Drupal Core Double Extension File Upload Vulnerability (CVE-2020-13671) Analysis and Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTDrupal Core versions prior to 9.0.8, 8.9.9, 8.8.11, and 7.74
Attackers can upload malicious files with double extensions (e.g., filename.php.txt) to execute arbitrary PHP code (RCE) under certain hosting configurations
Immediately upgrade Drupal Core to the patched versions (9.0.8, 8.9.9, 8.8.11, 7.74 or higher)
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
For Composer-managed Drupal installations, run the following commands to update to a secure version:
# Upgrade Drupal 8.x / 9.x core dependencies
composer update drupal/core-recommended --with-dependencies
# Upgrade Drupal 7.x using drush
drush pm-update drupal2. Temporary Workaround & Hardening Configuration
If patching cannot be applied immediately, restrict PHP execution within the public files directory using web server configuration rules.
# Nginx configuration to block PHP execution in the files directory
location ~* ^/sites/default/files/.*\.(php|phtml|php5)$ {
deny all;
return 403;
}3. Log Audit & IoC Verification Commands
Use the following commands to scan for potentially malicious double-extension files and audit web server access logs.
# Scan the files directory for suspicious double extensions containing .php
find /var/www/html/sites/default/files -type f -name '*.php.*'
# Search web server access logs for requests targeting double-extension files
grep -rE '\.php\.[a-zA-Z0-9]+' /var/log/nginx/access.log /var/log/apache2/access.logBlock Exploits Instantly with Managed Virtual Patching Rules
Can't patch production immediately? Deploy managed WAF rules at the edge to intercept RCE, SQLi, and deserialization payloads before they hit your origin server.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2020-13671) exists because Drupal Core does not properly sanitize filenames on uploaded files. When a file with multiple extensions is uploaded, the sanitization mechanism fails to recognize and strip dangerous intermediate extensions, leading to files being saved with potentially executable structures.
2. Exploitation Mechanics & Attack Scenario
In specific hosting environments (such as Apache with misconfigured AddHandler directives or Nginx passing requests to PHP-FPM without strict path validation), files with names like 'malicious.php.txt' may be interpreted and executed as PHP scripts. An attacker with basic file upload privileges can upload a web shell disguised with a double extension and execute it by directly requesting the file's URL.
3. Infrastructure & Business Impact
As this vulnerability is listed in the CISA KEV catalog, it represents a highly active threat vector. Successful exploitation leads to Remote Code Execution (RCE), allowing attackers to compromise the underlying server, exfiltrate database credentials, deface the website, or pivot into the internal network.
Official Vendor Advisories & References
Drupal core does not properly sanitize certain filenames on uploaded files, which can lead to files being interpreted as the incorrect extension and served as the wrong MIME type or executed as PHP for certain hosting configurations. This issue affects: Drupal Drupal Core 9.0 versions prior to 9.0.8, 8.9 versions prior to 8.9.9, 8.8 versions prior to 8.8.11, and 7 versions prior to 7.74.
Frequently Asked SecOps Questions (CVE-2020-13671)
FAQQ1. How critical is CVE-2020-13671 and is it actively exploited in the wild?
CVE-2020-13671 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 36.2%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-13671?
Primary impacted vendors include drupal across drupal_core. Scope summary: Drupal Core versions prior to 9.0.8, 8.9.9, 8.8.11, and 7.74
Q3. What temporary mitigations or workarounds exist for CVE-2020-13671 before patching?
[Mitigation] Immediately upgrade Drupal Core to the patched versions (9.0.8, 8.9.9, 8.8.11, 7.74 or higher) Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2020-13671
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