CVE-2021-20723Network attack vector

Browser script execution via unescaped character conversion

Published May 24, 2021 · Updated Aug 3, 2024

Reflected XSS in PHP Factory MailForm01 2014-12-12 through 2018-07-27 allows remote attackers to execute arbitrary script in a user's browser. In mail.php's full-width-to-half-width conversion function, mb_convert_kana() returns form input to the confirmation page without the h() HTML-escaping wrapper. Exploitation requires the nondefault hankaku conversion option to be enabled and a user to click an attacker-controlled link, after which the injected script runs in that user's browser.

CVSS severity4.7
Medium
EPSS probability0.78%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reflected XSS in PHP Factory MailForm01 2014-12-12 through 2018-07-27 allows remote attackers to execute arbitrary script in a user's browser. In mail.php's full-width-to-half-width conversion function, mb_convert_kana() returns form input to the confirmation page without the h() HTML-escaping wrapper. Exploitation requires the nondefault hankaku conversion option to be enabled and a user to click an attacker-controlled link, after which the injected script runs in that user's browser.

The record

CVE
CVE-2021-20723
Published
May 24, 2021
Updated
Aug 3, 2024
Vendor
PHP Factory
Product
MailForm01
Classifications
CWE-79, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 24, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=versions which the last updated date listed at the top of descriptions in the program file is from 2014 December 12 to 2018 July 27

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

PHP Factory · MailForm01version=versions which the last updated date listed at the top of descriptions in the program file is from 2014 December 12 to 2018 July 27

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
2.6JVNAV:N/AC:H/Au:N/C:N/I:P/A:N
4.7JVNCVSS:3.0/AV:N/AC:H/PR:N/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 exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1204.001Malicious Link

Public exploit references

No public exploit references are available in this record.

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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