CVE-2016-7076 in sudo
Summary
by MITRE
sudo before version 1.8.18p1 is vulnerable to a bypass in the sudo noexec restriction if application run via sudo executed wordexp() C library function with a user supplied argument. A local user permitted to run such application via sudo with noexec restriction could possibly use this flaw to execute arbitrary commands with elevated privileges.
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Analysis
by VulDB Data Team • 03/17/2023
The vulnerability identified as CVE-2016-7076 represents a critical privilege escalation flaw in the sudo command utility affecting versions prior to 1.8.18p1. This security weakness specifically targets the sudo noexec restriction mechanism, which is designed to prevent the execution of binaries from non-executable filesystems such as /tmp or /var/tmp. The flaw occurs when a user with sudo privileges attempts to execute an application that utilizes the wordexp() C library function with user-supplied arguments. The wordexp() function processes shell metacharacters and can interpret special characters like backticks, dollar signs, and other shell operators within the input string. When sudo's noexec restriction is in place, it typically prevents execution of programs from directories that are mounted with the noexec flag, thereby mitigating attacks that rely on executing malicious code from temporary or shared filesystems.
The technical exploitation of this vulnerability requires a specific combination of conditions that must be met for successful privilege escalation. The attacker must first have permission to execute a particular application through sudo with the noexec restriction enabled. Additionally, this application must internally invoke the wordexp() function and pass user-controlled input to it. The interaction between sudo's noexec protection and the wordexp() function creates a bypass scenario where the shell expansion operations performed by wordexp() can execute commands in contexts where they would normally be restricted. This occurs because the wordexp() function processes the user-supplied arguments and may execute shell commands before sudo's noexec restrictions can take effect. The vulnerability essentially allows attackers to circumvent the intended security boundary imposed by sudo's noexec mechanism, which is a fundamental protection against executing malicious code from untrusted sources.
The operational impact of CVE-2016-7076 is significant as it enables local users to escalate their privileges from standard user accounts to root access without requiring authentication or additional exploitation vectors. This flaw is particularly dangerous in multi-user environments where users have limited privileges but need to execute specific administrative commands through sudo. The vulnerability can be exploited by attackers who have already gained access to a system with limited user privileges, allowing them to gain complete system control. The bypass mechanism works by leveraging the timing and context of shell expansion operations within the wordexp() function, which occurs before sudo's security checks are fully enforced. This vulnerability aligns with CWE-254, which addresses security weaknesses in the implementation of access control mechanisms, and specifically relates to CWE-78, which covers the execution of system commands. The attack pattern follows ATT&CK technique T1068, which involves the exploitation of system privileges to gain elevated access, and T1059, which encompasses the use of command and scripting interpreters for execution.
Mitigation strategies for CVE-2016-7076 primarily focus on updating sudo to version 1.8.18p1 or later, which contains the necessary patches to address the bypass vulnerability. System administrators should prioritize patching all affected systems and verify that the updated sudo version properly implements the noexec restriction. Organizations should also implement additional monitoring and logging of sudo usage, particularly for applications that utilize the wordexp() function, to detect potential exploitation attempts. The patch addresses the core issue by modifying how sudo handles command execution when wordexp() is involved, ensuring that the noexec restrictions remain effective regardless of shell expansion operations. Security teams should also review sudoers configuration files to minimize the number of applications that require elevated privileges and eliminate unnecessary sudo permissions. Additionally, implementing principle of least privilege practices and regular security audits of sudo configurations can help prevent exploitation of similar vulnerabilities. The fix typically involves changes to the internal command execution flow within sudo, ensuring that noexec restrictions are enforced before any shell expansion operations occur, thereby preventing the bypass mechanism that allowed attackers to escalate privileges.