Astro: Server island encrypted parameters vulnerable to cross-component replay
Date: 2026-05
Affected: npm/astro
Attack vector: xss
Category: vulnerability-disclosure
Severity: Low
CVE: CVE-2026-45028
CWE: CWE-323
CWE-79
CVSS: 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N)

### Impact Astro versions prior to 6.1.10 used AES-GCM encryption to protect the confidentiality and integrity of server island props and slots parameters, but did not bind the ciphertext to its intended component or parameter type. An attacker could replay one component's encrypted props (`p`) value as another component's slots (`s`) value, or vice versa. Since slots contain raw unescaped HTML while props may contain user-controlled values, this could lead to XSS in applications that meet **all** of the following conditions: - The application uses server islands - Two different server island components share the same key name for a prop and a slot - An attacker has full control over the value of the overlapping prop (requires a dynamically rendered page) These conditions are very unlikely to occur in real-world production applications. ### Patches This has been patched in **astro@6.1.10**. The fix binds each encrypted parameter to its target component and purpose using AES-GCM authenticated additional data (AAD). Each ciphertext now includes context like `props:IslandName` or `slots:IslandName`, so encrypted data for one component cannot be replayed against a different component, and encrypted props cannot be reused as slots. ### References - Fix PR: https://github.com/withastro/astro/pull/16457 - Example demonstrating the vulnerability: https://github.com/CyberSecurityAustria/ACSC2026-web-astronomical


Mitigations:

Architectural layers (MAESTRO):