diff --git a/dom/crypto/WebCryptoTask.cpp b/dom/crypto/WebCryptoTask.cpp
index 57a7da186c..f5fc7b5bc9 100644
--- a/dom/crypto/WebCryptoTask.cpp
+++ b/dom/crypto/WebCryptoTask.cpp
@@ -716,6 +716,11 @@ private:
       return NS_ERROR_DOM_INVALID_ACCESS_ERR;
     }
 
+    // Check whether the integer addition would overflow.
+    if (std::numeric_limits<CryptoBuffer::size_type>::max() - 16 < mData.Length()) {
+      return NS_ERROR_DOM_DATA_ERR;
+    }
+
     // Initialize the output buffer (enough space for padding / a full tag)
     uint32_t dataLen = mData.Length();
     uint32_t maxLen = dataLen + 16;