add unsafe HMAC comparison query and qlhelp file

ruby/ql/src/experimental/cwe-208/UnsafeHMACComparison.ql

@@ -0,0 +1,73 @@
+private import codeql.ruby.AST
+import codeql.ruby.AST
+import codeql.ruby.DataFlow
+import codeql.ruby.ApiGraphs
+import codeql.ruby.dataflow.RemoteFlowSources
+import codeql.ruby.ast.Operation
+import codeql.ruby.TaintTracking
+import ruby
+
+/**
+ * @kind problem
+ */
+
+// A call to OpenSSL::HMAC.hexdigest
+class OpenSSLHMACHexdigest extends DataFlow::Node {
+  OpenSSLHMACHexdigest() {
+    this = API::getTopLevelMember("OpenSSL").getMember("HMAC").getAMethodCall("hexdigest")
+  }
+}
+
+// A call to OpenSSL::HMAC.to_s (which is an alias for OpenSSL::HMAC.hexdigest)
+class OpenSSLHMACtos extends DataFlow::Node {
+  OpenSSLHMACtos() {
+    this = API::getTopLevelMember("OpenSSL").getMember("HMAC").getAMethodCall("to_s")
+  }
+}
+
+// A call to OpenSSL::HMAC.digest
+class OpenSSLHMACdigest extends DataFlow::Node {
+  OpenSSLHMACdigest() {
+    this = API::getTopLevelMember("OpenSSL").getMember("HMAC").getAMethodCall("digest")
+  }
+}
+
+// A call to OpenSSL::HMAC.new
+class OpenSSLnewHMAC extends DataFlow::Node {
+  OpenSSLnewHMAC() {
+    this = API::getTopLevelMember("OpenSSL").getMember("HMAC").getAnInstantiation()
+  }
+}
+
+// A call to OpenSSL::HMAC.base64digest
+class OpenSSLHmacbase64digest extends DataFlow::Node {
+  OpenSSLHmacbase64digest() {
+    this = API::getTopLevelMember("OpenSSL").getMember("HMAC").getAMethodCall("base64digest")
+  }
+}
+
+class Configuration extends DataFlow::Configuration {
+  Configuration() { this = "UnsafeHMACComparison" }
+
+  override predicate isSource(DataFlow::Node source) {
+    source instanceof OpenSSLHMACHexdigest or
+    source instanceof OpenSSLnewHMAC or
+    source instanceof OpenSSLHmacbase64digest or
+    source instanceof OpenSSLHMACdigest or
+    source instanceof OpenSSLHMACtos
+  }
+
+  // Holds if a given sink is an Equality Operation (== or !=)
+  override predicate isSink(DataFlow::Node sink) {
+    exists(EqualityOperation eqOp |
+      eqOp.getLeftOperand() = sink.asExpr().getExpr()
+      or
+      eqOp.getRightOperand() = sink.asExpr().getExpr()
+    )
+  }
+}
+
+from DataFlow::Node source, DataFlow::Node sink, Configuration config
+where config.hasFlow(source, sink)
+select sink,
+  "An HMAC is being compared using the equality operator.  This may be vulnerable to a cryptographic timing attack because the equality operation does not occur in constant time."

ruby/ql/src/experimental/cwe-208/UnsafeHMACComparison.qlhelp

@@ -0,0 +1,21 @@
+<!DOCTYPE qhelp PUBLIC
+"-//Semmle//qhelp//EN"
+"qhelp.dtd">
+<qhelp>
+  <overview>
+    <p>
+      Using the `==` or `!=` operator to compare a known valid HMAC with a user-supplied HMAC digest could lead to a timing attack, as these operations do not occur in constant time.
+    </p>
+  </overview>
+  <recommendation>
+    <p>
+      Instead of using `==` or `!=` to compare a known valid HMAC with a user-supplied HMAC digest use Rack::Utils#secure_compare, ActiveSupport::SecurityUtils#secure_compare or OpenSSL.secure_compare
+    </p>
+  </recommendation>
+  <example>
+    <p>
+    In this example, the HMAC is validated using the `==` operation.
+    </p>
+    <sample src="./examples/unsafe_hmac_comparison.rb" />
+  </example>
+</qhelp>

ruby/ql/src/experimental/cwe-208/examples/unsafe_hmac_comparison.rb

@@ -0,0 +1,11 @@
+class UnsafeHmacComparison
+    def verify_hmac(host, hmac, salt)
+      sha1 = OpenSSL::Digest.new('sha1')
+      if OpenSSL::HMAC.digest(sha1, salt, host) == hmac
+        puts "HMAC verified"
+      else
+        puts "HMAC not verified"
+      end
+    end
+  end
+