CVE-2021-27279Public exploitNetwork attack vector

Stored XSS via nested email-tag reparsing

Published Feb 22, 2021 · Updated Aug 3, 2024

Stored XSS in MyBB before 1.8.25 allows remote attackers to execute browser-side script via nested [email] MyCode tags. The mycode_parse_email function in inc/class_parser.php leaves bracket characters in an href value, enabling a second MyCode parsing pass to insert unescaped, colliding quotation marks. A victim must render or preview the crafted message; exposure includes saved posts or private messages and guest submission where MyCode is enabled.

CVSS severity5.4
Medium
EPSS probability0.96%
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 MyBB before 1.8.25 allows remote attackers to execute browser-side script via nested [email] MyCode tags. The mycode_parse_email function in inc/class_parser.php leaves bracket characters in an href value, enabling a second MyCode parsing pass to insert unescaped, colliding quotation marks. A victim must render or preview the crafted message; exposure includes saved posts or private messages and guest submission where MyCode is enabled.

The record

CVE
CVE-2021-27279
Published
Feb 22, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-79, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 22, 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

3.5NVDAV:N/AC:M/Au:S/C:N/I:P/A:N
5.4NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/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')
T1190Exploit Public-Facing Application

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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