Security readout for executives and security teams
Plain-English summary
CVE-2021-47857 is a stored cross-site scripting issue in Moodle 3.10.3. A malicious calendar event subtitle can store JavaScript that runs when someone views the event. This can expose session data or alter page content in the victim’s browser. Public exploit material exists, but the supplied sources do not show active exploitation.
Executive priority
Treat this as high priority for any Moodle 3.10.3 site with broad user access. The business risk is account abuse and content manipulation through users’ browsers. Prioritize inventory and vendor-guided remediation, but do not assume active exploitation from the supplied evidence.
Technical view
Moodle 3.10.3 is reported vulnerable to CWE-79 persistent XSS in the calendar event subtitle field, described as a subtitle track label. The provided CVSS is 7.2, network exploitable, low complexity, scope changed, with low confidentiality and integrity impact. ExploitDB and VulnCheck reference the issue; no fixed version is identified in the supplied bundle.
Likely exposure
Exposure is limited to Moodle 3.10.3 instances, especially internet-facing sites or environments where untrusted users can create or influence calendar events. Other Moodle versions are not identified as affected in the provided sources.
Exploitation context
A public ExploitDB entry exists, indicating exploit details are available. The source bundle marks CISA KEV as false and provides no cited evidence of active exploitation. Impact is browser-context execution against users who view the malicious event.
Researcher notes
The record describes stored XSS in the calendar event subtitle field and references public exploit material. The supplied sources do not name a patch level, confirm vendor advisory details, or prove exploitation in the wild. Validate findings non-destructively and avoid relying on unverified affected-version assumptions.
Mitigation direction
- Identify any Moodle 3.10.3 deployments and prioritize review.
- Check Moodle’s official security guidance for fixed versions or workarounds.
- Upgrade if vendor guidance identifies a safe fixed release.
- Restrict calendar event creation to trusted users where operationally possible.
- Review content filtering and input sanitization controls for event subtitles.
Validation and detection
- Inventory Moodle versions across production and test environments.
- Confirm whether version 3.10.3 is present.
- Review who can create calendar events on affected sites.
- Inspect recent calendar events for suspicious subtitle or label content.
- Monitor web logs and user reports for anomalous script execution symptoms.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-47857 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.2 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N3.92.7Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.2HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- ExploitDB-49714CVE reference · exploit
- Official Moodle Project HomepageCVE reference · product
- VulnCheck Advisory: Moodle 3.10.3 - 'label' Persistent Cross Site ScriptingCVE reference · third-party-advisory
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.