diff --git a/ql/lib/codeql/ruby/security/UnsafeDeserializationCustomizations.qll b/ql/lib/codeql/ruby/security/UnsafeDeserializationCustomizations.qll
index 875081caa48..84e61bd1ece 100644
--- a/ql/lib/codeql/ruby/security/UnsafeDeserializationCustomizations.qll
+++ b/ql/lib/codeql/ruby/security/UnsafeDeserializationCustomizations.qll
@@ -25,6 +25,13 @@ module UnsafeDeserialization {
    */
   abstract class Sanitizer extends DataFlow::Node { }
 
+  /**
+   * Additional taint steps for "unsafe deserialization" vulnerabilities.
+   */
+  predicate isAdditionalTaintStep(DataFlow::Node fromNode, DataFlow::Node toNode) {
+    base64DecodeTaintStep(fromNode, toNode)
+  }
+
   /** A source of remote user input, considered as a flow source for unsafe deserialization. */
   class RemoteFlowSourceAsSource extends Source {
     RemoteFlowSourceAsSource() { this instanceof RemoteFlowSource }
@@ -59,4 +66,18 @@ module UnsafeDeserialization {
       this = API::getTopLevelMember("JSON").getAMethodCall(["load", "restore"]).getArgument(0)
     }
   }
+
+  /**
+   * `Base64.decode64` propagates taint from its argument to its return value.
+   */
+  predicate base64DecodeTaintStep(DataFlow::Node fromNode, DataFlow::Node toNode) {
+    exists(DataFlow::CallNode callNode |
+      callNode =
+        API::getTopLevelMember("Base64")
+            .getAMethodCall(["decode64", "strict_decode64", "urlsafe_decode64"])
+    |
+      fromNode = callNode.getArgument(0) and
+      toNode = callNode
+    )
+  }
 }
diff --git a/ql/lib/codeql/ruby/security/UnsafeDeserializationQuery.qll b/ql/lib/codeql/ruby/security/UnsafeDeserializationQuery.qll
index f8d19ab76f4..d08b73da936 100644
--- a/ql/lib/codeql/ruby/security/UnsafeDeserializationQuery.qll
+++ b/ql/lib/codeql/ruby/security/UnsafeDeserializationQuery.qll
@@ -27,4 +27,8 @@ class Configuration extends TaintTracking::Configuration {
     super.isSanitizer(node) or
     node instanceof UnsafeDeserialization::Sanitizer
   }
+
+  override predicate isAdditionalTaintStep(DataFlow::Node fromNode, DataFlow::Node toNode) {
+    UnsafeDeserialization::isAdditionalTaintStep(fromNode, toNode)
+  }
 }