CVE-2020-1934: Apache HTTP Server Memory Vulnerability

CVE-2020-1934 Overview

CVE-2020-1934 is an uninitialized memory use vulnerability [CWE-908] in the mod_proxy_ftp module of Apache HTTP Server versions 2.4.0 through 2.4.41. When the server proxies requests to a malicious FTP backend, the module may read from uninitialized memory regions during response processing. An attacker controlling the upstream FTP server can trigger the condition to influence proxy behavior or leak small fragments of process memory back through the HTTP response path. The flaw affects Apache HTTPD distributions shipped by Debian, Ubuntu, Fedora, openSUSE, and several Oracle products that embed Apache.

Critical Impact

A malicious FTP server reachable through mod_proxy_ftp can cause Apache HTTPD to operate on uninitialized memory, enabling limited information disclosure from the worker process.

Affected Products

• Apache HTTP Server 2.4.0 through 2.4.41
• Debian Linux 9, 10; Ubuntu 16.04 ESM, 18.04 LTS, 20.04 LTS; Fedora 31, 32; openSUSE Leap 15.1
• Oracle Communications Element Manager, Session Report Manager, Session Route Manager (8.1.1–8.2.1), Enterprise Manager Ops Center 12.4, Instantis EnterpriseTrack, and ZFS Storage Appliance Kit 8.8

Discovery Timeline

• 2020-04-01 - CVE-2020-1934 published to the National Vulnerability Database
• 2020-04-13 - NetApp publishes downstream security advisory
• 2020-07 - Oracle includes the fix in its July 2020 Critical Patch Update
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-1934

Vulnerability Analysis

The defect resides in mod_proxy_ftp, the Apache HTTPD module that allows the web server to act as a gateway between HTTP clients and remote FTP servers. When the module parses certain FTP control-channel responses, it consumes values from buffers or structures that were not fully initialized before use. The result is classified as CWE-908: Use of Uninitialized Resource.

Because the attacker controls the FTP server, they can craft replies that steer Apache's parsing logic into code paths where uninitialized stack or heap content is referenced. Depending on memory layout, the uninitialized bytes can be reflected in the HTTP response returned to the original client, producing an information disclosure primitive limited to confidentiality. The CVSS profile reflects this scope: confidentiality impact only, with no integrity or availability effect.

Root Cause

The root cause is a failure in mod_proxy_ftp to guarantee that all parsing buffers and response structures are zero-initialized or fully populated before being read. Specific code paths handling FTP server replies advance without setting fields that subsequent logic assumes are valid. The fix in Apache HTTPD 2.4.42 ensures the affected buffers are initialized before use.

Attack Vector

Exploitation requires that an Apache HTTPD instance has mod_proxy_ftp enabled and is configured to proxy FTP requests on behalf of clients. An attacker either operates a malicious FTP server or coerces the proxy into connecting to one through a controlled URL. The attacker then issues FTP responses designed to trigger the uninitialized read. Authentication is not required, and the attack is reachable over the network through normal HTTP proxy semantics. The vulnerability does not yield code execution and is constrained to leaking limited memory contents.

No verified public proof-of-concept code is available. Technical details are documented in the Apache HTTPD Vulnerability List and the upstream commit threads on the Apache developer mailing lists.

Detection Methods for CVE-2020-1934

Indicators of Compromise

• Outbound FTP control-channel connections from Apache HTTPD worker processes to untrusted or unexpected external hosts on TCP/21
• HTTP responses returned by mod_proxy_ftp containing unexpected binary fragments or non-ASCII filler in directory listings or error bodies
• Apache error_log entries referencing proxy_ftp parsing failures or malformed server replies

Detection Strategies

• Inventory Apache HTTPD installations and identify those running 2.4.0 through 2.4.41 with mod_proxy_ftp loaded via httpd -M or apachectl -M
• Review httpd.conf and included configuration fragments for ProxyPass or RewriteRule directives forwarding to ftp:// URLs
• Use vulnerability scanners with version-based plugins for CVE-2020-1934 against externally facing web servers

Monitoring Recommendations

• Alert on Apache worker processes initiating FTP connections to hosts outside an approved allowlist
• Forward Apache access and error logs to a centralized log platform and create rules for mod_proxy_ftp anomalies
• Track package versions of httpd, apache2, and vendor-rebranded Apache builds across the fleet to confirm patched releases are deployed

How to Mitigate CVE-2020-1934

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.43 or later, which contains the fix for mod_proxy_ftp
• Apply distribution-specific updates from Debian (DSA-4757), Ubuntu (USN-4458-1), Fedora, and openSUSE
• Apply Oracle's July 2020 Critical Patch Update for affected Communications, Enterprise Manager, and ZFS Storage Appliance products
• If mod_proxy_ftp is not required, disable the module to remove the attack surface entirely

Patch Information

The upstream fix is referenced in the Apache HTTPD 2.4 vulnerabilities list and shipped in Apache HTTP Server 2.4.43. Downstream patched packages are available through Ubuntu Security Notice USN-4458-1, Debian DSA-4757, the Fedora package announcement, the openSUSE advisory, and the NetApp advisory NTAP-20200413-0002.

Workarounds

• Unload mod_proxy_ftp by commenting out its LoadModule directive in httpd.conf when FTP proxying is not in use
• Restrict outbound FTP egress from Apache servers using host or network firewall rules so the proxy cannot reach attacker-controlled FTP endpoints
• Constrain ProxyPass and related directives to a hardcoded allowlist of trusted internal FTP backends

# Disable mod_proxy_ftp until patches can be applied
# Debian/Ubuntu
sudo a2dismod proxy_ftp
sudo systemctl restart apache2

# RHEL/Fedora: comment out the LoadModule line in /etc/httpd/conf.modules.d/00-proxy.conf
# LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
sudo systemctl restart httpd

# Verify the module is no longer loaded
httpd -M 2>/dev/null | grep -i proxy_ftp