GHSA-v8x7-r927-cc93 BIT-parse-2026-55778 CVE-2026-55778

parse-server: Stored XSS via non-standard file extension bypassing file upload extension blocklist

Low npm

Severity: 0.0 / 10; Low · CVSS 4.0
Exploit chance (EPSS): 0.55%; Chance of exploitation in the next 30 days. Higher than 46.24% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-434 Unrestricted Upload of File with Dangerous Type
Fix: 9.9.1-alpha.11; Upgrade parse-server to this version or later

Description

Impact

Parse Server's default fileUpload.fileExtensions blocklist is intended to prevent uploading files that browsers render as active content (such as HTML and SVG), which can be used to perform stored cross-site scripting (XSS) attacks against other users. The blocklist could be bypassed by uploading a file whose extension is not an exact match of a blocked extension (for example a non-standard or compound extension) together with a dangerous content type. On storage adapters that persist and serve the uploaded content type (such as S3 and GCS), the file is then served with the attacker-supplied content type, enabling stored XSS against users who open the file URL.

This affects the default configuration, in which authenticated users are allowed to upload files. The default GridFS/filesystem adapter sets the X-Content-Type-Options: nosniff response header, which mitigates browser rendering on that adapter, but the upload restriction itself is still bypassed. This is an incomplete-fix follow-up of GHSA-vr5f-2r24-w5hc and GHSA-7wqv-xjf3-x35v.

Patches

The file upload extension validation now also evaluates the request content type against the configured blocklist whenever the filename's extension is not a recognized type. As a result, a dangerous content type can no longer be preserved by uploading a file with a non-standard extension, and such uploads are rejected.

Workarounds

Configure fileUpload.fileExtensions as a strict allowlist of only the file extensions your application needs (for example ["^(png|jpe?g|gif|pdf)$"]) instead of relying on the default blocklist. Additionally, serve uploaded files from a separate domain than the application, so that any executed content is isolated from the application's origin.

Affected versions

npm

Package	Vulnerable	Fixed in
parse-server	`>= 9.0.0, < 9.9.1-alpha.11`	`9.9.1-alpha.11`
parse-server	`< 8.6.81`	`8.6.81`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

parse-server npm → `9.9.1-alpha.11`

npm

npm install parse-server@9.9.1-alpha.11

yarn

yarn add parse-server@9.9.1-alpha.11

pnpm

pnpm add parse-server@9.9.1-alpha.11

Severity breakdown

CVSS 4.0 0.0

CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N

Attack vector: Network
Attack complexity: Low
Attack requirements: Present
Privileges required: Low
User interaction: Passive
Confidentiality (vulnerable system): None
Integrity (vulnerable system): Low
Availability (vulnerable system): None
Confidentiality (subsequent systems): Low
Integrity (subsequent systems): Low
Availability (subsequent systems): None

Weakness

CWE-434: Unrestricted Upload of File with Dangerous Type

The product allows the upload or transfer of dangerous file types that are automatically processed within its environment.

How to prevent it

Architecture and Design: Generate a new, unique filename for an uploaded file instead of using the user-supplied filename, so that no external input is used at all.[REF-422] [REF-423]
Architecture and Design: When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
Architecture and Design: Consider storing the uploaded files outside of the web document root entirely. Then, use other mechanisms to deliver the files dynamically. [REF-423]
Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. For example, limiting filenames to alphanumeric characters can help to restrict the introduction of unintended file extensions.
Architecture and Design: Define a very limited set of allowable extensions and only generate filenames that end in these extensions. Consider the possibility of XSS (CWE-79) before allowing .html or .htm file types.
Implementation: Ensure that only one extension is used in the filename. Some web servers, including some versions of Apache, may process files based on inner extensions so that "filename.php.gif" is fed to the PHP interpreter.[REF-422] [REF-423]
Implementation: When running on a web server that supports case-insensitive filenames, perform case-insensitive evaluations of the extensions that are provided.
Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Implementation: Do not rely exclusively on sanity checks of file contents to ensure that the file is of the expected type and size. It may be possible for an attacker to hide code in some file segments that will still be executed by the server. For example, GIF images may contain a free-form comments field.
Implementation: Do not rely exclusively on the MIME content type or filename attribute when determining how to render a file. Validating the MIME content type and ensuring that it matches the extension is only a partial solution.
Architecture and Design, Operation: Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.
Architecture and Design, Operation: Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software. OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.

References

Packages

github.com/parse-community/parse-server

Timeline

Jun 19, 2026 Published
Oct 25, 2026 Last updated by the source
Oct 31, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-r899-h629-j84r Parse Server vulnerable to stored XSS via malformed Content-Type bypassing file upload extension blocklist	Low	Also affects parse-server
GHSA-97pr-9hgg-3p8r parse-server: LiveQuery discloses object data to a subscriber across an ACL read-access change	Low	Also affects parse-server
GHSA-cgxm-vr2f-6fj8 parse-server: Denial of service via exponential-time processing of deeply nested query operators	High	Also affects parse-server
GHSA-wmwx-jr2p-4j4r parse-server: Relation `$relatedTo` query bypasses `protectedFields` and owning-object ACL	Medium	Also affects parse-server
GHSA-75v4-m273-5j49 parse-server: Endpoints `/login` and `/verifyPassword` disclose MFA secrets and protected fields when `_User` get is denied	Medium	Also affects parse-server
GHSA-7wqv-xjf3-x35v parse-server: Stored XSS via trailing-dot filename bypassing file upload extension blocklist	Low	Also affects parse-server
GHSA-p84r-h6rx-f2xr parse-server: Server option routeAllowList is bypassable through batch sub-requests	Medium	Also affects parse-server
GHSA-8cph-rgr4-g5vj Parse Server's GraphQL "Did you mean ...?" validation suggestions disclose schema to unauthenticated callers	Medium	Also affects parse-server

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