2.9

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-45028

CVE-2026-45028: Astro: Server island encrypted parameters vulnerable to cross-component replay

Astro is a web framework. 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. This occurs when the application uses server islands, two different server island components share the same key name for a prop and a slot, and an attacker has full control over the value of the overlapping prop (requires a dynamically rendered page). This vulnerability is fixed in 6.1.10.

(GitHub Advisory)

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CVE-2026-45028

CVE-2026-45028:

2.9

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
astro	npm	< 6.1.10	6.1.10

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability exists in how Astro's server islands handle encryption for component parameters (props and slots). The core issue is a cryptographic weakness in the implementation of AES-GCM encryption. The functions encryptString and decryptString did not use the 'authenticated additional data' (AAD) feature of AES-GCM. This feature is designed to bind a ciphertext to its specific context.

Without AAD, a ciphertext generated for a component's 'props' could be intercepted by an attacker and re-submitted to the server as the 'slots' for the same or another component. The decryptString function would successfully decrypt this replayed data because it only checked the encryption key, not the context in which the data was intended to be used.

This is dangerous because 'slots' in Astro can contain raw, unescaped HTML, while 'props' might contain user-controlled data that is expected to be treated as simple data. By replaying a user-controlled prop as a slot, an attacker could inject malicious HTML and execute a Cross-Site Scripting (XSS) attack.

The patch addresses this by modifying encryptString and decryptString to accept an additionalData parameter. This parameter is used to pass context, such as props:ComponentName or slots:ComponentName, into the encryption and decryption process. This ensures that a ciphertext encrypted for a specific purpose can only be decrypted for that exact same purpose, effectively preventing the replay attack.

Vulnerable functions

encryptString

packages/astro/src/core/encryption.ts

The vulnerable version of `encryptString` does not include any authenticated additional data (AAD) when encrypting server island parameters. This means the resulting ciphertext is not bound to its intended context (e.g., a specific component or parameter type like 'props' or 'slots'). This allows an attacker to take a ciphertext generated for one context and replay it in another.

decryptString

packages/astro/src/core/encryption.ts

The vulnerable version of `decryptString` does not verify any authenticated additional data (AAD) when decrypting. This allows it to successfully decrypt a ciphertext that was generated for a different context. An attacker can exploit this by replaying an encrypted 'props' value as an encrypted 'slots' value, which could lead to XSS because slots may render raw HTML while props are typically treated as data.

ServerIslandComponent.render

packages/astro/src/runtime/server/render/server-islands.ts

This method uses the vulnerable `encryptString` function to encrypt component props and slots. The generated ciphertexts lack context, making them susceptible to being replayed in a different context by an attacker during a subsequent request to the server island endpoint.

createEndpoint

packages/astro/src/core/server-islands/endpoint.ts

This function acts as the endpoint for handling server island requests. It uses the vulnerable `decryptString` function to process incoming encrypted props and slots. An attacker can send a crafted request where a ciphertext originally intended for a 'prop' is placed in the 'slots' parameter. The function would decrypt it without error, potentially rendering unsafe user-controlled data as raw HTML, leading to XSS.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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