Add local file writes as external location sinks

Author: joefarebrother

csharp/ql/lib/semmle/code/csharp/security/dataflow/flowsinks/ExternalLocationSink.qll

@@ -6,7 +6,9 @@ import csharp
 private import Remote
 private import semmle.code.csharp.commons.Loggers
 private import semmle.code.csharp.frameworks.system.Web
+private import semmle.code.csharp.frameworks.system.IO
 private import semmle.code.csharp.dataflow.ExternalFlow
+private import semmle.code.csharp.dataflow.DataFlow
 
 /**
  * An external location sink.
@@ -63,3 +65,66 @@ class CookieStorageSink extends ExternalLocationSink, RemoteFlowSink {
     )
   }
 }
+
+private predicate isFileWriteCall(Expr stream, Expr data) {
+  exists(MethodCall mc, Method m | mc.getTarget() = m.getAnOverrider*() |
+    mc.getTarget().hasQualifiedName("System.IO", "Stream", ["Write", "WriteAsync"]) and
+    stream = mc.getQualifier() and
+    data = mc.getArgument(0)
+    or
+    mc.getTarget()
+        .hasQualifiedName("System.IO", "TextWriter",
+          ["Write", "WriteAsync", "WriteLine", "WriteLineAsync"]) and
+    stream = mc.getQualifier() and
+    data = mc.getArgument(0)
+    or
+    mc.getTarget().hasQualifiedName("System.Xml.Linq", "XDocument", ["Save", "SaveAsync"]) and
+    data = mc.getQualifier() and
+    stream = mc.getArgument(0)
+  )
+}
+
+private module LocalFileOutputStreamConfig implements DataFlow::ConfigSig {
+  predicate isSource(DataFlow::Node src) {
+    exists(MethodCall mc | mc = src.asExpr() |
+      mc.getTarget().hasQualifiedName("System.IO", "File", ["Open", "Create", "OpenWrite"])
+      or
+      mc.getTarget()
+          .hasQualifiedName("System.IO", "FileInfo",
+            ["AppendText", "Create", "CreateText", "Open", "OpenText", "OpenWrite"])
+    )
+    or
+    exists(ObjectCreation oc | oc = src.asExpr() |
+      oc.getObjectType() instanceof SystemIOStreamWriterClass and
+      oc.getArgument(0).getType() instanceof StringType
+    )
+  }
+
+  predicate isSink(DataFlow::Node sink) { isFileWriteCall(sink.asExpr(), _) }
+
+  predicate isBarrier(DataFlow::Node node) {
+    node.asExpr()
+        .(ObjectCreation)
+        .getObjectType()
+        .hasQualifiedName("System.Security.Cryptography", "CryptoStream")
+  }
+
+  predicate isAdditionalFlowStep(DataFlow::Node node1, DataFlow::Node node2) {
+    exists(ObjectCreation oc |
+      node2.asExpr() = oc and
+      node1.asExpr() = oc.getArgument(0) and
+      oc.getObjectType() instanceof SystemIOStreamWriterClass
+    )
+  }
+}
+
+private module LocalFileOutputStreamFlow = DataFlow::Make<LocalFileOutputStreamConfig>;
+
+class LocalFileOutputSink extends ExternalLocationSink {
+  LocalFileOutputSink() {
+    exists(DataFlow::Node streamSink |
+      LocalFileOutputStreamFlow::hasFlow(_, streamSink) and
+      isFileWriteCall(streamSink.asExpr(), this.asExpr())
+    )
+  }
+}