diff --git a/CHANGELOG.md b/CHANGELOG.md
index f960aa4aec..e519de1f0c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 
 ## Master
 
+- Prevent infinite loop when parsing corrupted lockfile with unterminated string
+
+  [#4965](https://github.com/yarnpkg/yarn/pull/4965) - [**Ryan Hendrickson**](https://github.com/rhendric)
+
 - Environment variables now have to **start** with `YARN_` (instead of just contain it) to be considered
 
   [#6518](https://github.com/yarnpkg/yarn/pull/6518) - [**Michael Gmelin**](https://blog.grem.de)
diff --git a/src/lockfile/parse.js b/src/lockfile/parse.js
index 4db0830ac4..2348746063 100644
--- a/src/lockfile/parse.js
+++ b/src/lockfile/parse.js
@@ -69,44 +69,43 @@ function* tokenise(input: string): Iterator<Token> {
     } else if (input[0] === '#') {
       chop++;
 
-      let val = '';
-      while (input[chop] !== '\n') {
-        val += input[chop];
-        chop++;
+      let nextNewline = input.indexOf('\n', chop);
+      if (nextNewline === -1) {
+        nextNewline = input.length;
       }
+      const val = input.substring(chop, nextNewline);
+      chop = nextNewline;
       yield buildToken(TOKEN_TYPES.comment, val);
     } else if (input[0] === ' ') {
       if (lastNewline) {
-        let indent = '';
-        for (let i = 0; input[i] === ' '; i++) {
-          indent += input[i];
+        let indentSize = 1;
+        for (let i = 1; input[i] === ' '; i++) {
+          indentSize++;
         }
 
-        if (indent.length % 2) {
+        if (indentSize % 2) {
           throw new TypeError('Invalid number of spaces');
         } else {
-          chop = indent.length;
-          yield buildToken(TOKEN_TYPES.indent, indent.length / 2);
+          chop = indentSize;
+          yield buildToken(TOKEN_TYPES.indent, indentSize / 2);
         }
       } else {
         chop++;
       }
     } else if (input[0] === '"') {
-      let val = '';
-
-      for (let i = 0; ; i++) {
-        const currentChar = input[i];
-        val += currentChar;
-
-        if (i > 0 && currentChar === '"') {
+      let i = 1;
+      for (; i < input.length; i++) {
+        if (input[i] === '"') {
           const isEscaped = input[i - 1] === '\\' && input[i - 2] !== '\\';
           if (!isEscaped) {
+            i++;
             break;
           }
         }
       }
+      const val = input.substring(0, i);
 
-      chop = val.length;
+      chop = i;
 
       try {
         yield buildToken(TOKEN_TYPES.string, JSON.parse(val));
@@ -118,10 +117,7 @@ function* tokenise(input: string): Iterator<Token> {
         }
       }
     } else if (/^[0-9]/.test(input)) {
-      let val = '';
-      for (let i = 0; /^[0-9]$/.test(input[i]); i++) {
-        val += input[i];
-      }
+      const val = /^[0-9]+/.exec(input)[0];
       chop = val.length;
 
       yield buildToken(TOKEN_TYPES.number, +val);
@@ -138,16 +134,15 @@ function* tokenise(input: string): Iterator<Token> {
       yield buildToken(TOKEN_TYPES.comma);
       chop++;
     } else if (/^[a-zA-Z\/-]/g.test(input)) {
-      let name = '';
-      for (let i = 0; i < input.length; i++) {
+      let i = 0;
+      for (; i < input.length; i++) {
         const char = input[i];
         if (char === ':' || char === ' ' || char === '\n' || char === '\r' || char === ',') {
           break;
-        } else {
-          name += char;
         }
       }
-      chop = name.length;
+      const name = input.substring(0, i);
+      chop = i;
 
       yield buildToken(TOKEN_TYPES.string, name);
     } else {