From 85c9ecac51819b1d91453e89f3bce2828e8641e9 Mon Sep 17 00:00:00 2001
From: Harry Altman <harry.j.altman@gmail.com>
Date: Wed, 16 Dec 2020 13:43:15 -0500
Subject: [PATCH 1/3] Prevent ABI objects from being mutated

---
 packages/web3-eth-abi/src/index.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/web3-eth-abi/src/index.js b/packages/web3-eth-abi/src/index.js
index 31248bc7b0f..34801939265 100644
--- a/packages/web3-eth-abi/src/index.js
+++ b/packages/web3-eth-abi/src/index.js
@@ -155,7 +155,7 @@ ABICoder.prototype.mapTypes = function (types) {
         // recognize former type. Solidity docs say `Function` is a bytes24
         // encoding the contract address followed by the function selector hash.
         if (typeof type === 'object' && type.type === 'function'){
-            type.type = "bytes24";
+            type = Object.assign({}, type, { type: "bytes24" });
         }
         if (self.isSimplifiedStructFormat(type)) {
             var structName = Object.keys(type)[0];

From 9019ebe8ff7cf6bf7d410ceb0c7fc5ea0b10a1a5 Mon Sep 17 00:00:00 2001
From: Harry Altman <harry.j.altman@gmail.com>
Date: Wed, 16 Dec 2020 14:05:39 -0500
Subject: [PATCH 2/3] Add test of ABI non-mutation

---
 test/abi.decodeParameter.js | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/test/abi.decodeParameter.js b/test/abi.decodeParameter.js
index c98930b3191..e79fab2c927 100644
--- a/test/abi.decodeParameter.js
+++ b/test/abi.decodeParameter.js
@@ -638,3 +638,18 @@ describe('lib/solidity/coder', function () {
                         '737472696e670000000000000000000000000000000000000000000000000000'})
     });
 });
+
+describe('/lib/solidity/coder', function() {
+    describe('decodeParam', function () {
+        it('should not alter inputs', function () {
+            const t = {
+                type: "function",
+                name: "f",
+                internalType: "function () external"
+            };
+            const copyOfT = Object.assign({}, t);
+            coder.decodeParameter(t, '063e4f349a9e91c6575aedab0e70087fab642ecac04062260000000000000000'); //must not alter t!
+            assert.deepEqual(t, copyOfT);
+        });
+    });
+});

From 54f56b905b4ffe1cc723068867adfd18c2525704 Mon Sep 17 00:00:00 2001
From: Harry Altman <harry.j.altman@gmail.com>
Date: Wed, 16 Dec 2020 14:19:42 -0500
Subject: [PATCH 3/3] Update changelog

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0f350308696..1f1d0fc68ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -310,3 +310,4 @@ Released with 1.0.0-beta.37 code base.
 - Fixed decoding bytes and string parameters for logs emitted with solc 0.4.x (#3724, #3738)
 - Grammar changes to inputAddressFormatter error message
 - Fixed vulnerable dependencies
+- Fixed mutation of inputs to encoding and decoding functions (#3748)