CVE-2021-26929: An XSS issue was discovered in Horde Groupware Webmail Edition through 5.2.22 (where the Horde_Text_Filter lib

Q: How do I fix CVE-2021-26929?

groupware: upgrade to newer than 5.2.22; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-26929 being exploited?

CVE-2021-26929 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-26929 affect?

CVE-2021-26929 affects groupware, debian linux. Severity is medium (CVSS 6.1). 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

Q: How urgent is CVE-2021-26929?

Priority 54/100. EPSS top 8%: very likely to be exploited; Public exploit reference; On your watchlist

This month

MEDIUM 6.1WATCHLISTUPGRADEMITIGATE

CVE-2021-26929 is a medium-severity flaw (CVSS 6.1) affecting groupware, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

EPSS top 8%: very likely to be exploited
Public exploit reference
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	4.9% (top 8%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-02-14
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · horde

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-02-14 04:15 UTCPublished CVE record published
2026-06-17 03:44 UTCUpdated Record last modified

horde groupware UPGRADE

Affected: <= 5.2.22

Fixed in: newer than 5.2.22

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy groupware || rpm -q groupware || groupware --version

2. UPGRADE to newer than 5.2.22 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade groupware

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade groupware

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Exploit-DB: Horde Groupware Webmail 5.2.22 - Stored XSS

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-26929?

groupware: upgrade to newer than 5.2.22; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-26929 being exploited?

CVE-2021-26929 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-26929 affect?

CVE-2021-26929 affects groupware, debian linux. Severity is medium (CVSS 6.1). 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

How urgent is CVE-2021-26929?

Priority 54/100. EPSS top 8%: very likely to be exploited; Public exploit reference; On your watchlist

Software pages

PHP vulnerabilities · Debian vulnerabilities

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References

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