How severe is CVE-2021-21376?

CVE-2021-21376 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21376?

CVE-2021-21376 affects the following packages: omero-web (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21376?

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.

How do I detect CVE-2021-21376 in my PyPI dependencies?

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.

How do I mitigate CVE-2021-21376 if there is no patch (or I can't update yet)?

Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Is CVE-2021-21376 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21376 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21376?

CVE-2021-21376 has an EPSS (Exploit Prediction Scoring System) score of 1.5%, placing it in the 74th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21376?

CVE-2021-21376 is classified as Information Exposure (CWE-200). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21376 published, and has it been updated?

CVE-2021-21376 was published on March 23, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2021-21376 — omero-web

MEDIUMFix: ome/omero-web@952f8e5

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

Also known asGHSA-gfp2-w5jm-955qPYSEC-2021-31

Published

Mar 23, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs74th percentile — riskier than 74% of all scored CVEsHighest risk

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

1 pkg affected

🐍omero-web

Real-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

Background

OMERO.web 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. Some additional information being loaded is not used by the webclient and is being removed in this release.

Impact

OMERO.web before 5.9.0

Patches

5.9.0

Workarounds

No workaround

References

For more information

If you have any questions or comments about this advisory:

Open an issue in omero-web
Email us at security

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`omero-web`	all versions	5.9.0`pip install --upgrade 'omero-web==5.9.0'`

Affected Products

1 product · 1 configurations

Application

omero.webopenmicroscopy

< 5.9.0

range

Detection & mitigation playbook

Open-source dependency

Detect
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
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Frequently Asked Questions

### Background OMERO.web 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. Some additional information being loaded is not used by the webclient and is being removed in this release. ### Impact OMERO.web before 5.9.0 ### Patches 5.9.0 ### Workarounds No workaround ### References ### For more information If you have any questions or comments about this advisory: * Open an issue in [omero-web](https://github.com/ome/omero-web) * Email us at [security](mailto:[email protected])