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Parse Server vulnerable to stored XSS via malformed Content-Type bypassing file upload extension blocklist

Low severity GitHub Reviewed Published Jun 25, 2026 in parse-community/parse-server • Updated Oct 29, 2026

Package

npm parse-server (npm)

Affected versions

>= 9.0.0, < 9.10.0-alpha.2
<= 8.6.83

Patched versions

9.10.0-alpha.2
8.6.84

Description

Impact

Parse Server's file upload validation uses the fileUpload.fileExtensions option (enabled by default with a blocklist) to reject files that browsers may render as active content, mitigating stored cross-site scripting (XSS). When an uploaded file's extension is not recognized by the mime package, Parse Server preserves the client-supplied Content-Type and passes it to the storage adapter. A malformed Content-Type that is not a valid type/subtype media type — for example image (no subtype), image/ (empty subtype), or image//svg+xml (invalid token) — bypassed the blocklist and was stored unchanged. On storage adapters that persist and serve the uploaded Content-Type (such as Amazon S3, Google Cloud Storage, or Azure Blob Storage), a browser cannot parse the malformed Content-Type and falls back to MIME-sniffing the file content; a file whose body begins with HTML markup is then rendered as HTML, executing any embedded script in the application's origin. This allows a user permitted to upload files to store content that executes as script against other users who open the file URL. The default GridFS storage adapter is not affected, as it recomputes the Content-Type from the filename and serves files with X-Content-Type-Options: nosniff.

Patches

The file upload handler now validates that the client-supplied Content-Type is a well-formed media type (type/subtype with non-empty tokens per RFC 9110 §5.6.2) whenever the filename extension is not recognized by mime. Malformed values are rejected instead of being stored, while well-formed types — including custom or vendor types that the mime package does not recognize — continue to be accepted; only blocklisted subtypes are rejected.

Workarounds

Configure fileUpload.fileExtensions as a strict allowlist of only the extensions the application needs (for example ["^(png|jpe?g|gif|pdf)$"]). Additionally, serve uploaded files from a domain separate from the application so that any rendered content is isolated from the application's origin, and configure the storage/CDN layer to send X-Content-Type-Options: nosniff.

References

References

@mtrezza mtrezza published to parse-community/parse-server Jun 25, 2026
Published to the GitHub Advisory Database Oct 29, 2026
Reviewed Oct 29, 2026
Last updated Oct 29, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required Low
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(36th percentile)

Weaknesses

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. Learn more on MITRE.

CVE ID

CVE-2026-61448

GHSA ID

GHSA-r899-h629-j84r

Source code

Credits

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