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CVE-2021-21809

Status: ModifiedCritical (9.1)—Exploit
Severity9.1Critical · CVSS 3.1
Exploitation probability24 %higher than 98% of CVEs
Active exploitationNonot in the CISA catalog
Public exploitAvailableProof of concept on GitHub (unverified) · Metasploit module (reliable, widely available exploit)

A command execution vulnerability exists in the default legacy spellchecker plugin in Moodle 3.10. A specially crafted series of HTTP requests can lead to command execution. An attacker must have administrator privileges to exploit this vulnerabilities.

CVSS

Base score
9.1
Version
3.1
Vector
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Exploitation probability (EPSS)

Probability of exploitation in the next 30 days
24 %
Percentile among all scored CVEs
98
Score date
10/25/2026

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

Public exploits

Exploit code or detection templates are publicly available. This is not the same as confirmed active exploitation (KEV), but it raises the risk: patch with priority.

⚠️ GitHub proofs of concept are not verified: some are fake or contain malware. Never run them outside an isolated lab.

Affected technologies (1)

Moodle — Moodle

CWEs

References

Show 4 references

Raw JSON (NVD)

Show
{
  "id": "CVE-2021-21809",
  "cveTags": [],
  "metrics": {
    "cvssMetricV2": [
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        "type": "Primary",
        "source": "nvd@nist.gov",
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          "accessVector": "NETWORK",
          "vectorString": "AV:N/AC:L/Au:S/C:C/I:C/A:C",
          "authentication": "SINGLE",
          "integrityImpact": "COMPLETE",
          "accessComplexity": "LOW",
          "availabilityImpact": "COMPLETE",
          "confidentialityImpact": "COMPLETE"
        },
        "acInsufInfo": false,
        "impactScore": 10,
        "baseSeverity": "HIGH",
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    "cvssMetricV30": [
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  "published": "2021-06-23T22:15:08.407",
  "references": [
    {
      "url": "http://packetstormsecurity.com/files/164481/Moodle-SpellChecker-Path-Authenticated-Remote-Command-Execution.html",
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      "source": "af854a3a-2127-422b-91ae-364da2661108"
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  "vulnStatus": "Modified",
  "weaknesses": [
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      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-78"
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  "descriptions": [
    {
      "lang": "en",
      "value": "A command execution vulnerability exists in the default legacy spellchecker plugin in Moodle 3.10. A specially crafted series of HTTP requests can lead to command execution. An attacker must have administrator privileges to exploit this vulnerabilities."
    },
    {
      "lang": "es",
      "value": "Se presenta una vulnerabilidad de ejecución de comandos en el plugin default legacy spellchecker en Moodle versión 3.10. Una serie de peticiones HTTP especialmente diseñadas pueden conllevar a una ejecución de comandos. Un atacante debe tener privilegios de administrador para explotar esta vulnerabilidades"
    }
  ],
  "lastModified": "2026-06-17T03:36:09.460",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
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  "sourceIdentifier": "talos-cna@cisco.com"
}