Critical nginx Vulnerabilities Expose Memory Leaks, DoS, and Arbitrary Code Execution Risks
Ubuntu security update USN-8375-1 addresses five critical vulnerabilities in nginx affecting SMTP authentication, TLS proxying, and media handling modules. Organizations running affected versions face risks including information disclosure, denial of service, and potential remote code execution.
TL;DR
- Five nginx vulnerabilities patched in USN-8375-1, spanning memory handling, TLS proxying, and module-specific flaws
- SMTP authentication module leaks sensitive data to auth servers; TLS proxy can inject plaintext into upstream responses
- DAV and MP4 modules vulnerable to DoS crashes and potential arbitrary code execution via malformed requests
- Mail auth module crashes on certain requests, enabling denial of service attacks
- Immediate patching recommended for production nginx deployments across all affected modules
Canonical has released security update USN-8375-1 addressing five vulnerabilities in nginx that span multiple modules and threat vectors. These flaws range from information disclosure in SMTP authentication flows to denial of service conditions and potential arbitrary code execution in media handling. Organizations relying on nginx for reverse proxying, mail services, or media delivery should prioritize patching to prevent exploitation.
The vulnerabilities affect core nginx functionality including the mail SMTP module, upstream TLS proxy handling, WebDAV operations, and MP4 file processing. Each flaw presents distinct attack surfaces: memory operations during authentication, protocol-level injection during proxying, and crash-inducing malformed input handling in specialized modules.
Information Disclosure and Authentication Risks
- CVE-2025-53859: ngx_mail_smtp_module mishandles memory during SMTP authentication, potentially exposing sensitive credentials or session data to the authentication server
- CVE-2026-1642: nginx TLS proxy incorrectly handles upstream connections, allowing attackers to inject plaintext data into proxied responses—bypassing encryption expectations
- Both flaws compromise confidentiality in mail and proxying workflows, affecting organizations using nginx for secure relay or authentication delegation
Denial of Service and Code Execution Vectors
- CVE-2026-27651: ngx_mail_auth_http_module crashes on malformed requests, enabling trivial DoS attacks against mail services
- CVE-2026-27654: ngx_http_dav_module processes destination URIs unsafely, causing crashes and potentially allowing path traversal to modify files outside the document root
- CVE-2026-27654 (partial): ngx_http_mp4_module mishandles MP4 file parsing, risking DoS or arbitrary code execution when processing untrusted media files
- Specialized modules present elevated risk in deployments serving media content or exposing WebDAV interfaces
Remediation and Deployment Guidance
- Apply USN-8375-1 patches immediately to all Ubuntu systems running affected nginx versions
- Prioritize patching for production reverse proxies, mail gateways, and media servers handling untrusted input
- Review nginx module configuration to disable unnecessary modules (DAV, MP4, mail auth) if not in use
- Monitor upstream TLS connections and SMTP authentication logs for signs of exploitation post-patch
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