CVE-2016-1247 in nginx
Summary
by MITRE
The nginx package before 1.6.2-5+deb8u3 on Debian jessie, the nginx packages before 1.4.6-1ubuntu3.6 on Ubuntu 14.04 LTS, before 1.10.0-0ubuntu0.16.04.3 on Ubuntu 16.04 LTS, and before 1.10.1-0ubuntu1.1 on Ubuntu 16.10, and the nginx ebuild before 1.10.2-r3 on Gentoo allow local users with access to the web server user account to gain root privileges via a symlink attack on the error log.
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Analysis
by VulDB Data Team • 10/16/2024
The vulnerability described in CVE-2016-1247 represents a critical privilege escalation flaw in the nginx web server software that affects multiple Linux distributions including Debian, Ubuntu, and Gentoo. This vulnerability stems from improper handling of symbolic links within the nginx error logging mechanism, creating a pathway for local attackers with web server user access to escalate their privileges to root level. The issue specifically manifests when nginx processes error logs that contain symbolic links, allowing malicious users to manipulate the logging process and potentially execute arbitrary code with elevated privileges. This flaw directly impacts the principle of least privilege and demonstrates a failure in proper access control mechanisms within the web server's logging subsystem.
The technical implementation of this vulnerability involves the interaction between nginx's logging functionality and the filesystem permissions model. When nginx writes error logs, it follows symbolic links to determine the actual log file location, but fails to properly validate or restrict access to these symbolic link targets. An attacker with access to the web server user account can create a symbolic link pointing to a critical system file such as /etc/shadow or a setuid binary, and then trigger an error condition that causes nginx to follow this link and write to the target file. This creates a race condition where the attacker can effectively overwrite or modify system-critical files. The vulnerability is classified under CWE-59 as improper link resolution, and aligns with ATT&CK technique T1068 which covers privilege escalation through local exploitation.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally compromises the security boundary between the web server process and the underlying operating system. Systems running affected nginx versions become vulnerable to attacks that could result in complete system compromise, data exfiltration, or persistent backdoor installation. The vulnerability is particularly dangerous in shared hosting environments or multi-tenant deployments where multiple users might have access to the web server process. Organizations using nginx as their primary web server are at risk of unauthorized root access, which could lead to complete infrastructure compromise, making this a critical security concern for any deployment that does not immediately patch the affected versions.
Mitigation strategies for CVE-2016-1247 focus on immediate patching and configuration hardening measures. The primary recommendation is to update nginx to versions that contain the security fix, specifically nginx 1.6.2-5+deb8u3 for Debian jessie, the appropriate Ubuntu versions mentioned in the CVE, and the corresponding Gentoo ebuild. Additionally, system administrators should implement proper logging directory permissions and ensure that the web server user cannot create or modify symbolic links within the logging hierarchy. This includes setting restrictive permissions on the nginx log directory and implementing proper file system controls. The vulnerability also highlights the importance of following security best practices such as running web servers with minimal privileges, regularly auditing file permissions, and implementing monitoring solutions to detect unauthorized symbolic link creation. Organizations should also consider implementing network segmentation and intrusion detection systems to detect potential exploitation attempts. This vulnerability serves as a reminder of the critical importance of proper input validation and access control in server applications, particularly those handling user-provided data through logging mechanisms.