CVE-2019-18634 in sudo
Summary
by MITRE
In Sudo through 1.8.29, if pwfeedback is enabled in /etc/sudoers, users can trigger a stack-based buffer overflow in the privileged sudo process. (pwfeedback is a default setting in Linux Mint and elementary OS; however, it is NOT the default for upstream and many other packages, and would exist only if enabled by an administrator.) The attacker needs to deliver a long string to the stdin of getln() in tgetpass.c.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 05/16/2025
The vulnerability CVE-2019-18634 represents a critical stack-based buffer overflow in the sudo command version 1.8.29 and earlier, specifically when the pwfeedback option is enabled in the /etc/sudoers configuration file. This flaw exists within the privileged sudo process and creates a significant security risk that can be exploited by malicious actors. The pwfeedback feature, which provides visual feedback during password entry, is enabled by default in certain Linux distributions such as Linux Mint and elementary OS, making these systems particularly vulnerable. The vulnerability stems from improper input validation in the getln() function within the tgetpass.c source file, where the sudo utility fails to properly handle excessively long input strings delivered through standard input. This buffer overflow occurs because the sudo process does not adequately check the length of input data before copying it into fixed-size buffers on the stack, creating a predictable exploitation vector.
The technical implementation of this vulnerability involves the exploitation of a classic buffer overflow condition that allows attackers to overwrite adjacent memory locations within the sudo process's stack space. When pwfeedback is enabled and an attacker provides a sufficiently long string to the stdin of the getln() function, the input exceeds the allocated buffer size, causing memory corruption that can be leveraged for privilege escalation. The attacker's control over the input flow enables them to overwrite return addresses, function pointers, or other critical stack variables, potentially allowing execution of arbitrary code with elevated privileges. This vulnerability directly maps to CWE-121, which describes stack-based buffer overflow conditions where insufficient bounds checking permits buffer overflows. The attack vector requires minimal privileges since the vulnerability exists within the sudo utility itself, which is designed to be executed with elevated permissions, and the attacker only needs to provide malicious input to the getln() function.
The operational impact of CVE-2019-18634 extends beyond simple privilege escalation, as it represents a fundamental flaw in authentication handling that can be exploited in various attack scenarios. An attacker who successfully exploits this vulnerability can gain root access to systems where sudo is used, potentially leading to complete system compromise and unauthorized access to sensitive data. The vulnerability's presence in default configurations of popular Linux distributions means that a significant number of systems may be exposed without administrator knowledge, as the pwfeedback feature is enabled by default in these environments. This makes the exploit particularly dangerous because it does not require complex attack preparation or specialized knowledge of system configurations. The attack can be executed through standard input redirection or by crafting malicious commands that feed oversized input to sudo's password prompt, making it accessible to attackers with basic command-line knowledge.
Mitigation strategies for CVE-2019-18634 focus on immediate remediation through software updates and configuration changes. The most effective solution involves upgrading to sudo version 1.8.30 or later, where the buffer overflow has been patched and proper bounds checking has been implemented. Administrators should also disable the pwfeedback option in /etc/sudoers if it is not required for their specific use cases, as this removes the attack surface entirely. The patch addresses the vulnerability by implementing proper input length validation and ensuring that input strings are truncated or rejected if they exceed safe buffer limits. Organizations should conduct comprehensive system audits to identify all installations where pwfeedback is enabled and ensure that all systems are updated to patched versions of sudo. Additionally, security monitoring should be implemented to detect potential exploitation attempts, and access controls should be reviewed to minimize the impact of any successful attacks. This vulnerability demonstrates the importance of proper input validation and buffer management in privileged system components, aligning with ATT&CK technique T1068 which covers exploit for privilege escalation through local system exploits. The vulnerability also underscores the necessity of regular security updates and the risks associated with default configurations that may introduce unnecessary security risks in otherwise secure systems.