diff --git a/src/butil/third_party/modp_b64/modp_b64_rvv.cc b/src/butil/third_party/modp_b64/modp_b64_rvv.cc
index 10fd8847dd..4c76177918 100644
--- a/src/butil/third_party/modp_b64/modp_b64_rvv.cc
+++ b/src/butil/third_party/modp_b64/modp_b64_rvv.cc
@@ -150,11 +150,14 @@ size_t rvv_modp_b64_decode(char* dest, const char* src, size_t len)
 {
     if (len == 0) return 0;
 
-    /* Strip trailing '=' padding (up to 2 chars for valid base64) */
-    while (len > 0 && src[len-1] == '=') {
-        len--;
+    /* Match the padded scalar decoder's length and padding rules. */
+    if (len < 4 || len % 4 != 0) return MODP_B64_RVV_ERROR;
+    if (src[len-1] == '=') {
+        --len;
+        if (src[len-1] == '=') {
+            --len;
+        }
     }
-    if (len == 0) return 0;
 
     size_t i = 0;
     uint8_t* p = (uint8_t*)dest;
diff --git a/test/base64_unittest.cc b/test/base64_unittest.cc
index 40a3906a97..3f49642315 100644
--- a/test/base64_unittest.cc
+++ b/test/base64_unittest.cc
@@ -24,4 +24,16 @@ TEST(Base64Test, Basic) {
   EXPECT_EQ(kText, decoded);
 }
 
+TEST(Base64Test, RejectsMalformedPaddingAndLengths) {
+  for (const char* input : {"A", "AA", "AAA", "AAAAA", "=", "====",
+                            "A===", "AA======", "AA=A", "AA==AAAA"}) {
+    std::string decoded;
+    EXPECT_FALSE(butil::Base64Decode(input, &decoded)) << input;
+  }
+  for (const char* input : {"", "AA==", "AAA=", "AAAA"}) {
+    std::string decoded;
+    EXPECT_TRUE(butil::Base64Decode(input, &decoded)) << input;
+  }
+}
+
 }  // namespace butil