🛡️ GHSA-82jv-9wjw-pqh6 — derby

Prototype pollution in emit function ### Summary A prototype pollution in derby can crash the application, if the application author has atypical HTML templates that feed user input into an object key. Attribute keys are almost always developer-controlled, not end-user-controlled, so this shouldn't be an issue in practice for most applications. ### Details ``` emit(context: Context, target: T) { const node = traverseAndCreate(context.controller, this.segments); node[this.lastSegment] = target; this.addListeners(target, node, this.lastSegment); } ``` The emit() function in src/templates/templates.ts is called without sanitizing the variable `this.lastSegment `. The variable `this.lastSegment ` can be set to `__proto__`, and this will pollute the prototype of Javascipt Object (`node['__proto__'] = target`). ### PoC To reproduce this vulnerability, you can adjust the test case `ignores DOM mutations in components\' create()` in `test/dom/ComponentHarness.mocha.js`. ``` it('ignores DOM mutations in components\' create()', function() { function Box() {} Box.view = { is: 'box', - source: '' + source: '' }; Box.prototype.create = function() { this.boxElement.className = 'box-changed-in-create'; }; var harness = runner.createHarness('', Box); expect(harness).to.render(''); }); ``` When `as` attribute is controlled by attackers, the variable in `this.lastSegment` will exactly take value` __proto__` and prototype pollution happens. ### Patch Add a check on `this.lastSegment` can prevent this attack. ``` emit(context: Context, target: T) { const node = traverseAndCreate(context.controller, this.segments); + if (this.lastSegment.includes('__proto__') || this.lastSegment.includes('prototype')) { + throw new Error('Unsafe code detected'); + } node[this.lastSegment] = target; this.addListeners(target, node, this.lastSegment); } ```