Secure ApiUser::GetByAuthHeader() against timing attacks

(cherry picked from commit 9558ebc0f46febc7692bbb65394708b78b276d46)

lib/base/utility.cpp

@@ -26,6 +26,7 @@
 #include "base/utility.hpp"
 #include "base/json.hpp"
 #include "base/objectlock.hpp"
+#include <cstdint>
 #include <mmatch.h>
 #include <boost/lexical_cast.hpp>
 #include <boost/thread/tss.hpp>
@@ -1949,3 +1950,38 @@ String Utility::GetFromEnvironment(const String& env)
 	return String();
 #endif /* _WIN32 */
 }
+
+/**
+ * Compare the password entered by a client with the actual password.
+ * The comparision is safe against timing attacks.
+ */
+bool Utility::ComparePasswords(const String& enteredPassword, const String& actualPassword)
+{
+	volatile const char * volatile enteredPasswordCStr = enteredPassword.CStr();
+	volatile size_t enteredPasswordLen = enteredPassword.GetLength();
+
+	volatile const char * volatile actualPasswordCStr = actualPassword.CStr();
+	volatile size_t actualPasswordLen = actualPassword.GetLength();
+
+	volatile uint_fast8_t result = enteredPasswordLen == actualPasswordLen;
+
+	if (result) {
+		auto cStr (actualPasswordCStr);
+		auto len (actualPasswordLen);
+
+		actualPasswordCStr = cStr;
+		actualPasswordLen = len;
+	} else {
+		auto cStr (enteredPasswordCStr);
+		auto len (enteredPasswordLen);
+
+		actualPasswordCStr = cStr;
+		actualPasswordLen = len;
+	}
+
+	for (volatile size_t i = 0; i < enteredPasswordLen; ++i) {
+		result &= uint_fast8_t(enteredPasswordCStr[i] == actualPasswordCStr[i]);
+	}
+
+	return result;
+}

lib/base/utility.hpp

@@ -149,6 +149,8 @@ public:
 
 	static String GetFromEnvironment(const String& env);
 
+	static bool ComparePasswords(const String& enteredPassword, const String& actualPassword);
+
 #ifdef I2_DEBUG
 	static void SetTime(double);
 	static void IncrementTime(double);

lib/remote/apiuser.cpp

@@ -22,6 +22,7 @@
 #include "base/configtype.hpp"
 #include "base/base64.hpp"
 #include "base/tlsutility.hpp"
+#include "base/utility.hpp"
 
 using namespace icinga;
 
@@ -63,7 +64,7 @@ ApiUser::Ptr ApiUser::GetByAuthHeader(const String& auth_header)
 	 */
 	if (!user || password.IsEmpty())
 		return nullptr;
-	else if (user && user->GetPassword() != password)
+	else if (user && !Utility::ComparePasswords(password, user->GetPassword()))
 		return nullptr;
 
 	return user;