Summary
Ghost does not properly sanitize data inserted into the JSON-LD HTML output generated by the {{ghost_head}} helper. An authenticated user with limited privileges may inject content that executes as script when a published page is viewed, potentially compromising a staff user's administrative session.
Affected context
The vulnerability affects Ghost installations using vulnerable versions of the {{ghost_head}} helper to emit JSON-LD content. It becomes risky when a limited-privilege authenticated user can place controllable data into content or metadata that is later rendered on a published page without adequate output encoding.
Exploit preconditions
- Attack surface: Published Ghost pages containing JSON-LD output generated by the
{{ghost_head}}helper - Required access: An authenticated account with permissions to influence the relevant content or metadata
- Trigger point: A staff user or other privileged visitor views the affected published page
- Why exploitation is possible: User-controlled data is emitted into an HTML script context without sufficient sanitization or context-aware encoding
Why this happens
The underlying flaw is improper neutralization of user-controlled data before it is placed into an HTML script element. JSON-LD content must be safely serialized and encoded for its surrounding HTML context; otherwise, attacker-controlled values can alter the interpreted markup or script content. This behavior maps to CWE-79: Improper Neutralization of Input During Web Page Generation, commonly known as cross-site scripting.
Potential impact
- Arbitrary script execution in a staff user's browser
- Theft or misuse of accessible session information
- Unauthorized actions performed through a compromised administrative session
- Modification of site content or configuration
- Exposure of data available to the affected staff user
General mitigation guidance
Mitigation generally focuses on ensuring that all data placed in JSON-LD and other HTML script contexts is serialized and encoded using context-appropriate, secure-by-default mechanisms.
Common defensive patterns include:
- Applying strict output encoding at the point where data enters the HTML context
- Using well-maintained serializers that safely emit JSON-LD for HTML embedding
- Reducing trust in metadata and content supplied by lower-privilege users
- Enforcing least privilege for accounts that can modify published content or metadata
- Applying a restrictive Content Security Policy where practical as defense in depth
- Validating rendered output through security-focused automated and manual testing
How teams detect issues like this
Organizations can look for this weakness through:
- Static analysis rules identifying unescaped values in HTML or script contexts
- Code review checks for JSON serialization followed by unsafe HTML interpolation
- Tests that place representative special characters in user-controlled metadata and inspect rendered pages
- Content security policy reports indicating unexpected script execution or violations
- Policy enforcement requiring context-aware output encoding for all templated HTML
- Automated security review tools that flag stored cross-site scripting patterns
