diff --git a/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirect.qll b/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirect.qll
index 5102b53f0ce..f7493f44131 100644
--- a/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirect.qll
+++ b/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirect.qll
@@ -50,5 +50,9 @@ module ClientSideUrlRedirect {
       g instanceof DocumentUrl and
       succ.(DataFlow::PropRead).accesses(pred, "href")
     }
+
+    override predicate isSanitizerGuard(TaintTracking::SanitizerGuardNode guard) {
+      guard instanceof HostnameSanitizerGuard
+    }
   }
 }
diff --git a/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirectCustomizations.qll b/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirectCustomizations.qll
index 8c2211b086f..bb213e227e2 100644
--- a/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirectCustomizations.qll
+++ b/javascript/ql/src/semmle/javascript/security/dataflow/ClientSideUrlRedirectCustomizations.qll
@@ -6,7 +6,7 @@
 
 import javascript
 import semmle.javascript.security.dataflow.RemoteFlowSources
-import UrlConcatenation
+private import UrlConcatenation
 
 module ClientSideUrlRedirect {
   private import Xss::DomBasedXss as DomBasedXss
diff --git a/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirect.qll b/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirect.qll
index ac69497eada..f2d6cfb1652 100644
--- a/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirect.qll
+++ b/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirect.qll
@@ -34,7 +34,8 @@ module ServerSideUrlRedirect {
     }
 
     override predicate isSanitizerGuard(TaintTracking::SanitizerGuardNode guard) {
-      guard instanceof LocalUrlSanitizingGuard
+      guard instanceof LocalUrlSanitizingGuard or
+      guard instanceof HostnameSanitizerGuard
     }
   }
 }
diff --git a/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirectCustomizations.qll b/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirectCustomizations.qll
index 8806e1811e1..3b6e7db7322 100644
--- a/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirectCustomizations.qll
+++ b/javascript/ql/src/semmle/javascript/security/dataflow/ServerSideUrlRedirectCustomizations.qll
@@ -6,7 +6,7 @@
 
 import javascript
 import RemoteFlowSources
-import UrlConcatenation
+private import UrlConcatenation
 
