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Sveltia CMS: Stored XSS in Markdown/RichText preview via unsandboxed same-origin iframe

Low severity GitHub Reviewed Published Jun 18, 2026 in sveltia/sveltia-cms

Package

npm @sveltia/cms (npm)

Affected versions

< 0.167.3

Patched versions

0.167.3

Description

Impact

A stored cross-site scripting (XSS) vulnerability affected the Markdown/RichText field preview renderer in Sveltia CMS.

The DOMPurify sanitization configuration used for Markdown previews explicitly permitted iframe elements without enforcing a sandbox attribute or restricting iframe sources. Sanitized Markdown output was then inserted into the CMS preview DOM as raw HTML. Because no sandboxing or source validation was applied, a Markdown field containing an iframe whose src pointed to a same-origin uploaded or publicly accessible HTML asset would render an unsandboxed iframe. JavaScript in the framed document could then access the parent CMS window via the browser’s same-origin policy, allowing it to read and write the parent window’s state, DOM, and browser storage, and to trigger actions in the CMS context.

The practical impact is limited in currently supported Sveltia CMS usage because the CMS is intended for a single developer or a small trusted team, and open authoring / untrusted multi-user authoring is not currently implemented. Exploitation requires the ability to place malicious content into the repository or content source that the CMS loads.

Patches

The issue has been patched by replacing the direct DOMPurify call in the Markdown preview renderer with a two-pass sanitization pipeline:

  1. DOMPurify processes the raw HTML as before.
  2. A post-processing step validates every iframe element in the sanitized output. Any iframe whose src does not use an https:// URL, uses a same-origin URL, uses a relative path, or uses any other non-HTTPS scheme (javascript:, data:, blob:, , etc.) is removed from the output. Surviving iframes (HTTPS cross-origin only) have a sandbox attribute enforced with at minimum allow-scripts allow-same-origin, which confines the framed document to its own origin and prevents it from accessing the parent CMS window.

Users should upgrade to Sveltia CMS v0.167.3 or later.

Workarounds

If upgrading is not immediately possible, avoid loading CMS content from untrusted authors and review Markdown field content for iframe elements pointing to same-origin or relative paths.

Administrators can also reduce exposure by limiting repository or content source write access to trusted users only.

References

References

@kyoshino kyoshino published to sveltia/sveltia-cms Jun 18, 2026
Published to the GitHub Advisory Database Jun 19, 2026
Reviewed Jun 19, 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 None
Privileges Required Low
User interaction Active
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
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:N/PR:L/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:U

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-h5jc-78hr-3pc9

Source code

Credits

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