CVE-2026-81887 in Livewire
Summary
by MITRE • 08/31/2026
Livewire is a full-stack framework for Laravel. From 3.0.0-beta.1 until 3.8.3 and 4.3.4, the dot-notated query-string parser in js/plugins/history/index.js, including fromQueryString() and insertDotNotatedValueIntoData(), accepts the __proto__, constructor, and prototype path segments and creates inherited objects. Client-side state handlers then access effects.html, effects.js, effects.xjs, and effects.scripts without Object.prototype.hasOwnProperty.call(), allowing inherited attacker-controlled state to be treated as trusted effects. An unauthenticated attacker can craft a URL that, when opened by a user, executes arbitrary JavaScript in the affected application's origin. Exploitation requires user interaction and does not bypass server-side authorization or grant privileges beyond the affected user. This issue is fixed in versions 3.8.3 and 4.3.4.
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Analysis
by VulDB Data Team • 08/31/2026
The vulnerability identified within the Livewire framework, specifically affecting versions from 3.0.0-beta.1 through 3.8.3 and version 4.3.4, represents a critical client-side security flaw rooted in improper handling of query string parameters during state deserialization. Livewire is a full-stack framework for Laravel that facilitates dynamic interface updates without requiring traditional page reloads by managing application state on the server while maintaining synchronization with the browser's Document Object Model. The core technical deficiency lies within the JavaScript module responsible for parsing history-based URL states, specifically in the functions fromQueryString and insertDotNotatedValueIntoData located in js/plugins/history/index.js. These components are designed to parse dot-notation paths found in URLs to reconstruct application state objects on the client side. However, the parser fails to adequately sanitize or validate property names that correspond to standard JavaScript object prototype properties such as _proto_, constructor, and prototype.
When an attacker crafts a malicious URL containing these specific path segments within its query string parameters, the parsing logic inadvertently triggers JavaScript's prototype pollution mechanism. Instead of treating these keys as simple data fields, the browser's native object creation processes interpret them as instructions to modify or access the Object.prototype chain. This results in the injection of inherited properties into all objects created subsequently by that script execution context. The severity of this issue is compounded by how Livewire handles client-side state effects. Subsequent operations within the framework attempt to process these polluted states without employing strict property existence checks, such as Object.prototype.hasOwnProperty.call(). Consequently, attacker-controlled data injected via prototype pollution is erroneously treated as trusted application configuration or effect definitions rather than external input.
The operational impact of this vulnerability allows for Cross-Site Scripting attacks against users who visit the crafted URL. Because the malicious state is processed by client-side handlers that execute scripts associated with effects.html, effects.js, effects.xjs, and effects.scripts, an unauthenticated attacker can achieve arbitrary JavaScript execution within the context of the affected application's origin. This exploitation vector does not require bypassing server-side authorization mechanisms or escalating privileges beyond what the victim user already possesses. The attack relies entirely on social engineering to induce a target user to open the malicious link, after which the browser executes the injected code with full access to cookies, session tokens, and other sensitive data associated with that domain. This aligns closely with CWE-1321: Improperly Controlled Modification of Object Prototype Attributes, as well as CWE-79: Improper Neutralization of Input During Web Page Generation which is commonly known as Cross-site Scripting.
From a threat intelligence perspective, this vulnerability maps to the MITRE ATT&CK technique T1059.007: JavaScript, specifically under the subcategory of Client-side Execution. The attack lifecycle involves Initial Access via phishing or malicious links followed by Command and Control through script injection that can exfiltrate data back to an attacker-controlled server. Mitigation strategies must prioritize immediate upgrading to patched versions 3.8.3 and 4.3.4 where these parsing logic flaws have been corrected. For environments unable to upgrade immediately, defensive coding practices should be implemented within custom Livewire components or middleware that validate all incoming state data against a whitelist of expected properties before processing. Additionally, developers should ensure that any manual manipulation of object states utilizes hasOwnProperty checks to prevent prototype pollution from affecting the application's runtime environment. Implementing Content Security Policy headers can also help mitigate the impact by restricting script execution sources and preventing unauthorized inline scripts from running even if injection occurs.