CVE-2026-86550 in NebulaOS
Summary
by MITRE • 09/08/2026
NuBrowser lacks protocol whitelist validation for the S.browser_fallback_url field of intent://, allowing attackers to inject javascript: URLs via 302 redirects. This results in a universal cross‑site scripting (UXSS) vulnerability that enables script execution within the origin of arbitrary websites.
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Analysis
by VulDB Data Team • 09/08/2026
The NuBrowser application contains a critical security flaw related to its handling of intent-based navigation and protocol validation mechanisms. Specifically, the browser fails to enforce a strict whitelist for protocols when processing the S.browser_fallback_url field associated with intent:// schemes. This oversight allows an attacker to manipulate the fallback URL parameter during a redirection sequence, effectively bypassing standard security controls that are designed to restrict user agents from executing arbitrary code or navigating to non-web protocols without explicit user consent and validation. The vulnerability stems from insufficient input sanitization and protocol verification logic within the browser's intent handling subsystem, which should strictly limit accepted schemes to safe web-based protocols such as HTTP and HTTPS while rejecting dangerous ones like javascript:, data:, or file:.
This architectural weakness enables a Universal Cross-Site Scripting attack vector that operates through 302 redirects. An attacker can craft a malicious webpage or link that triggers an intent:// navigation with a specifically crafted fallback URL containing a javascript: URI scheme. When the browser processes this request, it follows the redirect chain and ultimately executes the injected JavaScript code within the context of the target website's origin. Unlike traditional cross-site scripting vulnerabilities that are often constrained by same-origin policies, this UXSS vulnerability allows script execution across different origins because the flaw resides in the browser engine itself rather than a specific web application. Consequently, any website visited after such an attack can be compromised, allowing for session hijacking, credential theft, or further malware distribution under the guise of legitimate site content.
The operational impact of this vulnerability is severe due to its universal nature and the high privileges associated with script execution in the browser context. Attackers can exploit this flaw to bypass Content Security Policies, steal sensitive user data such as cookies and local storage contents, and perform actions on behalf of the authenticated user without their knowledge. Since the attack relies on standard HTTP redirect mechanisms (302), it is difficult for users or basic security filters to detect malicious intent based solely on URL structure alone. The ability to execute scripts within arbitrary website origins means that even secure sites with robust server-side protections remain vulnerable if a user interacts with a compromised third-party link, thereby undermining the fundamental trust model of web browsing.
To mitigate this risk, developers must implement rigorous protocol validation for all fallback URLs processed by intent handlers. This involves maintaining an allowlist of safe protocols and explicitly rejecting any scheme that is not on the list, particularly javascript:, data:, vbscript:, and file:. Additionally, input sanitization routines should be strengthened to detect encoded or obfuscated attempts to inject malicious schemes through URL encoding or other evasion techniques. Security testing frameworks should include automated checks for intent-based XSS vectors to ensure that fallback mechanisms do not become attack surfaces. Users are advised to keep the application updated if a patch is available and exercise caution when interacting with links from untrusted sources, as client-side browser vulnerabilities often require vendor intervention for complete resolution rather than simple user awareness alone.