Stored XSS in Parse Server via File Upload Extension Bypass

Executive Summary

This vulnerability affects Parse Server versions prior to 9.9.1-alpha.11 and 8.6.81. It allows an attacker to bypass the default fileUpload.fileExtensions blocklist by uploading files with non-standard or compound extensions combined with dangerous content types. As a result, storage adapters like S3 and GCS may serve attacker-supplied active content, enabling stored cross-site scripting (XSS) attacks.

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Impact Analysis

The vulnerability can lead to stored cross-site scripting attacks, where malicious scripts supplied by an attacker are stored and later executed in the context of users accessing the content. This can compromise user data, session tokens, or lead to other malicious actions performed on behalf of the user.

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Mitigation Strategies

To mitigate this vulnerability, you should upgrade Parse Server to version 9.9.1-alpha.11 or later, or version 8.6.81 or later.

This update fixes the issue where the default fileUpload.fileExtensions blocklist could be bypassed by uploading files with non-standard or compound extensions and dangerous content types.

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Compliance Impact

CVE-2026-55778 is a stored Cross-Site Scripting (XSS) vulnerability that allows attackers to bypass file upload extension blocklists and upload malicious files that can execute arbitrary scripts. This can lead to unauthorized script execution when users access these files.

Such a vulnerability could potentially impact compliance with common standards and regulations like GDPR and HIPAA because it may lead to unauthorized access or manipulation of user data, compromise of user privacy, and exposure to cross-site scripting attacks that can be used to steal sensitive information or credentials.

However, the provided information does not explicitly discuss the direct impact of this vulnerability on compliance with these standards or any regulatory requirements.

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Detection Guidance

This vulnerability involves bypassing file upload extension blocklists by using non-standard or compound file extensions combined with dangerous content types, allowing stored cross-site scripting (XSS). Detection involves monitoring file uploads for suspicious file extensions that attempt to bypass the blocklist, such as files with extensions like .svg~, .html.old, .svg.tmp, or other unusual suffixes appended to normally blocked extensions.

To detect potential exploitation attempts on your system or network, you can scan uploaded files or logs for filenames with non-standard or compound extensions that match known dangerous types (e.g., SVG, HTML) but have additional suffixes.

• Use command-line tools to find suspicious files in your upload directories, for example:
• find /path/to/uploads -type f \( -name '*.svg~' -o -name '*.html.old' -o -name '*.svg.tmp' -o -name '*.xhtml.bak' \)
• Check server logs or application logs for upload requests containing suspicious filenames or Content-Type headers that do not match the file extension.
• Use network monitoring tools or web application firewalls (WAF) to alert on HTTP POST requests with file uploads having unusual extensions or mismatched MIME types.

Since the vulnerability is related to the file upload validation logic in Parse Server, ensuring your Parse Server version is updated to 9.9.1-alpha.11 or 8.6.81 will mitigate the issue by enforcing stricter validation on both filename extensions and Content-Type headers.

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