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– How CVE-2026-81887 Works
Livewire is a full-stack framework for Laravel that enables dynamic UI updates without page reloads by synchronizing server-side state with the browser’s DOM. In versions 3.0.0-beta.1 through 3.8.2 and 4.0.0-beta.1 through 4.3.3, a DOM-based XSS vulnerability exists in the client-side state handling mechanism. The flaw resides in the dot‑notated query‑string parser found in js/plugins/history/index.js, specifically within the `fromQueryString()` and `insertDotNotatedValueIntoData()` functions.
These functions parse dot‑notation paths from URL query strings to reconstruct application state objects on the client side. However, the parser fails to properly 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, the parsing logic inadvertently triggers JavaScript prototype pollution. 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, injecting inherited properties into all objects created subsequently by that script execution context.
The severity is compounded by how Livewire handles client‑side state effects. Client‑side state handlers subsequently access properties such as effects.html, effects.js, effects.xjs, and `effects.scripts` without employing `Object.prototype.hasOwnProperty.call()` to verify property ownership. Consequently, attacker‑controlled data injected via prototype pollution is erroneously treated as trusted application configuration or effect definitions rather than external input.
An unauthenticated attacker can craft a URL that, when opened by a victim, executes arbitrary JavaScript within the origin of the affected application. Exploitation requires user interaction (the victim must click the malicious link) but does not require authentication or prior access to the application. The attack does not bypass server‑side authorization and grants no privileges beyond those the affected user already holds. The vulnerability is rooted in CWE‑1321: Improperly Controlled Modification of Object Prototype Attributes and CWE‑79: Cross‑site Scripting, and maps to MITRE ATT&CK technique T1059.007: Command and Scripting Interpreter – JavaScript.
DailyCVE Form:
Platform: Livewire (Laravel)
Version: 3.0.0‑beta.1 – 3.8.2, 4.0.0‑beta.1 – 4.3.3
Vulnerability: DOM‑based XSS (Prototype Pollution)
Severity: Moderate (CVSS 5.1)
Date: August 31, 2026
Prediction: Patch expected September 2, 2026
What Undercode Say:
Check installed Livewire version via Composer composer show livewire/livewire Identify vulnerable versions Vulnerable: >= 3.0.0-beta.1, <= 3.8.2 and >= 4.0.0-beta.1, <= 4.3.3 Patched: 3.8.3 and 4.3.4 Upgrade to patched version immediately composer require livewire/livewire:^3.8.3 or for v4 users: composer require livewire/livewire:^4.3.4 Verify upgrade composer show livewire/livewire
// Vulnerable code snippet (js/plugins/history/index.js) // fromQueryString() and insertDotNotatedValueIntoData() accept <strong>proto</strong>, // constructor, and prototype path segments without sanitization // Example of missing hasOwnProperty check: // State handlers access effects. without Object.prototype.hasOwnProperty.call() // This allows inherited attacker-controlled state to be treated as trusted
Audit dependencies for known vulnerabilities composer audit Check for specific CVE composer audit --cve CVE-2026-81887
Exploit: (Educational Purposes!)
An attacker can craft a malicious URL containing prototype pollution payloads in query parameters:
https://target-app.com/?__proto__[bash][html]=<img src=x onerror=alert('XSS')>
Or using dot‑notation:
https://target-app.com/?__proto__.effects.html=<img src=x onerror=alert(document.cookie)>
When a victim opens this URL, Livewire’s query‑string parser processes `__proto__` as a valid path segment, polluting `Object.prototype` with an attacker‑controlled `effects.html` property. Client‑side state handlers then read `effects.html` without `hasOwnProperty` check, treating the inherited property as a trusted effect and executing the injected JavaScript in the victim’s browser.
// Proof-of-concept payload structure // The parser accepts <strong>proto</strong>, constructor, and prototype path segments // Effects accessed: effects.html, effects.js, effects.xjs, effects.scripts // No hasOwnProperty() validation → prototype pollution leads to XSS
Protection from this CVE
- Immediate Upgrade – Upgrade Livewire to v3.8.3 or v4.3.4 immediately. There is no known workaround.
- Content Security Policy (CSP) – Deploy a strict CSP that disallows `unsafe-inline` and
unsafe-eval, restricting script sources to trusted origins. - Input Sanitization – Review and sanitize custom query‑string handling in your application, stripping unexpected path segments like
__proto__,constructor, and `prototype` to prevent prototype inheritance abuse. - Dependency Audit – Run `composer audit` regularly to detect vulnerable dependencies.
Impact
- Confidentiality – Attacker can execute JavaScript to steal session cookies, tokens, and sensitive data from the victim’s browser.
- Integrity – Attacker can manipulate page content, perform actions on behalf of the user, or redirect to malicious sites.
- Availability – Attacker can disrupt application functionality or cause denial of service through script execution.
- Scope – Affects all Laravel applications using Livewire v3 (≤ 3.8.2) and v4 (≤ 4.3.3). Exploitation requires user interaction and grants no privileges beyond those of the victim user.
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Sources:
Reported By: github.com
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