What is CVE-2021-20875?

CVE-2021-20875 is a medium-severity vulnerability in Groupsession, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 6.1/10. Published 2021-12-24.

How severe is CVE-2021-20875?

Medium severity. CVSS v3 base score is 6.1 out of 10.

Open Redirect in Groupsession

CVE-2021-20875

Open redirect vulnerability in GroupSession Free edition ver5.1.1 and earlier, GroupSession byCloud ver5.1.1 and earlier, and GroupSession ZION ver5.1.1 and earlier allows a remote unauthenticated attacker to redirect users to arbitrary web sites and conduct phishing attacks by having a user to access a specially crafted URL.

Vulnerability class: Open Redirect

Published 2021-12-24 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.008 (55.6th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 56th percentile — 55.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.008 · newest: 0.008 · change: 0.000

Affected products

Groupsession
Japan Total System Co.,ltd. Groupsession Free Edition, Bycloud, Zion — versions GroupSession Free edition ver5.1.1 and earlier, GroupSession byCloud ver5.1.1 and earlier, and GroupSession ZION ver5.1.1 and earlier

Weakness classification (CWE)

CWE-601 — URL Redirection to Untrusted Site (Open Redirect)

References

groupsession.jp/info/info-news/security20211220 (Vendor Advisory)
jvn.jp/en/jp/JVN79798166/index.html (Third Party Advisory)

Frequently asked questions

What is CVE-2021-20875?: CVE-2021-20875 is a medium-severity vulnerability in Groupsession, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 6.1/10. Published 2021-12-24.
How severe is CVE-2021-20875?: Medium severity. CVSS v3 base score is 6.1 out of 10.