---
id: CVE-2021-21377
aliases:
  - GHSA-g4rf-pc26-6hmr
  - PYSEC-2021-32
title: OMERO webclient does not validate URL redirects on login or switching group.
summary: OMERO webclient does not validate URL redirects on login or switching group.
severity: medium
cvss: 4.8
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:N/A:N'
vendor: omero-web
product: omero-web
ecosystem: pip
affected:
  - omero-web < 5.9.0
patched:
  - omero-web 5.9.0
published: '2021-03-23'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-g4rf-pc26-6hmr'
references:
  - url: 'https://github.com/ome/omero-web/security/advisories/GHSA-g4rf-pc26-6hmr'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-21377'
  - url: >-
      https://github.com/ome/omero-web/commit/952f8e5d28532fbb14fb665982211329d137908c
  - url: 'https://github.com/ome/omero-web'
  - url: 'https://github.com/ome/omero-web/blob/master/CHANGELOG.md#590-march-2021'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/omero-web/PYSEC-2021-32.yaml
  - url: 'https://pypi.org/project/omero-web'
  - url: 'https://www.openmicroscopy.org/security/advisories/2021-SV2'
tags:
  - osv
  - pip
epss: 0.00826
epssPercentile: 0.5558
ingestedAt: '2026-07-08T18:25:49.225Z'
---

## Overview

### 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-web)
* Email us at [security](mailto:security@openmicroscopy.org.uk)

## Affected packages

- `omero-web < 5.9.0`

## Remediation

Upgrade to a patched release:

- `omero-web 5.9.0`