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Denied Extension Bypass in Theme Serving via URL Encoding

Moderate
acburdine published GHSA-6v8p-3jx9-8chr Aug 13, 2026

Package

npm ghost (npm)

Affected versions

<v6.20.0

Patched versions

v6.20.0

Description

Impact

An unauthenticated user could read theme templates and metadata due to a bug in file extension filtering.

Vulnerable versions

This vulnerability is present in Ghost versions prior to v6.20.0.

Patches

v6.20.0 contains a fix for this issue.

How to update

For self-hosters using Docker, find Docker's official Ghost image here. Updating a Docker-based Ghost instance is documented here.

If your Ghost is a Ghost-CLI install see our documentation on updating it to the latest version here.

References

We thank Sho Odagiri of GMO Cybersecurity by Ierae, Inc. for disclosing this vulnerability responsibly.

For more information

If you have any questions or comments about this advisory, email us at security@ghost.org.

Severity

Moderate

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 v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

CVE ID

No known CVE

Weaknesses

Improper Handling of Alternate Encoding

The product does not properly handle when an input uses an alternate encoding that is valid for the control sphere to which the input is being sent. Learn more on MITRE.

Exposure of Sensitive Information to an Unauthorized Actor

The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. Learn more on MITRE.