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How CVE-2026-55778 Works
Parse Server’s default `fileUpload.fileExtensions` blocklist is designed to prevent users from uploading files that browsers interpret as active content, such as HTML or SVG. This measure is critical for preventing stored cross-site scripting (XSS) attacks, where a malicious file, when accessed by other users, could execute scripts in their browsers.
The vulnerability arises because the blocklist checks only for exact matches of forbidden extensions. An attacker can bypass this protection by uploading a file with a non-standard or compound extension (e.g., malicious.svg.txt) while simultaneously setting a dangerous `Content-Type` header, such as image/svg+xml.
The issue is particularly critical for storage adapters like Amazon S3 and Google Cloud Storage (GCS). These services persist and serve the file with the user-supplied Content-Type. Consequently, when a victim opens the file URL, the browser interprets it based on the attacker-controlled Content-Type, executing any embedded scripts and enabling a stored XSS attack.
This vulnerability affects the default Parse Server configuration where authenticated users are permitted to upload files. The default GridFS/filesystem adapter is somewhat mitigated because it sets the `X-Content-Type-Options: nosniff` header, which prevents browsers from MIME-sniffing. However, the fundamental bypass of the upload restriction persists. This advisory is a follow-up to previous incomplete fixes for `GHSA-vr5f-2r24-w5hc` (CVE-2026-35200) and GHSA-7wqv-xjf3-x35v.
The patch implements a more robust validation process. It now evaluates the request’s `Content-Type` against the blocklist whenever the filename’s extension is not a recognized type. This prevents an attacker from preserving a dangerous content type by using a non-standard extension, and such uploads are now rejected.
DailyCVE Form
Platform: Parse Server
Version: prior 8.6.79, 9.0.0-9.9.1-alpha.3
Vulnerability: Stored XSS
Severity: Low
Date: 2026-06-16
Prediction: Patch available
What Undercode Say
The vulnerability stems from a fundamental design flaw: relying on a blocklist for security. As the Parse Server team notes, `fileUpload.fileExtensions` is intended to be an allowlist, not a denylist. A blocklist is inherently reactive and can never be exhaustive, as new dangerous extensions and MIME types will continue to emerge.
The analytics point to a clear pattern of incomplete fixes. The current CVE is a direct follow-up to `GHSA-vr5f-2r24-w5hc` (CVE-2026-35200) and GHSA-7wqv-xjf3-x35v, indicating that previous patches did not fully address the root cause. The real solution is a shift in security posture from a reactive blocklist to a proactive allowlist.
Server Configuration (Parse Server)
// Instead of relying on the default blocklist, use a strict allowlist.
// Example: Only allow PNG, JPEG, GIF, and PDF uploads.
{
"fileUpload": {
"fileExtensions": ["^(png|jpe?g|gif|pdf)$"]
}
}
Cloud Storage (e.g., AWS S3 Bucket Policy)
To further mitigate risks, one can enforce `X-Content-Type-Options: nosniff` at the CDN or storage level.
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AddResponseHeader",
"Effect": "Allow",
"Principal": "",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket/",
"Condition": {
"StringEquals": {
"s3:x-amz-meta-response-headers": "X-Content-Type-Options=nosniff"
}
}
}
]
}
Exploit
An attacker can bypass the blocklist by uploading a file with a compound extension (e.g., poc.svg.txt) and a malicious `Content-Type` (e.g., image/svg+xml). If the storage adapter serves the file with the attacker-controlled Content-Type, a victim’s browser will render it as an SVG, executing any embedded JavaScript.
Example Attack Payload (SVG):
<svg xmlns="http://www.w3.org/2000/svg">
<script>alert('XSS')</script>
</svg>
cURL Command:
curl -X POST \ -H "X-Parse-Application-Id: YOUR_APP_ID" \ -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \ -F "[email protected];type=image/svg+xml" \ https://your-parse-server.com/files/evil.svg
Protection
- Upgrade Parse Server: Immediately upgrade to version `8.6.79` or `9.9.1-alpha.4` or later.
- Use an Allowlist: Configure `fileUpload.fileExtensions` as a strict allowlist of only the extensions your application requires, as shown in the code example above.
- Isolate File Serving: Serve uploaded files from a separate domain (e.g.,
files.your-app.com) to isolate any executed content from your main application’s origin. - Configure Storage Adapter: For S3/GCS, configure the storage adapter or CDN to derive the `Content-Type` from the filename extension instead of trusting the stored value.
Impact
- Stored Cross-Site Scripting (XSS): An attacker can inject malicious scripts that are stored on the server and executed in the browsers of other users who view the uploaded file.
- Session Hijacking: The XSS can be used to steal session tokens, user credentials, or other sensitive data from the browser’s local storage.
- Data Theft: The attacker can perform actions on behalf of the victim, leading to data theft or unauthorized actions.
- Reputational Damage: A successful XSS attack can severely damage the trust users place in the application.
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Sources:
Reported By: github.com
Extra Source Hub:
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