diff --git a/pom.xml b/pom.xml
index 0d84b0d2..da2e14da 100644
--- a/pom.xml
+++ b/pom.xml
@@ -150,20 +150,8 @@
     </dependency>
     <dependency>
       <groupId>it.unimi.dsi</groupId>
-      <artifactId>dsiutils</artifactId>
-      <version>2.2.8</version>
-      <scope>compile</scope>
-      <exclusions>
-        <exclusion>
-          <groupId>ch.qos.logback</groupId>
-          <artifactId>logback-classic</artifactId>
-        </exclusion>
-        <!-- exclude the vulnerable commons-collections v3.2.1 -->
-        <exclusion>
-          <groupId>commons-collections</groupId>
-          <artifactId>commons-collections</artifactId>
-        </exclusion>
-      </exclusions>
+      <artifactId>fastutil</artifactId>
+      <version>7.0.10</version>
     </dependency>
 
     <!-- explicitly require a patched commons-collections to avoid vulnerable v3.2.1 -->
diff --git a/src/main/java/org/archive/url/UsableURIFactory.java b/src/main/java/org/archive/url/UsableURIFactory.java
index d44b5c84..3dfc33a7 100644
--- a/src/main/java/org/archive/url/UsableURIFactory.java
+++ b/src/main/java/org/archive/url/UsableURIFactory.java
@@ -20,7 +20,6 @@
 
 import gnu.inet.encoding.IDNA;
 import gnu.inet.encoding.IDNAException;
-import it.unimi.dsi.lang.MutableString;
 
 import java.io.UnsupportedEncodingException;
 import java.util.BitSet;
@@ -485,7 +484,7 @@ private String fixup(String uri, final URI base, final String charset)
 
         // Preallocate.  The '1's and '2's in below are space for ':',
         // '//', etc. URI characters.
-        MutableString s = new MutableString(
+        StringBuilder s = new StringBuilder(
             ((uriScheme != null)? uriScheme.length(): 0)
             + 1 // ';' 
             + ((uriAuthority != null)? uriAuthority.length(): 0)
@@ -707,7 +706,7 @@ private String checkPort(String uriAuthority)
      * @param substr Suffix or prefix to use if <code>str</code> is not null.
      * @param suffix True if <code>substr</code> is a suffix.
      */
-    private void appendNonNull(MutableString b, String str, String substr,
+    private void appendNonNull(StringBuilder b, String str, String substr,
             boolean suffix) {
         if (str != null && str.length() > 0) {
             if (!suffix) {