 module ServerSideUrlRedirect {
   /**
diff --git a/javascript/ql/src/semmle/javascript/security/dataflow/UrlConcatenation.qll b/javascript/ql/src/semmle/javascript/security/dataflow/UrlConcatenation.qll
index 8d6a3014822..e32826b79be 100644
--- a/javascript/ql/src/semmle/javascript/security/dataflow/UrlConcatenation.qll
+++ b/javascript/ql/src/semmle/javascript/security/dataflow/UrlConcatenation.qll
@@ -96,3 +96,17 @@ predicate hostnameSanitizingPrefixEdge(DataFlow::Node source, DataFlow::Node sin
     hasHostnameSanitizingSubstring(StringConcatenation::getOperand(operator, [0 .. n - 1]))
   )
 }
+
+/**
+ * A check that sanitizes the hostname of a URL.
+ */
+class HostnameSanitizerGuard extends TaintTracking::SanitizerGuardNode, StringOps::StartsWith {
+  HostnameSanitizerGuard() {
+    hasHostnameSanitizingSubstring(getSubstring())
+  }
+
+  override predicate sanitizes(boolean outcome, Expr e) {
+    outcome = getPolarity() and
+    e = getBaseString().asExpr()
+  }
+}
diff --git a/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/ClientSideUrlRedirect.expected b/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/ClientSideUrlRedirect.expected
index 542731ea9e4..02cf0c015d5 100644
--- a/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/ClientSideUrlRedirect.expected
+++ b/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/ClientSideUrlRedirect.expected
@@ -4,12 +4,6 @@ nodes
 | sanitizer.js:2:15:2:25 | window.name |
 | sanitizer.js:4:27:4:29 | url |
 | sanitizer.js:4:27:4:29 | url |
-| sanitizer.js:7:27:7:29 | url |
-| sanitizer.js:7:27:7:29 | url |
-| sanitizer.js:10:27:10:29 | url |
-| sanitizer.js:10:27:10:29 | url |
-| sanitizer.js:13:27:13:29 | url |
-| sanitizer.js:13:27:13:29 | url |
 | sanitizer.js:16:27:16:29 | url |
 | sanitizer.js:16:27:16:29 | url |
 | sanitizer.js:19:27:19:29 | url |
@@ -22,8 +16,6 @@ nodes
 | sanitizer.js:28:27:28:29 | url |
 | sanitizer.js:31:27:31:29 | url |
 | sanitizer.js:31:27:31:29 | url |
-| sanitizer.js:34:27:34:29 | url |
-| sanitizer.js:34:27:34:29 | url |
 | sanitizer.js:37:27:37:29 | url |
 | sanitizer.js:37:27:37:29 | url |
 | tst2.js:2:7:2:33 | href |
@@ -109,12 +101,6 @@ nodes
 edges
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:4:27:4:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:4:27:4:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:7:27:7:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:7:27:7:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:10:27:10:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:10:27:10:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:13:27:13:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:13:27:13:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:16:27:16:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:16:27:16:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:19:27:19:29 | url |
@@ -127,8 +113,6 @@ edges
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:28:27:28:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:31:27:31:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:31:27:31:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:34:27:34:29 | url |
-| sanitizer.js:2:9:2:25 | url | sanitizer.js:34:27:34:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:37:27:37:29 | url |
 | sanitizer.js:2:9:2:25 | url | sanitizer.js:37:27:37:29 | url |
 | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:2:9:2:25 | url |
@@ -209,16 +193,12 @@ edges
 | tst.js:6:34:6:55 | documen ... on.href | tst.js:6:20:6:56 | indirec ... n.href) |
 #select
 | sanitizer.js:4:27:4:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:4:27:4:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
-| sanitizer.js:7:27:7:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:7:27:7:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
-| sanitizer.js:10:27:10:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:10:27:10:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
-| sanitizer.js:13:27:13:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:13:27:13:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:16:27:16:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:16:27:16:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:19:27:19:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:19:27:19:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:22:27:22:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:22:27:22:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:25:27:25:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:25:27:25:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:28:27:28:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:28:27:28:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:31:27:31:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:31:27:31:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
-| sanitizer.js:34:27:34:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:34:27:34:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | sanitizer.js:37:27:37:29 | url | sanitizer.js:2:15:2:25 | window.name | sanitizer.js:37:27:37:29 | url | Untrusted URL redirection due to $@. | sanitizer.js:2:15:2:25 | window.name | user-provided value |
 | tst2.js:4:21:4:55 | href.su ... '?')+1) | tst2.js:2:14:2:28 | window.location | tst2.js:4:21:4:55 | href.su ... '?')+1) | Untrusted URL redirection due to $@. | tst2.js:2:14:2:28 | window.location | user-provided value |
 | tst6.js:4:21:4:28 | redirect | tst6.js:2:18:2:45 | $locati ... irect') | tst6.js:4:21:4:28 | redirect | Untrusted URL redirection due to $@. | tst6.js:2:18:2:45 | $locati ... irect') | user-provided value |
diff --git a/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/sanitizer.js b/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/sanitizer.js
index 247f668f9b8..f8d5b805f4b 100644
--- a/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/sanitizer.js
+++ b/javascript/ql/test/query-tests/Security/CWE-601/ClientSideUrlRedirect/sanitizer.js
@@ -4,13 +4,13 @@ function f() {
         window.location = url; // NOT OK - can be example.com.evil.com
     }
     if (url.startsWith('https://example.com/')) {
-        window.location = url; // OK - but flagged anyway
+        window.location = url; // OK
     }
     if (url.startsWith('https://example.com//')) {
-        window.location = url; // OK - but flagged anyway
+        window.location = url; // OK
     }
     if (url.startsWith('https://example.com/foo')) {
-        window.location = url; // OK - but flagged anyway 
+        window.location = url; // OK 
     }
     if (url.startsWith('https://')) {
         window.location = url; // NOT OK - does not restrict hostname
@@ -31,9 +31,13 @@ function f() {
         window.location = url; // NOT OK - can be //example.com.evil.com
     }
     if (url.startsWith('//example.com/')) {
-        window.location = url; // OK - but flagged anyway
+        window.location = url; // OK
     }
     if (url.endsWith('https://example.com/')) {
         window.location = url; // NOT OK - could be evil.com?x=https://example.com/
     }
+    let basedir = whatever() ? 'foo' : 'bar';
+    if (url.startsWith('https://example.com/' + basedir)) {
+        window.location = url; // OK - the whole prefix is not known, but enough to restrict hostname
+    }
 }