CVE-2025-43864 is a medium-severity vulnerability affecting the react-router package, a widely used routing library for React applications developed by remix-run. The vulnerability exists in versions from 7.2.0 up to but not including 7.5.2. React-router supports both Single Page Application (SPA) mode and Server-Side Rendering (SSR). The issue arises when an attacker crafts a request with a specific header that forces the application to switch from SSR to SPA mode unexpectedly. This forced mode switch triggers an unhandled error that corrupts the rendered page output. When a caching mechanism is present, the erroneous response can be cached and subsequently served to legitimate users, effectively poisoning the cache. This cache poisoning leads to degraded application availability as users receive corrupted or broken pages instead of the intended content. The root cause is classified under CWE-755, indicating improper handling of exceptional conditions, specifically the failure to gracefully handle the forced mode switch and resulting error. Although no known exploits are currently reported in the wild, the vulnerability poses a risk to applications relying on react-router versions within the affected range, especially those employing SSR combined with caching layers such as CDNs or reverse proxies. The issue was addressed and patched in react-router version 7.5.2, which properly handles the exceptional condition and prevents cache poisoning. Organizations using vulnerable versions should prioritize upgrading to the patched release to mitigate this risk.

For European organizations, the impact of this vulnerability primarily affects web applications built with React that utilize react-router for routing and implement SSR with caching layers. The cache poisoning can lead to widespread availability issues, causing users to receive corrupted pages, which can disrupt business operations, degrade user experience, and potentially lead to loss of customer trust. In sectors such as e-commerce, finance, public services, and media—where React-based SSR applications are common—this can translate into significant operational downtime and reputational damage. Additionally, organizations relying on CDNs or reverse proxies to cache SSR responses are at higher risk, as the poisoned cache can propagate the corrupted content globally, amplifying the impact. While this vulnerability does not directly compromise confidentiality or integrity of data, the availability degradation can indirectly affect service reliability and business continuity. Given the reliance on React in modern web development across Europe, the vulnerability could affect a broad range of organizations, especially those slow to update dependencies or with complex caching architectures.

1. Immediate upgrade of react-router to version 7.5.2 or later, where the vulnerability is patched, is the most effective mitigation. 2. Review and audit caching configurations, particularly for SSR responses, to ensure that error responses are not cached. Implement cache-control headers that prevent caching of error or exceptional responses. 3. Introduce validation or filtering of incoming request headers to detect and block suspicious headers that attempt to force SPA mode switching. 4. Implement monitoring and alerting on application error rates and cache hit anomalies to detect potential exploitation attempts early. 5. For applications that cannot immediately upgrade, consider disabling SSR temporarily or isolating SSR responses from caching layers until the patch can be applied. 6. Conduct security testing focused on header manipulation and cache poisoning scenarios to validate the effectiveness of mitigations. 7. Educate development and operations teams about this vulnerability to ensure awareness and prompt response to related incidents.