CVE-2021-26929Public exploitNetwork attack vector

Stored XSS via forgeable filter placeholders

Published Feb 14, 2021 · Updated Aug 3, 2024

Stored XSS in Horde Groupware Webmail Edition through 5.2.22 allows remote attackers to execute JavaScript via crafted plain-text email. Horde_Text_Filter preProcess uses predictable NUL and control-byte markers to preserve generated links, then base64-decodes attacker-supplied marker contents without authenticating their origin. Execution occurs when a webmail user displays the message, allowing code to run in the webmail application context and act with that user session.

CVSS severity6.1
Medium
EPSS probability4.94%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in Horde Groupware Webmail Edition through 5.2.22 allows remote attackers to execute JavaScript via crafted plain-text email. Horde_Text_Filter preProcess uses predictable NUL and control-byte markers to preserve generated links, then base64-decodes attacker-supplied marker contents without authenticating their origin. Execution occurs when a webmail user displays the message, allowing code to run in the webmail application context and act with that user session.

The record

CVE
CVE-2021-26929
Published
Feb 14, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-79, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.1NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
4.3NVDAV:N/AC:M/Au:N/C:N/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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