhirCodeSystemProviderContext; overload;
     function getParent(ctxt : TFhirCodeSystemConceptW) : TFhirCodeSystemConceptW;
     procedure FilterCodes(dest : TFhirCodeSystemProviderFilterContext; source : TFhirCodeSystemConceptListW; filter : TSearchFilterText);
-    procedure iterateCodes(base: TFhirCodeSystemConceptW; list: TFhirCodeSystemProviderFilterContext; filter : TCodeSystemCodeFilterProc; context : pointer; includeRoot : boolean; exception : TFhirCodeSystemConceptW = nil);
+    procedure iterateCodes(opContext : TTxOperationContext; op : String; base: TFhirCodeSystemConceptW; list: TFhirCodeSystemProviderFilterContext; filter : TCodeSystemCodeFilterProc; context : pointer; includeRoot : boolean; exception : TFhirCodeSystemConceptW = nil);
     function locateParent(ctxt: TFHIRCodeSystemConceptW; code: String): String;
     function locCode(list: TFhirCodeSystemConceptListW; code, synonym: String; altOpt : TAlternateCodeOptions): TFhirCodeSystemConceptW;
     function getProperty(code : String) : TFhirCodeSystemPropertyW;
@@ -214,7 +214,7 @@   TFhirCodeSystemProvider = class (TCodeSystemProvider)
     procedure iterateConceptsByRegex(src : TFhirCodeSystemConceptListW; regex: string; list: TFhirCodeSystemProviderFilterContext);
     procedure iterateConceptsByEquality(positive : boolean; src : TFhirCodeSystemConceptListW; code: string; list: TFhirCodeSystemProviderFilterContext);
 
-    procedure listChildrenByProperty(code : String; list, children : TFhirCodeSystemConceptListW);
+    procedure listChildrenByProperty(opContext : TTxOperationContext; op : String; code : String; list, children : TFhirCodeSystemConceptListW);
   protected
     function sizeInBytesV(magic : integer) : cardinal; override;
   public
@@ -1353,16 +1353,17 @@ function {TFhirCodeSystemProvider.}nonLeafCodes(context : pointer; concept: TFhi
   result := concept.conceptList.Count > 0;
 end;
 
-procedure TFhirCodeSystemProvider.listChildrenByProperty(code: String; list, children: TFhirCodeSystemConceptListW);
+procedure TFhirCodeSystemProvider.listChildrenByProperty(opContext : TTxOperationContext; op : String; code: String; list, children: TFhirCodeSystemConceptListW);
 var
   item : TFhirCodeSystemConceptW;
 begin
   for item in list do
   begin
+    deadCheck(opContext, 'listChildrenByProperty', op);
     if conceptHasProperty(item, 'http://hl7.org/fhir/concept-properties#parent', code) then
       children.Add(item.link)
     else if item.HasConcepts then
-      listChildrenByProperty(code, item.conceptList, children);
+      listChildrenByProperty(opContext, op, code, item.conceptList, children);
   end;
 end;
 
@@ -1446,7 +1447,7 @@ function TFhirCodeSystemProvider.version: String;
    result := FCs.CodeSystem.version;
 end;
 
-procedure TFhirCodeSystemProvider.iterateCodes(base : TFhirCodeSystemConceptW; list : TFhirCodeSystemProviderFilterContext; filter : TCodeSystemCodeFilterProc; context : pointer; includeRoot : boolean; exception : TFhirCodeSystemConceptW = nil);
+procedure TFhirCodeSystemProvider.iterateCodes(opContext : TTxOperationContext; op : String; base : TFhirCodeSystemConceptW; list : TFhirCodeSystemProviderFilterContext; filter : TCodeSystemCodeFilterProc; context : pointer; includeRoot : boolean; exception : TFhirCodeSystemConceptW = nil);
 var
   i : integer;
   el : TFslList<TFHIRObject>;
@@ -1467,14 +1468,20 @@ procedure TFhirCodeSystemProvider.iterateCodes(base : TFhirCodeSystemConceptW; l
 
