add model for the unified and remark libraries

javascript/ql/src/semmle/javascript/frameworks/Markdown.qll

@@ -48,3 +48,61 @@ private class ShowDownStep extends TaintTracking::AdditionalTaintStep, DataFlow:
     pred = this.getArgument(0)
   }
 }
+
+/**
+ * Classes and predicates for modelling taint steps in `unified` and `remark`.
+ */
+private module Unified {
+  /**
+   * The creation of a parser from `unified`.
+   * The `remark` module is a shorthand that initializes `unified` with a markdown parser.
+   */
+  DataFlow::CallNode unified() { result = DataFlow::moduleImport(["unified", "remark"]).getACall() }
+
+  /**
+   * A chain of method calls that process an input with `unified`.
+   */
+  class UnifiedChain extends DataFlow::CallNode {
+    DataFlow::CallNode root;
+
+    UnifiedChain() {
+      root = unified() and
+      this = root.getAChainedMethodCall(["process", "processSync"])
+    }
+
+    /**
+     * Gets a plugin that is used in this chain.
+     */
+    DataFlow::Node getAUsedPlugin() { result = root.getAChainedMethodCall("use").getArgument(0) }
+
+    /**
+     * Gets the input that is processed.
+     */
+    DataFlow::Node getInput() { result = getArgument(0) }
+
+    /**
+     * Gets the processed output.
+     */
+    DataFlow::Node getOutput() {
+      this.getCalleeName() = "process" and result = getABoundCallbackParameter(1, 1)
+      or
+      this.getCalleeName() = "processSync" and result = this
+    }
+  }
+
+  /**
+   * A taint step for the `unified` library.
+   */
+  class UnifiedStep extends TaintTracking::AdditionalTaintStep, UnifiedChain {
+    UnifiedStep() {
+      // sanitizer. Mostly looking for `rehype-sanitize`, but also other plugins with `sanitize` in their name.
+      not this.getAUsedPlugin().getALocalSource() =
+        DataFlow::moduleImport(any(string s | s.matches("%sanitize%")))
+    }
+
+    override predicate step(DataFlow::Node pred, DataFlow::Node succ) {
+      pred = getInput() and
+      succ = getOutput()
+    }
+  }
+}