What is CVE-2021-26929?

CVE-2021-26929 is a medium-severity vulnerability in Horde Groupware, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2021-02-14.

How severe is CVE-2021-26929?

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

Is CVE-2021-26929 known to be exploited?

7 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

XSS in Horde Groupware

CVE-2021-26929

An XSS issue was discovered in Horde Groupware Webmail Edition through 5.2.22 (where the Horde_Text_Filter library before 2.3.7 is used). The attacker can send a plain text e-mail message, with JavaScript encoded as a link or email that is mishandled by preProcess in Text2html.php, because bespoke use of \x00\x00\x00 and \x01\x01\x01 interferes with XSS defenses.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-02-14 · 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: 0.049 (92.1th percentile), scored 2026-10-24.

Low probability of exploitation in the next 30 days, but still tracked. 92nd percentile — 92.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.049 · newest: 0.049 · change: 0.000

Affected products

Horde Groupware
Debian Linux — versions 9.0

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

Public proof-of-concept exploits

References

github.com/horde/webmail/releases (Third Party Advisory)
www.horde.org/apps/webmail (Vendor Advisory)
www.alexbirnberg.com/horde-xss.html (Exploit, Third Party Advisory)
lists.horde.org/archives/announce/2021/001298.html (Mailing List, Vendor Advisory)
lists.debian.org/debian-lts-announce/2021/02/msg00028.html (mailing-list, Mailing List, Third Party Advisory)
packetstormsecurity.com/files/162187/Webmail-Edition-5.2.22-XSS-Remote-Code-Exe… (Exploit, VDB Entry, Third Party Advisory)
packetstormsecurity.com/files/162194/Horde-Groupware-Webmail-5.2.22-Cross-Site-… (Third Party Advisory)

Frequently asked questions

What is CVE-2021-26929?: CVE-2021-26929 is a medium-severity vulnerability in Horde Groupware, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2021-02-14.
How severe is CVE-2021-26929?: Medium severity. CVSS v3 base score is 6.1 out of 10.
Is CVE-2021-26929 known to be exploited?: 7 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.