Astro: Cache Poisoning due to incorrect error handling when if-match header is malformed

Because the testing setup configures if-match as part of the cache key, the exploit no longer affects the production application. Prior to that change, the CDN Point of Presence would become cache-poisoned, and any client visiting the affected pages without cached files through the same PoP would receive broken pages. This was reproduced by creating test URLs and visiting them in a browser only after triggering the exploit. The exploited resources returned 5xx errors instead of the original CSS/JS content, breaking the application.

Details

The findings were analyzed with an LLM, which identified the following file as the likely source: serve-static.ts

// Lines 129-153

let forwardError = false;

stream.on('error', (err) => {
    if (forwardError) {
        console.error(err.toString());
        res.writeHead(500);
        res.end('Internal server error');
        return;
    }
    // File not found, forward to the SSR handler
    ssr();
});
stream.on('headers', (_res: ServerResponse) => {
    // assets in dist/_astro are hashed and should get the immutable header
    if (normalizedPathname.startsWith(`/${app.manifest.assetsDir}/`)) {
        // This is the "far future" cache header, used for static files whose name includes their digest hash.
        // 1 year (31,536,000 seconds) is convention.
        // Taken from https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control#immutable
        _res.setHeader('Cache-Control', 'public, max-age=31536000, immutable');
    }
});
stream.on('file', () => {
    forwardError = true;
});
stream.pipe(res);

LLM analysis:

send handles conditional request headers such as If-Match internally. When a file is found but the precondition fails (ETag mismatch), send:

1. Emits file (the file exists) → forwardError = true
2. Emits headers → Cache-Control: public, max-age=31536000, immutable is set on res
3. Emits error with a PreconditionFailedError (status 412)

However, the error handler does not inspect the error's status code:

stream.on('error', (err) => {
    if (forwardError) {
        console.error(err.toString());
        res.writeHead(500);   // ← always 500, regardless of the actual error
        res.end('Internal server error');
        return;
    }
    ssr();
});

Because Cache-Control was already set during the headers event, the response is sent as:

HTTP/1.1 500 Internal Server Error
Cache-Control: public, max-age=31536000, immutable

Impact

Cache Poisoning — An attacker can force edge servers to cache an error page instead of the actual content, rendering one or more assets unavailable to legitimate users until the cache expires.