CVE-2021-21376 is a medium-severity (CVSS 6.5) Information Exposure vulnerability in omero-web. A fix is available for omero-web — see the affected versions and patch details below.
OMERO.web exposes some unnecessary session information in the page
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21376 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
omero-webReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
OMERO.web is open source Django-based software for managing microscopy imaging. OMERO.web before version 5.9.0 loads various information about the current user such as their id, name and the groups they are in, and these are available on the main webclient pages. This represents an information exposure vulnerability. Some additional information being loaded is not used by the webclient and is being removed in this release. This is fixed in version 5.9.0.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | omero-web | all versions | 5.9.0pip install --upgrade 'omero-web==5.9.0' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for omero-web, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update omero-web to 5.9.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21376 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21376 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21376. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-21376 in your dependencies?
O3 Security finds CVE-2021-21376 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.