CVE-2021-4120 in snapd
Summary
by MITRE • 02/18/2022
snapd 2.54.2 fails to perform sufficient validation of snap content interface and layout paths, resulting in the ability for snaps to inject arbitrary AppArmor policy rules via malformed content interface and layout declarations and hence escape strict snap confinement. Fixed in snapd versions 2.54.3+18.04, 2.54.3+20.04 and 2.54.3+21.10.1
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Analysis
by VulDB Data Team • 02/19/2022
The vulnerability identified as CVE-2021-4120 represents a critical security flaw in the snapd package management system affecting versions up to 2.54.2. This issue stems from insufficient validation mechanisms within the snapd daemon responsible for managing snap packages on Ubuntu systems. The flaw specifically targets the content interface and layout path validation processes that govern how snaps interact with system resources and define their filesystem access patterns. When snaps attempt to declare content interfaces or layout paths, the system fails to properly sanitize or validate these declarations, creating a pathway for malicious actors to inject unauthorized AppArmor policy rules.
The technical exploitation of this vulnerability occurs through carefully crafted content interface and layout declarations within snap packages. These malformed declarations allow attackers to bypass the strict confinement mechanisms that normally protect snap applications from accessing unauthorized system resources. The flaw essentially enables privilege escalation by permitting snap packages to inject arbitrary AppArmor rules that can expand their access privileges beyond the intended confinement boundaries. This represents a fundamental failure in the security model of snapd where input validation is insufficient to prevent malicious payload injection.
The operational impact of CVE-2021-4120 is severe as it undermines the core security premise of snap confinement. Attackers can leverage this vulnerability to escape the sandboxed environment typically enforced by snapd, potentially gaining access to sensitive system resources, files, or processes that should remain isolated. The vulnerability affects systems running Ubuntu 18.04, 20.04, and 21.10.1, making it particularly concerning for enterprise environments that rely on these platforms. The ability to inject AppArmor rules means that malicious actors could potentially create persistent backdoors or elevate their privileges to system-level access, depending on the specific implementation details of the compromised system.
The remediation for CVE-2021-4120 requires immediate upgrade to snapd versions 2.54.3 or later across all affected Ubuntu releases. This update implements proper validation mechanisms for content interface and layout path declarations, ensuring that all input is properly sanitized before being processed by the snapd daemon. System administrators should prioritize patching this vulnerability as it represents a direct threat to system integrity and confinement security. The fix addresses the root cause by implementing stricter validation protocols that prevent malformed declarations from being processed, thereby maintaining the security boundaries that snap confinement is designed to enforce. Organizations should also conduct comprehensive security audits to ensure that no malicious snaps have already been installed on systems prior to patching, as the vulnerability could have been exploited to establish persistent access to affected systems.
This vulnerability aligns with CWE-20, which describes improper input validation, and maps to ATT&CK techniques including privilege escalation and defense evasion. The flaw demonstrates how insufficient input sanitization can lead to complete bypass of confinement mechanisms, making it a critical concern for containerized and sandboxed application environments.