CVE-2021-47857
Published: 21 January 2026
Summary
CVE-2021-47857 is a high-severity Cross-site Scripting (CWE-79) vulnerability in Moodle Moodle. Its CVSS base score is 7.2 (High).
Operationally, ranked at the 11.1th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
Threat & Defense Details
Likely Mitigating ControlsAI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Penetration testing submits XSS payloads to web applications, detecting cross-site scripting flaws for subsequent remediation.
Validates web inputs to reject script-related content that could produce XSS.
Output validation against expected content can reject or sanitize script content in generated web pages, reducing XSS exploitability.
NVD Description
Moodle 3.10.3 contains a persistent cross-site scripting vulnerability in the calendar event subtitle field that allows attackers to inject malicious scripts. Attackers can craft a calendar event with malicious JavaScript in the subtitle track label to execute arbitrary code when…
more
users view the event.
Deeper analysisAI
CVE-2021-47857 is a persistent cross-site scripting (XSS) vulnerability, classified under CWE-79, affecting Moodle version 3.10.3. The flaw exists in the calendar event subtitle field, specifically the subtitle track label, which fails to properly sanitize user input. This allows attackers to inject malicious JavaScript scripts into calendar events.
Unauthenticated attackers can exploit this vulnerability remotely over the network with low attack complexity and no user interaction required. By crafting a calendar event with malicious JavaScript in the subtitle track label, attackers cause the script to execute arbitrary code in the browser of any user who views the event. The vulnerability has a CVSS v3.1 base score of 7.2 (AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N), enabling low confidentiality and integrity impacts with a changed scope.
Advisories and resources include the official Moodle site (https://moodle.org/), a proof-of-concept exploit on Exploit-DB (https://www.exploit-db.com/exploits/49714), and a detailed advisory from Vulncheck (https://www.vulncheck.com/advisories/moodle-label-persistent-cross-site-scripting). The CVE was published on 2026-01-21T18:16:16.567, with the Exploit-DB entry indicating public exploit availability.
Details
- CWE(s)