CVE-2011-4211 in App Engine Python SDK
Summary
by MITRE
The FakeFile implementation in the sandbox environment in the Google App Engine Python SDK before 1.5.4 does not properly control the opening of files, which allows local users to bypass intended access restrictions and create arbitrary files via ALLOWED_MODES and ALLOWED_DIRS changes within the code parameter to _ah/admin/interactive/execute, a different vulnerability than CVE-2011-1364.
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Analysis
by VulDB Data Team • 01/22/2018
The vulnerability identified as CVE-2011-4211 represents a critical sandbox escape mechanism within the Google App Engine Python SDK that existed prior to version 1.5.4. This flaw specifically targets the FakeFile implementation component that governs file system operations within the sandboxed environment where applications execute. The sandbox architecture was designed to isolate application code from the underlying operating system and prevent unauthorized access to system resources, but this vulnerability undermines those protective measures through improper file handling controls.
The technical exploitation occurs through manipulation of the ALLOWED_MODES and ALLOWED_DIRS parameters within the code parameter of the _ah/admin/interactive/execute endpoint. This endpoint provides an interactive debugging interface that should normally be restricted to authorized administrative operations, yet the flawed FakeFile implementation fails to properly validate or sanitize these parameters before processing file operations. The vulnerability stems from inadequate input validation and parameter handling that allows attackers to modify the allowed file access patterns and directory restrictions dynamically during runtime execution.
This weakness creates a significant operational impact by enabling local users to bypass intended access restrictions and create arbitrary files anywhere within the system's file hierarchy. The attack vector leverages the administrative interface's trust model, where legitimate administrative operations are permitted but the underlying validation logic fails to properly enforce security boundaries. Attackers can potentially escalate privileges, create malicious files, or manipulate the system's file structure in ways that would normally be prevented by the sandbox's access controls.
The vulnerability aligns with CWE-22, which describes improper limitation of a pathname to a restricted directory, and CWE-73, which covers external control of file name or path. From an ATT&CK perspective, this maps to T1059.001 (Command and Scripting Interpreter: PowerShell) and T1548.002 (Abuse Elevation Control Mechanism: Bypass User Account Control) as it allows for privilege escalation through sandbox bypass techniques. The exploit demonstrates a classic example of insecure parameter handling that can lead to arbitrary code execution and unauthorized file system manipulation.
Mitigation strategies should focus on implementing proper input validation and parameter sanitization for all administrative interfaces, particularly those that handle file system operations. The recommended approach includes enforcing strict access controls on the interactive execution endpoint, implementing comprehensive parameter validation before processing file operations, and ensuring that the sandbox environment maintains strict isolation between application code and system resources. Additionally, regular security audits of administrative interfaces and proper code reviews focusing on parameter handling can prevent similar vulnerabilities from being introduced in future releases.