diff --git a/src/main/java/com/google/crypto/tink/subtle/Base64.java b/src/main/java/com/google/crypto/tink/subtle/Base64.java
index 5c6563ead..9f06b1de3 100644
--- a/src/main/java/com/google/crypto/tink/subtle/Base64.java
+++ b/src/main/java/com/google/crypto/tink/subtle/Base64.java
@@ -395,11 +395,26 @@ public boolean process(byte[] input, int offset, int len, boolean finish) {
           this.state = 6;
           return false;
         case 2:
+          // The final quantum carries twelve bits but only eight are used, so
+          // the low four bits must be zero: RFC 4648 section 3.5 requires the
+          // canonical encoding.  Without this check a non-canonical encoding of
+          // the same bytes is accepted, which makes two distinct strings decode
+          // to one value (signature malleability for JWS inputs).
+          if ((value & 0xF) != 0) {
+            this.state = 6;
+            return false;
+          }
           // Read two extra input bytes, enough to emit 1 more
           // output byte.  Fine.
           output[op++] = (byte) (value >> 4);
           break;
         case 3:
+          // The final quantum carries eighteen bits but only sixteen are used,
+          // so the low two bits must be zero (RFC 4648 section 3.5).
+          if ((value & 0x3) != 0) {
+            this.state = 6;
+            return false;
+          }
           // Read three extra input bytes, enough to emit 2 more
           // output bytes.  Fine.
           output[op++] = (byte) (value >> 10);
diff --git a/src/test/java/com/google/crypto/tink/subtle/Base64StrictDecodingTest.java b/src/test/java/com/google/crypto/tink/subtle/Base64StrictDecodingTest.java
new file mode 100644
index 000000000..0663aacdd
--- /dev/null
+++ b/src/test/java/com/google/crypto/tink/subtle/Base64StrictDecodingTest.java
@@ -0,0 +1,56 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+////////////////////////////////////////////////////////////////////////////////
+
+package com.google.crypto.tink.subtle;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/**
+ * RFC 4648 section 3.5 requires decoders to reject a final quantum whose unused bits are not zero.
+ * Without that check a non-canonical string decodes to the same bytes as its canonical
+ * equivalent, which for JWS/JWT inputs means two distinct compact serialisations verify against one
+ * signature (signature malleability).
+ */
+@RunWith(JUnit4.class)
+public final class Base64StrictDecodingTest {
+
+  // RFC 7515 Appendix A.1.1 signature. The two strings differ only in the unused
+  // trailing bits of the final character ('k' has zero, 'l' does not).
+  private static final String CANONICAL = "dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk";
+  private static final String NON_CANONICAL = "dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXl";
+
+  @Test
+  public void urlSafeDecode_canonicalSignature_isAccepted() {
+    assertThat(Base64.urlSafeDecode(CANONICAL)).hasLength(32);
+  }
+
+  @Test
+  public void urlSafeDecode_nonCanonicalTrailingBits_isRejected() {
+    assertThrows(IllegalArgumentException.class, () -> Base64.urlSafeDecode(NON_CANONICAL));
+  }
+
+  @Test
+  public void urlSafeDecode_validShortForms_stillDecode() {
+    assertThat(Base64.urlSafeDecode("QQ")).hasLength(1);
+    assertThat(Base64.urlSafeDecode("QUI")).hasLength(2);
+    assertThat(Base64.urlSafeDecode("QUJD")).hasLength(3);
+  }
+}