What is CVE-2021-22878?

CVE-2021-22878 is a medium-severity vulnerability in Nextcloud Server, classified under Cross-site Scripting. CVSS score: 4.8/10. Published 2021-03-03.

How severe is CVE-2021-22878?

Medium severity. CVSS v3 base score is 4.8 out of 10.

XSS in Nextcloud Server

CVE-2021-22878

Nextcloud Server prior to 20.0.6 is vulnerable to reflected cross-site scripting (XSS) due to lack of sanitization in `OC.Notification.show`.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-03-03 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.011 (63.1th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 63rd percentile — 63.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.011 · newest: 0.011 · change: 0.000

Affected products

Nextcloud Server
Fedoraproject Fedora — versions 34
Nextcloud Server — versions Fixed in 20.0.6

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

hackerone.com/reports/896522 (Exploit, Third Party Advisory, Issue Tracking)
nextcloud.com/security/advisory/ (Vendor Advisory)
github.com/nextcloud/server/pull/25234 (Patch, Third Party Advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)

Frequently asked questions

What is CVE-2021-22878?: CVE-2021-22878 is a medium-severity vulnerability in Nextcloud Server, classified under Cross-site Scripting. CVSS score: 4.8/10. Published 2021-03-03.
How severe is CVE-2021-22878?: Medium severity. CVSS v3 base score is 4.8 out of 10.