diff --git a/src/crypto/cfb.js b/src/crypto/cfb.js
index 9686c5e8..adc6c162 100644
--- a/src/crypto/cfb.js
+++ b/src/crypto/cfb.js
@@ -45,7 +45,7 @@ export default {
     const block_size = cipherfn.blockSize;
 
     let blocki = new Uint8Array(block_size);
-    const blockc = iv;
+    const blockc = iv.slice();
     let pos = 0;
     const ciphertext = new Uint8Array(plaintext.length);
     let i;