   // 1. Add children in the heirarchy
   for i := 0 to base.conceptList.count - 1 do
-    iterateCodes(base.conceptList[i], list, filter, context, true);
+  begin
+    deadCheck(opContext, 'iterate-codes-1', op);
+    iterateCodes(opContext, op, base.conceptList[i], list, filter, context, true);
+  end;
 
   // 2. find any codes that identify this as a parent in their properties
   cl := TFhirCodeSystemConceptListW.Create;
   try
-    listChildrenByProperty(base.code, FCs.CodeSystem.conceptList, cl);
+    listChildrenByProperty(opContext, op, base.code, FCs.CodeSystem.conceptList, cl);
     for i := 0 to cl.count - 1 do
-      iterateCodes(cl[i], list, filter, context, true);
+    begin
+      deadCheck(opContext, 'iterate-codes-2', op);
+      iterateCodes(opContext, op, cl[i], list, filter, context, true);
+    end;
   finally
     cl.free;
   end;
@@ -1484,10 +1491,11 @@ procedure TFhirCodeSystemProvider.iterateCodes(base : TFhirCodeSystemConceptW; l
     for e in el do
     begin
       ex := FFactory.wrapExtension(e.Link);
-      try
+      try           
+        deadCheck(opContext, 'iterate-codes-3', op);
         ctxt := doLocate(ex.value.primitiveValue, nil);
         try
-          iterateCodes(TFhirCodeSystemProviderContext(ctxt).concept, list, filter, context, true);
+          iterateCodes(opContext, op, TFhirCodeSystemProviderContext(ctxt).concept, list, filter, context, true);
         finally
           ctxt.free;
         end;
@@ -1677,6 +1685,7 @@ function TFhirCodeSystemProvider.filter(opContext : TTxOperationContext; forExpa
   cc : TFhirCodeSystemConceptW;
   includeRoot : boolean;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   if (op in [foIsA, foDescendentOf]) and ((prop = 'concept') or (prop = 'code')) then
   begin
     code := doLocate(value, nil);
@@ -1690,7 +1699,7 @@ function TFhirCodeSystemProvider.filter(opContext : TTxOperationContext; forExpa
           includeRoot := false;
         result := TFhirCodeSystemProviderFilterContext.Create;
         try
-          iterateCodes(code.concept, result as TFhirCodeSystemProviderFilterContext, allCodes, nil, includeRoot);
+          iterateCodes(opContext, 'filter('+prop+CODES_TFhirFilterOperator[op]+value+')', code.concept, result as TFhirCodeSystemProviderFilterContext, allCodes, nil, includeRoot);
           result.link;
         finally
           result.free;
@@ -1711,7 +1720,7 @@ function TFhirCodeSystemProvider.filter(opContext : TTxOperationContext; forExpa
         result := TFhirCodeSystemProviderFilterContext.Create;
         try
           for cc in FCs.CodeSystem.conceptList do
-            iterateCodes(cc, result as TFhirCodeSystemProviderFilterContext, allCodes, code.concept, true);
+            iterateCodes(opContext, 'filter('+prop+CODES_TFhirFilterOperator[op]+value+')', cc, result as TFhirCodeSystemProviderFilterContext, allCodes, code.concept, true);
           result.link;
         finally
           result.free;
@@ -1732,6 +1741,7 @@ function TFhirCodeSystemProvider.filter(opContext : TTxOperationContext; forExpa
         begin
           code := doLocate(value, nil);
           try
+            deadCheck(opContext, 'filter-1', 'filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
             if code = nil then
               raise ETerminologyError.Create('Unable to locate code '+value, itUnknown)
             else
@@ -1754,9 +1764,9 @@ function TFhirCodeSystemProvider.filter(opContext : TTxOperationContext; forExpa
     try
       for cc in FCs.CodeSystem.conceptList do
         if value = 'true' then
-          iterateCodes(cc, result as TFhirCodeSystemProviderFilterContext, nonLeafCodes, nil, true)
+          iterateCodes(opContext, 'filter('+prop+CODES_TFhirFilterOperator[op]+value+')', cc, result as TFhirCodeSystemProviderFilterContext, nonLeafCodes, nil, true)
         else
-          iterateCodes(cc, result as TFhirCodeSystemProviderFilterContext, leafCodes, nil, true);
+          iterateCodes(opContext, 'filter('+prop+CODES_TFhirFilterOperator[op]+value+')', cc, result as TFhirCodeSystemProviderFilterContext, leafCodes, nil, true);
       result.link;
     finally
       result.free;
diff --git a/library/ftx/ftx_lang.pas b/library/ftx/ftx_lang.pas
index 430473fbf..de47a92c0 100644
--- a/library/ftx/ftx_lang.pas
+++ b/library/ftx/ftx_lang.pas
@@ -34,7 +34,7 @@ interface
 
