CVE-2021-21996 in Salt
Summary
by MITRE • 09/08/2021
An issue was discovered in SaltStack Salt before 3003.3. A user who has control of the source, and source_hash URLs can gain full file system access as root on a salt minion.
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Analysis
by VulDB Data Team • 09/10/2021
The vulnerability identified as CVE-2021-21996 represents a critical privilege escalation flaw within the SaltStack Salt configuration management system that affects versions prior to 3003.3. This vulnerability stems from inadequate validation of source and source_hash URL parameters during file transfer operations, creating a path traversal condition that allows malicious actors with control over these parameters to execute arbitrary file system operations with root privileges on targeted salt minions. The flaw exists in the remote execution and file management components of the SaltStack ecosystem, specifically within the salt-minion process that handles file transfers from salt master servers.
The technical implementation of this vulnerability relies on the improper handling of user-supplied URLs during file synchronization operations. When salt minions receive file transfer instructions from a salt master, they validate the source and source_hash parameters to determine where to fetch files from. However, the validation mechanism fails to properly sanitize or restrict these URLs, allowing attackers to craft malicious URLs that can traverse the file system beyond intended boundaries. This weakness falls under CWE-22 Path Traversal, which specifically addresses vulnerabilities where untrusted input is used to reference files or directories without proper validation, allowing access to resources outside the intended scope. The vulnerability is particularly dangerous because it operates at the minion level where the salt minion process typically runs with elevated privileges, often including root access, to perform system configuration tasks.
The operational impact of this vulnerability extends far beyond simple unauthorized file access, as it provides attackers with complete control over the affected system's file system. An attacker who can manipulate the source or source_hash parameters in salt states can potentially overwrite critical system files, install backdoors, modify system configurations, or extract sensitive data from the targeted system. This vulnerability is particularly concerning in enterprise environments where salt minions are often deployed across critical infrastructure components, as it could enable attackers to escalate privileges and gain persistent access to multiple systems simultaneously. The attack vector is particularly insidious because it requires minimal privileges to exploit, as the attacker only needs to control the salt master or have the ability to inject malicious state files that contain crafted source URLs, which is often achievable in environments where the salt master is compromised or when attackers can manipulate state management processes.
Mitigation strategies for CVE-2021-21996 should focus on immediate version upgrades to SaltStack Salt 3003.3 or later, which includes patches that properly validate and sanitize source and source_hash URL parameters. Organizations should also implement network segmentation and access controls to limit exposure of salt masters and minions to untrusted networks or users. Additional protective measures include enabling strict source validation in salt states, implementing monitoring for unusual file transfer patterns, and conducting regular security audits of salt configurations to ensure that source parameters are properly validated. From a defensive standpoint, this vulnerability aligns with ATT&CK technique T1059 Command and Scripting Interpreter, as it enables adversaries to execute arbitrary commands through the salt minion's file handling mechanisms, and T1548 Abuse of System Permissions, since it allows privilege escalation through the manipulation of file system access controls. The vulnerability also demonstrates the importance of principle of least privilege in distributed systems, where the salt minion process should not operate with unnecessary elevated privileges, and proper input validation should be enforced at all levels of the system architecture.