How severe is CVE-2021-21377?

CVE-2021-21377 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21377?

CVE-2021-21377 affects the following packages: omero-web (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21377?

Update omero-web to 5.9.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21377 is resolved across your whole dependency graph.

How do I detect CVE-2021-21377 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-2021-21377 if there is no patch (or I can't update yet)?

Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Is CVE-2021-21377 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21377 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-2021-21377?

CVE-2021-21377 has an EPSS (Exploit Prediction Scoring System) score of 0.8%, placing it in the 57th 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-2021-21377?

CVE-2021-21377 is classified as Open Redirect (CWE-601). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21377 published, and has it been updated?

CVE-2021-21377 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-2021-21377 — omero-web

MEDIUMFix: ome/omero-web@952f8e5

CVE-2021-21377 is a medium-severity (CVSS 5.4) Open Redirect vulnerability in omero-web. A fix is available for omero-web — see the affected versions and patch details below.

OMERO webclient does not validate URL redirects on login or switching group.

Also known asGHSA-g4rf-pc26-6hmrPYSEC-2021-32

Published

Mar 23, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs57th percentile — riskier than 57% 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 supports redirection to a given URL after performing login or switching the group context. These URLs are not validated, allowing redirection to untrusted sites. OMERO.web 5.9.0 adds URL validation before redirecting. External URLs are not considered valid, unless specified in the omero.web.redirect_allowed_hosts setting.

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-2021-21377 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

### Background OMERO.web supports redirection to a given URL after performing login or switching the group context. These URLs are not validated, allowing redirection to untrusted sites. OMERO.web 5.9.0 adds URL validation before redirecting. External URLs are not considered valid, unless specified in the ``omero.web.redirect_allowed_hosts`` setting. ### 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-w