 uses
   SysUtils, Classes, Generics.Collections,
-  fsl_utilities, fsl_stream, fsl_base, fsl_http, fsl_lang,
+  fsl_utilities, fsl_stream, fsl_base, fsl_http, fsl_lang, fsl_threads,
   fhir_objects, fhir_common, fhir_features, fhir_uris,
   ftx_service;
 
@@ -299,6 +299,7 @@ function TIETFLanguageCodeServices.filter(opContext : TTxOperationContext; forEx
 var
   i : integer;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   i := StringArrayIndexOfSensitive(CODES_TIETFLanguageComponent, prop);
   if (i >= 0) and (op = foExists) and ((value = 'true') or (value = 'false')) then
     result := TIETFLanguageCodeFilter.Create(TIETFLanguageComponent(i), value = 'true')
diff --git a/library/ftx/ftx_loinc_services.pas b/library/ftx/ftx_loinc_services.pas
index c7e4ffa11..329843407 100644
--- a/library/ftx/ftx_loinc_services.pas
+++ b/library/ftx/ftx_loinc_services.pas
@@ -1141,6 +1141,7 @@ function TLOINCServices.filter(opContext : TTxOperationContext; forExpansion, fo
   s : string;
   d : String;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   d := prop+' '+CODES_TFhirFilterOperator[op]+' '+value;
   c := FDB.getConnection('filterBySQL');
   try
diff --git a/library/ftx/ftx_sct_services.pas b/library/ftx/ftx_sct_services.pas
index 3f44a3988..2073db16b 100644
--- a/library/ftx/ftx_sct_services.pas
+++ b/library/ftx/ftx_sct_services.pas
@@ -5262,6 +5262,7 @@ function TSnomedProvider.filter(opContext : TTxOperationContext; forExpansion, f
 var
   id : UInt64;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   result := nil;
   if (prop = 'concept') and FSct.StringIsId(value, id) then
     if op = foIsA then
diff --git a/library/ftx/ftx_service.pas b/library/ftx/ftx_service.pas
index bb86252cd..b8e3ec076 100644
--- a/library/ftx/ftx_service.pas
+++ b/library/ftx/ftx_service.pas
@@ -67,6 +67,7 @@   ETerminologyError = class (EFslException) // problem in terminology operation
   TTxOperationContext = class abstract (TFslObject)
   public
     procedure log(note : String); virtual; abstract;
+    function deadCheck(var time : integer) : boolean; virtual; abstract;
   end;
 
   TCodeSystemProviderContext = class (TFslObject)
@@ -240,6 +241,7 @@   TCodeSystemProvider = class abstract (TFslObject)
     FLanguages : TIETFLanguageDefinitions;
     FI18n : TI18nSupport;
     procedure setDefLang(value : TIETFLang);
+    procedure deadCheck(opContext: TTxOperationContext; place, op: String);
   public
     constructor Create(languages : TIETFLanguageDefinitions; i18n : TI18nSupport);
     destructor Destroy; override;
@@ -902,6 +904,14 @@ procedure TCodeSystemProvider.setDefLang(value: TIETFLang);
   FDefLang := value;
 end;
 
