Cross-Site Scripting (XSS) vulnerabilities have long been a persistent challenge in web security, affecting applications from legacy systems to modern cloud-based architectures. Despite advancements in security protocols, XSS remains a significant threat. The Microsoft Security Response Center (MSRC) has recently disclosed that it has addressed over 900 XSS vulnerabilities across its extensive range of services and applications.
Prevalence and Impact of XSS Vulnerabilities
Between July 2024 and July 2025, XSS vulnerabilities constituted 15% of all Important or Critical security cases managed by the MSRC. During this period, the center resolved 265 specific XSS cases, with 263 classified as Important and two as Critical. To acknowledge the efforts of security researchers who identified these flaws, Microsoft awarded a total of $912,300 in bounties for XSS vulnerabilities. Notably, the highest single bounty paid for a high-impact XSS attack, such as one involving token theft or a zero-click exploit, was $20,000.
Distribution Across Microsoft Services
These vulnerabilities were not confined to a single product but were reported across a wide array of Microsoft’s major services. Bounty programs for Microsoft Copilot, Microsoft 365, Dynamics 365, Microsoft Identity, Microsoft Azure, and Xbox all received XSS submissions. Reports from both internal and external researchers often detailed methods for bypassing sanitization logic and exploiting behaviors in modern web frameworks.
Severity Assessment and Prioritization
Not all XSS vulnerabilities carry the same risk. Microsoft prioritizes issues based on their real-world impact on customers. Factors such as the potential for data exposure, the level of user interaction required for an exploit, and overall exploitability determine a vulnerability’s severity. The MSRC uses a matrix that combines data classification with exploit conditions to assign a severity rating:
– Critical Severity: A zero-click XSS that compromises highly confidential data, like session tokens or sensitive cookies, is rated as Critical.
– Important Severity: If an XSS requires some user interaction but can still expose confidential information, it is typically rated as Important.
– Moderate/Low Severity: XSS on public pages with no sensitive data exposure, or scenarios that require the user to perform the attack on themselves (self-XSS), are considered lower severity.
Out-of-Scope Vulnerabilities
Microsoft has also clarified which types of XSS vulnerabilities are considered out of scope for servicing. These include self-XSS, which requires a user to manually paste a payload into their browser’s developer console, and vulnerabilities that only execute in non-standard or outdated browsers like Internet Explorer. Similarly, JavaScript execution within a PDF’s restricted environment does not typically qualify unless it can escape into a more privileged context.
Guidance for Security Researchers
To aid security researchers, the MSRC provided a checklist for submitting XSS reports, emphasizing the need for clear, reproducible steps, a proof-of-concept that works without developer tools, and a detailed explanation of the potential impact. This structured approach ensures that vulnerabilities are effectively communicated and addressed.
Broader Context of XSS Vulnerabilities
The persistence of XSS vulnerabilities is not unique to Microsoft. For instance, in the WordPress ecosystem, XSS accounted for approximately 53.3% of all security flaws discovered, highlighting the widespread nature of this issue. This underscores the importance of continuous vigilance and proactive security measures across all platforms.
Conclusion
The ongoing efforts by Microsoft to identify and mitigate XSS vulnerabilities reflect the company’s commitment to security. However, the prevalence of these issues serves as a reminder of the need for continuous improvement in security practices. Organizations and developers must remain vigilant, adopting robust security measures and staying informed about potential threats to protect their systems and users effectively.
