CVE-2021-20280Public exploitNetwork attack vector

Stored XSS and blind SSRF via decoded answers

Published Mar 15, 2021 · Updated Aug 3, 2024

Stored XSS and blind SSRF in Moodle LMS before 3.10.2, 3.9.5, 3.8.8, and 3.5.17 allow remote authenticated users to inject active content. The feedback module's complete_form.php and responses_table.php paths decode stored answer text instead of escaping it, causing encoded markup to be interpreted in form defaults and downloads. A low-privileged account must submit a crafted answer and another user must render it; impact is limited to script execution in that user's browser and blind outbound requests.

CVSS severity5.4
Medium
EPSS probability1.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS and blind SSRF in Moodle LMS before 3.10.2, 3.9.5, 3.8.8, and 3.5.17 allow remote authenticated users to inject active content. The feedback module's complete_form.php and responses_table.php paths decode stored answer text instead of escaping it, causing encoded markup to be interpreted in form defaults and downloads. A low-privileged account must submit a crafted answer and another user must render it; impact is limited to script execution in that user's browser and blind outbound requests.

The record

CVE
CVE-2021-20280
Published
Mar 15, 2021
Updated
Aug 3, 2024
Vendor
Moodle
Product
Moodle LMS
Classifications
CWE-79, T1190
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 15, 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=Fixed in 3.10.2, 3.9.5, 3.8.8, 3.5.17

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Moodle · Moodle LMSversion=Fixed in 3.10.2, 3.9.5, 3.8.8, 3.5.17

Published CVSS scores

5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
3.5NIST NVDAV:N/AC:M/Au:S/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
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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