Website accessibility is essential for inclusiveness and regulatory compliance. Although third-party advertisements (ads) are a vital revenue source for free web services, they introduce significant accessibility challenges. Leasing a website's space to ad-serving technologies, like DoubleClick, results in developers losing control over ad content accessibility. Even on highly accessible websites, third-party ads can undermine adherence to Web Content Accessibility Guidelines (WCAG). We conduct the first-of-its-kind large-scale investigation of 430 K website elements, including nearly 100 K ad elements, to understand the accessibility of ads on websites. We seek to understand the prevalence of inaccessible ads and their overall impact on the accessibility of websites. Our findings show that 67 % of websites experience increased accessibility violations due to ads, with common violations including Focus Visible (WCAG 2.4.7) and On Input (WCAG 3.2.2). Popular ad-serving technologies like Taboola, DoubleClick, and RevContent often serve ads that fail to comply with WCAG standards. Even when ads are WCAG compliant, 27 % of them have alternative text in ad images that misrepresents information, potentially deceiving users. Manual inspection of a sample of these misleading ads revealed that user-identifiable data is collected on 94 % of websites through interactions, such as hovering. Since users with disabilities often rely on tools like screen readers that require hover events to access website content, they have no choice but to compromise their privacy to navigate website ads. Based on our findings, we further dissect the root cause of these violations and provide design guidelines to both website developers and ad-serving technologies to achieve WCAG-compliant ad integration.