Astro's server source code is exposed to the public if sourcemaps are enabled

Summary

A bug in the build process allows any unauthenticated user to read parts of the server source code.

Details

During build, along with client assets such as css and font files, the sourcemap files for the server code are moved to a publicly-accessible folder. https://github.com/withastro/astro/blob/176fe9f113fd912f9b61e848b00bbcfecd6d5c2c/packages/astro/src/core/build/static-build.ts#L139

Any outside party can read them with an unauthorized HTTP GET request to the same server hosting the rest of the website.

While some server files are hashed, making their access obscure, the files corresponding to the file system router (those in src/pages) are predictably named. For example. the sourcemap file for src/pages/index.astro gets named dist/client/pages/index.astro.mjs.map.

PoC

Here is one example of an affected open-source website: https://creatorsgarten.org/pages/index.astro.mjs.map

<image width="500" height="263" src="https://github.com/user-attachments/assets/773c5532-87af-42b8-838e-8f5472bf9f68"/>

The file can be saved and opened using https://evanw.github.io/source-map-visualization/ to reconstruct the source code.

<image width="500" height="271" src="https://github.com/user-attachments/assets/7d35d0ca-3a29-4666-be21-cfefe311ac9d"/>

The above accurately mirrors the source code as seen in the repository: https://github.com/creatorsgarten/creatorsgarten.org/blob/main/src/pages/index.astro

<image width="500" height="298" src="https://github.com/user-attachments/assets/39e77197-8382-4556-a024-c526dacccc1c"/>

The above was found as the 4th result (and the first one on Astro 5.0+) when making the following search query on GitHub.com (search results link):

path:astro.config.mjs @sentry/astro

This vulnerability is the root cause of https://github.com/withastro/astro/issues/12703, which links to a simple stackblitz project demonstrating the vulnerability. Upon build, notice the contents of the dist/client (referred to as config.build.client in astro code) folder. All astro servers make the folder in question accessible to the public internet without any authentication. It contains .map files corresponding to the code that runs on the server.

Impact

All server-output (SSR) projects on Astro 5 versions v5.0.3 through v5.0.6 (inclusive), that have sourcemaps enabled, either directly or through an add-on such as sentry, are affected. The fix for server-output projects was released in astro@5.0.7.

Additionally, all static-output (SSG) projects built using Astro 4 versions 4.16.17 or older, or Astro 5 versions 5.0.7 or older, that have sourcemaps enabled are also affected. The fix for static-output projects was released in astro@5.0.8, and backported to Astro v4 in astro@4.16.18.

The immediate impact is limited to source code. Any secrets or environment variables are not exposed unless they are present verbatim in the source code.

There is no immediate loss of integrity within the the vulnerable server. However, it is possible to subsequently discover another vulnerability via the revealed source code .

There is no immediate impact to availability of the vulnerable server. However, the presence of an unsafe regular expression, for example, can quickly be exploited to subsequently compromise the availability.

• Network attack vector.
• Low attack complexity.
• No privileges required.
• No interaction required from an authorized user.
• Scope is limited to first party. Although the source code of closed-source third-party software may also be exposed.

Remediation

The fix for server-output projects was released in astro@5.0.7, and the fix for static-output projects was released in astro@5.0.8 and backported to Astro v4 in astro@4.16.18. Users are advised to update immediately if they are using sourcemaps or an integration that enables sourcemaps.