CVE-2021-29484

DOM XSS in Theme Preview

Workarounds

• Log out of Ghost Admin
• Suspend any users who cannot log out
• Block access to /ghost/preview

Example of how to block access in nginx:

location ~ /ghost/preview {
    rewrite ^(\/(.*\/)?ghost\/)(.*)$ $1 redirect;
}

This redirects the endpoint to /ghost/, which mimics the behaviour after the patch is applied.

References

• Forum Post

For more information

If you have any questions or comments about this advisory:

• email us at [email protected]

Credits: Paul Gerste, SonarSource (https://www.sonarsource.com/)

An unused endpoint added during the development of 4.0.0 has left sites vulnerable to untrusted users gaining access to Ghost Admin. Attackers can gain access by getting logged in users to click a link containing malicious code. Users do not need to enter credentials and may not know they've visited a malicious site.

Ghost(Pro) has already been patched. We can find no evidence that the issue was exploited on Ghost(Pro) prior to the patch being added.

Self-hosters are impacted if running Ghost a version between 4.0.0 and 4.3.2. Immediate action should be taken to secure your site - see patches & workarounds below.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2021-29484 has a CVSS score of 6.8 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.3.3); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Fixed in 4.3.3, all 4.x sites should upgrade as soon as possible.
As the endpoint is unused, the patch simply removes it.

1. What is CVE-2021-29484? CVE-2021-29484 is a medium-severity cross-site scripting (XSS) vulnerability in ghost (npm), affecting versions >= 4.0.0, < 4.3.3. It is fixed in 4.3.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2021-29484? CVE-2021-29484 has a CVSS score of 6.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of ghost are affected by CVE-2021-29484? ghost (npm) versions >= 4.0.0, < 4.3.3 is affected.
4. Is there a fix for CVE-2021-29484? Yes. CVE-2021-29484 is fixed in 4.3.3. Upgrade to this version or later.
5. Is CVE-2021-29484 exploitable, and should I be worried? Whether CVE-2021-29484 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-29484 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-29484? Upgrade ghost to 4.3.3 or later.

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