CVE-2025-53859: F5 Nginx Plus Information Disclosure Flaw

CVE-2025-53859 Overview

CVE-2025-53859 is an out-of-bounds read vulnerability affecting NGINX Open Source and NGINX Plus within the ngx_mail_smtp_module. This security flaw allows an unauthenticated attacker to over-read NGINX SMTP authentication process memory, potentially causing the server to leak arbitrary bytes sent in a request to the authentication server.

The vulnerability occurs during the NGINX SMTP authentication process and requires specific conditions to be exploited. An attacker must make preparations against the target system to extract the leaked data. The issue affects NGINX installations only when three conditions are met: (1) the system is built with the ngx_mail_smtp_module, (2) the smtp_auth directive is configured with method "none," and (3) the authentication server returns the "Auth-Wait" response header.

Critical Impact

Successful exploitation could lead to information disclosure of sensitive memory contents from the NGINX SMTP authentication process, potentially exposing data from other authentication requests.

Affected Products

• F5 NGINX Plus R30 through R34 (including patch releases)
• F5 NGINX Open Source (versions with ngx_mail_smtp_module enabled)
• Systems with smtp_auth directive configured with method "none"

Discovery Timeline

• 2025-08-13 - CVE-2025-53859 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2025-53859

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-bounds Read), a memory corruption flaw that occurs when the ngx_mail_smtp_module reads data beyond the intended buffer boundaries during the SMTP authentication process. The network-accessible nature of this vulnerability allows remote attackers to trigger the memory over-read without requiring authentication, though successful exploitation requires specific server configurations and preparatory actions against the target system.

The information disclosure impact is limited to confidentiality, as the vulnerability allows reading of arbitrary bytes from process memory without the ability to modify data or disrupt service availability. The leaked data is transmitted to the authentication server, requiring the attacker to have some mechanism to capture or access this data flow.

Root Cause

The root cause of CVE-2025-53859 lies in improper bounds checking within the ngx_mail_smtp_module when processing SMTP authentication requests. When the smtp_auth directive is set to "none" and the authentication server responds with the "Auth-Wait" header, the module fails to properly validate memory boundaries, resulting in an out-of-bounds read condition. This allows the server to access and potentially transmit memory contents beyond the intended buffer.

Attack Vector

The attack vector for CVE-2025-53859 is network-based, allowing remote exploitation without authentication. However, successful exploitation requires several preconditions:

1. The target NGINX installation must have the ngx_mail_smtp_module compiled and enabled
2. The smtp_auth directive must be configured with the "none" method
3. The authentication server must be configured to return the "Auth-Wait" response header
4. The attacker must have a means to capture or access the leaked data sent to the authentication server

The vulnerability mechanism involves sending crafted SMTP authentication requests that trigger the memory over-read condition. When the authentication process encounters the specific combination of configuration settings, arbitrary bytes from process memory are included in requests sent to the authentication server. Refer to the F5 Support Article for detailed technical information.

Detection Methods for CVE-2025-53859

Indicators of Compromise

• Unusual or malformed SMTP authentication requests targeting NGINX mail servers
• Unexpected data patterns in authentication server logs that may indicate memory leakage
• Anomalous network traffic between NGINX mail servers and authentication backends
• Multiple failed or unusual SMTP connection attempts from single sources

Detection Strategies

• Monitor SMTP authentication logs for abnormal request patterns or unexpected data content
• Implement network traffic analysis to detect unusual communication between NGINX and authentication servers
• Review NGINX configuration files to identify vulnerable smtp_auth "none" configurations
• Deploy application-layer firewalls capable of inspecting SMTP protocol traffic for anomalies

Monitoring Recommendations

• Enable detailed logging for the ngx_mail_smtp_module to capture authentication events
• Set up alerts for unusual volumes of SMTP authentication attempts
• Monitor authentication server logs for requests containing unexpected binary or malformed data
• Implement baseline monitoring for normal SMTP traffic patterns to detect deviations

How to Mitigate CVE-2025-53859

Immediate Actions Required

• Review NGINX configurations to identify systems using smtp_auth "none" with the ngx_mail_smtp_module
• Apply vendor patches as soon as they become available from F5
• Consider temporarily disabling the smtp_auth "none" configuration if not strictly required
• Implement network segmentation to limit access to SMTP authentication services

Patch Information

F5 has released security guidance for this vulnerability. Organizations should consult the F5 Support Article K000152786 for official patch information and remediation guidance. Additional technical discussion is available on the Openwall OSS-Security mailing list.

Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated by the vendor.

Workarounds

• Change the smtp_auth directive from "none" to a more secure authentication method
• Restrict network access to SMTP services using firewall rules to limit exposure
• Disable the ngx_mail_smtp_module if SMTP proxy functionality is not required
• Implement additional authentication layers between NGINX and backend authentication servers

# Configuration example - Modify smtp_auth to use a secure method
# In nginx.conf mail block, change:
# smtp_auth none;
# To a secure authentication method:
mail {
    server {
        listen 25;
        protocol smtp;
        smtp_auth login plain;  # Use authenticated methods instead of "none"
    }
}