diff --git a/web/src/main/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilter.java b/web/src/main/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilter.java
index 0a77d570a2..2ab32598ce 100755
--- a/web/src/main/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilter.java
+++ b/web/src/main/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilter.java
@@ -143,6 +143,8 @@ public abstract class AbstractPreAuthenticatedProcessingFilter extends GenericFi
         logger.debug("Pre-authenticated principal has changed to " + principal + " and will be reauthenticated");
 
         if (invalidateSessionOnPrincipalChange) {
+            SecurityContextHolder.clearContext();
+
             HttpSession session = request.getSession(false);
 
             if (session != null) {
diff --git a/web/src/test/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilterTests.java b/web/src/test/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilterTests.java
index a61f1436fa..cd4cdd907c 100644
--- a/web/src/test/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilterTests.java
+++ b/web/src/test/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilterTests.java
@@ -7,6 +7,7 @@ import static org.mockito.Mockito.*;
 import javax.servlet.FilterChain;
 import javax.servlet.http.HttpServletRequest;
 
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.mockito.invocation.InvocationOnMock;
@@ -16,6 +17,7 @@ import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.security.authentication.AuthenticationManager;
 import org.springframework.security.authentication.BadCredentialsException;
+import org.springframework.security.authentication.TestingAuthenticationToken;
 import org.springframework.security.core.Authentication;
 import org.springframework.security.core.context.SecurityContextHolder;
 
@@ -33,7 +35,12 @@ public class AbstractPreAuthenticatedProcessingFilterTests {
                 return "doesntmatter";
             }
         };
-        SecurityContextHolder.getContext().setAuthentication(null);
+        SecurityContextHolder.clearContext();
+    }
+
+    @After
+    public void tearDown() {
+        SecurityContextHolder.clearContext();
     }
 
     @Test
@@ -80,6 +87,31 @@ public class AbstractPreAuthenticatedProcessingFilterTests {
         testDoFilter(false);
     }
 
+    // SEC-1968
+    @Test
+    public void nullPreAuthenticationClearsPreviousUser() throws Exception {
+        SecurityContextHolder.getContext().setAuthentication(new TestingAuthenticationToken("oldUser", "pass","ROLE_USER"));
+        ConcretePreAuthenticatedProcessingFilter filter = new ConcretePreAuthenticatedProcessingFilter();
+        filter.principal = null;
+        filter.setCheckForPrincipalChanges(true);
+
+        filter.doFilter(new MockHttpServletRequest(), new MockHttpServletResponse(), new MockFilterChain());
+
+        assertEquals(null, SecurityContextHolder.getContext().getAuthentication());
+    }
+
+    @Test
+    public void nullPreAuthenticationPerservesPreviousUserCheckPrincipalChangesFalse() throws Exception {
+        TestingAuthenticationToken authentication = new TestingAuthenticationToken("oldUser", "pass","ROLE_USER");
+        SecurityContextHolder.getContext().setAuthentication(authentication);
+        ConcretePreAuthenticatedProcessingFilter filter = new ConcretePreAuthenticatedProcessingFilter();
+        filter.principal = null;
+
+        filter.doFilter(new MockHttpServletRequest(), new MockHttpServletResponse(), new MockFilterChain());
+
+        assertEquals(authentication, SecurityContextHolder.getContext().getAuthentication());
+    }
+
     private void testDoFilter(boolean grantAccess) throws Exception {
         MockHttpServletRequest req = new MockHttpServletRequest();
         MockHttpServletResponse res = new MockHttpServletResponse();
@@ -107,8 +139,9 @@ public class AbstractPreAuthenticatedProcessingFilterTests {
     }
 
     private static class ConcretePreAuthenticatedProcessingFilter extends AbstractPreAuthenticatedProcessingFilter {
+        private String principal = "testPrincipal";
         protected Object getPreAuthenticatedPrincipal(HttpServletRequest httpRequest) {
-            return "testPrincipal";
+            return principal;
         }
         protected Object getPreAuthenticatedCredentials(HttpServletRequest httpRequest) {
             return "testCredentials";