CVE-2019-5736 in runc
Summary
by MITRE
runc through 1.0-rc6, as used in Docker before 18.09.2 and other products, allows attackers to overwrite the host runc binary (and consequently obtain host root access) by leveraging the ability to execute a command as root within one of these types of containers: (1) a new container with an attacker-controlled image, or (2) an existing container, to which the attacker previously had write access, that can be attached with docker exec. This occurs because of file-descriptor mishandling, related to /proc/self/exe.
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Analysis
by VulDB Data Team • 10/01/2025
The vulnerability identified as CVE-2019-5736 represents a critical privilege escalation flaw in the runc container runtime that affects Docker versions prior to 18.09.2 and other containerization platforms. This vulnerability stems from improper handling of file descriptors, specifically involving the /proc/self/exe symbolic link that points to the currently running process executable. The flaw allows attackers with limited access to containerized environments to escalate privileges to the host system level, effectively obtaining root access to the underlying host machine.
The technical mechanism behind this vulnerability involves the manipulation of file descriptor inheritance during container execution. When runc executes container processes, it maintains file descriptors open to its own binary file. An attacker who can execute commands within a container can exploit this by creating a new container with a malicious image or by accessing an existing container where they previously had write permissions. The exploit leverages the fact that /proc/self/exe can be used to reference the running runc binary, and through careful manipulation of file descriptor inheritance, attackers can cause the runc binary to be overwritten with malicious code. This occurs because the container runtime does not properly close or sanitize file descriptors before executing the containerized process, creating a window where the attacker-controlled process can manipulate the host runc binary.
The operational impact of CVE-2019-5736 is severe and far-reaching, as it fundamentally undermines the security isolation that containers are designed to provide. Once an attacker successfully exploits this vulnerability, they gain complete control over the host system, enabling them to access all data, services, and resources available on the host machine. This includes the ability to escalate privileges to root access, access other containers running on the same host, and potentially move laterally within a network infrastructure. The vulnerability is particularly dangerous because it can be exploited through two distinct attack vectors: creating new containers with malicious images or by leveraging previously compromised containers that allow write access. This dual exploitation method increases the attack surface and makes the vulnerability more difficult to prevent entirely.
From a cybersecurity perspective, this vulnerability maps directly to CWE-362, which describes Concurrent Execution using Shared Resource with Improper Synchronization, and aligns with several ATT&CK techniques including privilege escalation through exploitation of software vulnerabilities and persistence mechanisms. The flaw demonstrates how containerization security models can be subverted when underlying runtime components contain critical implementation errors. Organizations should immediately update their container runtime environments to versions that address this vulnerability, ensuring that all Docker installations are updated to 18.09.2 or later. Additional mitigations include implementing strict container image policies, limiting container privileges through proper security contexts, and monitoring for suspicious file operations on critical system binaries. The vulnerability also highlights the importance of proper file descriptor management in security-critical software components and serves as a reminder that even seemingly simple file operations can have catastrophic security implications when not properly implemented with security considerations in mind.