CVE-2021-39338Public exploitNetwork attack vector

Script execution via unsanitized settings parameters

Published Oct 15, 2021 · Updated Mar 31, 2025

Stored XSS in MyBB Cross-Poster 1.0 and earlier allows remote authenticated users with administrator access to inject scripts through settings. MyBBXPSettings.php accepts several settings parameters without sufficient validation or sanitization, allowing their values to persist as executable page content. Exploitation requires administrative access and a WordPress configuration that disables unfiltered_html; loading affected output executes the stored script.

CVSS severity5.5
Medium
EPSS probability0.95%
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 Cross-Poster 1.0 and earlier allows remote authenticated users with administrator access to inject scripts through settings. MyBBXPSettings.php accepts several settings parameters without sufficient validation or sanitization, allowing their values to persist as executable page content. Exploitation requires administrative access and a WordPress configuration that disables unfiltered_html; loading affected output executes the stored script.

The record

CVE
CVE-2021-39338
Published
Oct 15, 2021
Updated
Mar 31, 2025
Vendor
Jochen Gererstorfer
Product
MyBB Cross-Poster
Classifications
CWE-79, T1059.007
Attack vector
network
Privileges
admin

Timeline

How it unfolded

  1. Oct 15, 2021CVE publishedPublication date reported by the CVE source.
  2. Mar 31, 2025Record 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=1.0 <=1.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesadmin

What is affected?

Jochen Gererstorfer · MyBB Cross-Posterversion=1.0 <=1.0

Published CVSS scores

2.1NIST NVDAV:N/AC:H/Au:S/C:N/I:P/A:N
5.5WordfenceCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N
4.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:H/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.

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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
Privilegesadmin
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1059.007JavaScript/JScript

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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