Compliance Impact

The vulnerability allows attackers to upload files with dangerous extensions by bypassing the default file upload extension blocklist, leading to stored cross-site scripting (XSS) attacks when victims access the malicious files.

Such stored XSS vulnerabilities can lead to unauthorized actions or data theft, which may result in exposure of personal or sensitive data.

Exposure or unauthorized access to personal data due to this vulnerability could impact compliance with data protection regulations such as GDPR or HIPAA, which require safeguarding personal and sensitive information against unauthorized access or breaches.

Therefore, if exploited, this vulnerability could undermine an organization's ability to meet security and privacy requirements mandated by these standards.

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Executive Summary

CVE-2026-53724 is a stored cross-site scripting (XSS) vulnerability in Parse Server that affects versions prior to 8.6.79 and 9.9.1-alpha.4. The issue arises because the default file upload extension blocklist can be bypassed by appending a trailing dot to a filename (e.g., poc.svg.).

The trailing dot causes the extension parser to extract an empty string as the file extension, which short-circuits the blocklist check. As a result, the attacker-controlled Content-Type is forwarded unchanged to storage adapters like S3 or GCS.

These storage adapters then serve the file with an active Content-Type such as image/svg+xml, enabling stored XSS attacks when a victim opens the file URL. The default GridFS adapter is not affected because it sets the X-Content-Type-Options: nosniff header on responses.

The vulnerability has been patched in versions 8.6.79 and 9.9.1-alpha.4 by treating filenames ending in a dot as extensionless and validating Content-Type subtypes against the blocklist.

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Impact Analysis

This vulnerability allows attackers to upload files with dangerous extensions that bypass the default blocklist by using filenames with trailing dots.

When victims access the URLs of these uploaded files, the malicious content can execute arbitrary JavaScript code in their browsers, leading to stored cross-site scripting (XSS) attacks.

Such stored XSS attacks can result in unauthorized actions, data theft, session hijacking, or other malicious activities impacting users and the security of your application.

The vulnerability requires user interaction (victims opening the malicious file URL) and has a low severity score (CVSS 2.1), but it still poses a risk especially if unpatched.

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Detection Guidance

This vulnerability can be detected by monitoring file uploads for filenames that end with a trailing dot, such as "poc.svg." which bypass the extension blocklist.

You can scan your storage or logs for files with suspicious trailing dots in their extensions to identify potential exploit attempts.

• Use commands to find files with trailing dots in filenames, for example in a Unix-like system: `find /path/to/uploads -name '*.'`
• Check HTTP request logs for upload attempts with filenames containing trailing dots.
• Inspect Content-Type headers forwarded to storage adapters to see if they contain unexpected or attacker-controlled values.

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Mitigation Strategies

Immediate mitigation steps include upgrading Parse Server to version 8.6.79 or 9.9.1-alpha.4 or later, where the vulnerability has been patched.

If upgrading is not immediately possible, configure your storage adapters (such as S3 or GCS) to derive Content-Type from filenames rather than trusting the forwarded Content-Type header.

Alternatively, replace the default file extension blocklist with an allowlist to restrict uploads to only safe file types.

Ensure that your storage adapters or servers set security headers like X-Content-Type-Options: nosniff to prevent content sniffing.

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