+procedure TCodeSystemProvider.deadCheck(opContext: TTxOperationContext; place, op: String);
+var
+  time : integer;
+begin
+  if opContext.deadCheck(time) then
+    raise ETooCostly.create(FI18n.translate('CODESYSTEM_TOO_COSTLY_TIME', nil, [inttostr(time), op, place]));
+end;
+
 constructor TCodeSystemProvider.Create(languages: TIETFLanguageDefinitions; i18n : TI18nSupport);
 begin
   inherited Create;
diff --git a/library/ftx/ftx_ucum_services.pas b/library/ftx/ftx_ucum_services.pas
index dc4f1bf76..b9fbe7b2a 100644
--- a/library/ftx/ftx_ucum_services.pas
+++ b/library/ftx/ftx_ucum_services.pas
@@ -34,7 +34,7 @@
 
 Uses
   SysUtils, Classes,
-  fsl_base, fsl_utilities, fsl_collections, fsl_stream, fsl_xml, fsl_ucum, fsl_http, fsl_lang, fsl_i18n,
+  fsl_base, fsl_utilities, fsl_collections, fsl_stream, fsl_xml, fsl_ucum, fsl_http, fsl_lang, fsl_i18n, fsl_threads,
   ftx_ucum_handlers, ftx_ucum_validators, ftx_ucum_expressions, ftx_ucum_base,
   fhir_common, fhir_features, fhir_uris,
   fhir_cdshooks,
@@ -1053,6 +1053,7 @@ function TUcumServices.TotalCount: integer;
 
 function TUcumServices.filter(opContext : TTxOperationContext; forExpansion, forIteration : boolean; prop: String; op: TFhirFilterOperator; value: String; prep : TCodeSystemProviderFilterPreparationContext): TCodeSystemProviderFilterContext;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   if (prop = 'canonical') and (op in [foEqual]) then
     result := TUcumFilterContext.create(value)
   else
diff --git a/release-notes.md b/release-notes.md
index b3d636a04..e47537a9b 100644
--- a/release-notes.md
+++ b/release-notes.md
@@ -1,7 +1,7 @@
 ## Change Notes:
 
-* 
+* Add denial of service protection inside CodeSystem iteration
 
 ## Conformance Notes:
 
-* 
\ No newline at end of file
+* tx.fhir.org passed 300 HL7 terminology service tests (mode 'tx.fhir.org', tests v1.7.5, runner v6.5.8)
\ No newline at end of file
diff --git a/server/tx/tx_areacode.pas b/server/tx/tx_areacode.pas
index 037089b52..59b991e80 100644
--- a/server/tx/tx_areacode.pas
+++ b/server/tx/tx_areacode.pas
@@ -34,7 +34,7 @@ interface
 
 uses
   SysUtils, Classes,
-  fsl_utilities, fsl_http, fsl_lang, fsl_base, fsl_stream, fsl_i18n,
+  fsl_utilities, fsl_http, fsl_lang, fsl_base, fsl_stream, fsl_i18n, fsl_threads,
   fhir_objects, fhir_common, fhir_features,
   ftx_service;
 
@@ -544,6 +544,7 @@ function TAreaCodeServices.filter(opContext : TTxOperationContext; forExpansion,
   res : TAreaCodeConceptFilter;
   c : TAreaCodeConcept;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   if ((prop = 'type') or (prop = 'class')) and (op = foEqual) then
   begin
     res := TAreaCodeConceptFilter.Create;
diff --git a/server/tx/tx_countrycode.pas b/server/tx/tx_countrycode.pas
index d0a8ac118..53c3f7eb3 100644
--- a/server/tx/tx_countrycode.pas
+++ b/server/tx/tx_countrycode.pas
@@ -34,7 +34,7 @@ interface
 
 uses
   SysUtils, Classes, 
-  fsl_utilities, fsl_base, fsl_stream, fsl_http, fsl_fpc, fsl_lang, fsl_regex, fsl_i18n,
+  fsl_utilities, fsl_base, fsl_stream, fsl_http, fsl_fpc, fsl_lang, fsl_regex, fsl_i18n, fsl_threads,
   fhir_objects, fhir_common, fhir_features, fhir_uris,
   ftx_service;
 
@@ -1058,6 +1058,7 @@ function TCountryCodeServices.filter(opContext : TTxOperationContext; forExpansi
   list : TCountryCodeConceptFilter;
   concept : TCountryCodeConcept;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   if (op = foRegex) and (prop = 'code') then
   begin
     list := TCountryCodeConceptFilter.Create;
diff --git a/server/tx/tx_cpt.pas b/server/tx/tx_cpt.pas
index ff6cbf92f..1ed1e4b2f 100644
--- a/server/tx/tx_cpt.pas
+++ b/server/tx/tx_cpt.pas
@@ -857,6 +857,7 @@ function TCPTServices.filter(opContext : TTxOperationContext; forExpansion, forI
   item : TCPTConcept;
   b : boolean;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   // filters supported
   //  * modified = false
   //  * modifier = true / false
diff --git a/server/tx/tx_iso_4217.pas b/server/tx/tx_iso_4217.pas
index ac5ee850e..40b9d3d7d 100644
--- a/server/tx/tx_iso_4217.pas
+++ b/server/tx/tx_iso_4217.pas
@@ -35,7 +35,7 @@ interface
 
 uses
   SysUtils, Classes,
-  fsl_utilities, fsl_base, fsl_stream, fsl_lang, fsl_http, fsl_i18n,
+  fsl_utilities, fsl_base, fsl_stream, fsl_lang, fsl_http, fsl_i18n, fsl_threads,
   fhir_objects, fhir_common, fhir_features,
   ftx_service;
 
@@ -250,6 +250,7 @@ function TIso4217Services.filter(opContext : TTxOperationContext; forExpansion,
   res : TIso4217ConceptFilter;
   c : TIso4217Currency;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   if (prop = 'decimals') and (op = foEqual) then
   begin
     res := TIso4217ConceptFilter.Create;
diff --git a/server/tx/tx_ndc.pas b/server/tx/tx_ndc.pas
index 12b4d67dc..d25b2785d 100644
--- a/server/tx/tx_ndc.pas
+++ b/server/tx/tx_ndc.pas
@@ -1141,6 +1141,7 @@ function TNDCServices.filter(opContext : TTxOperationContext; forExpansion, forI
   ctxt : TNDCFilterPreparationContext;
   res : TNDCFilterContext;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   res := nil;
   try
     ctxt := prep as TNDCFilterPreparationContext;
diff --git a/server/tx/tx_omop.pas b/server/tx/tx_omop.pas
index a0f2582e4..6f4ce25c1 100644
--- a/server/tx/tx_omop.pas
+++ b/server/tx/tx_omop.pas
@@ -375,6 +375,7 @@ function TOMOPServices.filter(opContext : TTxOperationContext; forExpansion, for
 var
   f : TOMOPFilter;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   if (prop = 'domain') and (op = foEqual) then
   begin
     f := TOMOPFilter.Create;
diff --git a/server/tx/tx_rxnorm.pas b/server/tx/tx_rxnorm.pas
index 4a2d57a8f..98ff26ec9 100644
--- a/server/tx/tx_rxnorm.pas
+++ b/server/tx/tx_rxnorm.pas
@@ -878,6 +878,7 @@ function TUMLSServices.filter(opContext : TTxOperationContext; forExpansion, for
   res : TUMLSFilter;
   ok : boolean;
 begin
+  SetThreadStatus(ClassName+'.filter('+prop+CODES_TFhirFilterOperator[op]+value+')');
   prop := prop.toUpper;
   res := TUMLSFilter.Create;
   try