CVE-2017-7572 in Back In Time
Summary
by MITRE
The _checkPolkitPrivilege function in serviceHelper.py in Back In Time (aka backintime) 1.1.18 and earlier uses a deprecated polkit authorization method (unix-process) that is subject to a race condition (time of check, time of use). With this authorization method, the owner of a process requesting a polkit operation is checked by polkitd via /proc/<pid>/status, by which time the requesting process may have been replaced by a different process with the same PID that has different privileges then the original requester.
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Analysis
by VulDB Data Team • 08/24/2024
The vulnerability identified as CVE-2017-7572 resides within the Back In Time backup application version 1.1.18 and earlier, specifically in the serviceHelper.py module where the _checkPolkitPrivilege function implements a deprecated polkit authorization mechanism. This flaw represents a classic time-of-check to time-of-use race condition that fundamentally undermines the security assurances of the authorization process. The vulnerability stems from the application's reliance on the unix-process authorization method, which is inherently flawed due to its dependency on process information retrieved through the /proc filesystem interface.
The technical implementation of this vulnerability exploits a fundamental weakness in how polkit handles process ownership verification during privilege checks. When the _checkPolkitPrivilege function executes, it queries the /proc/<pid>/status file to determine the original owner of a process requesting polkit operations. This approach creates a temporal window where the process identifier can be reused by a different process with potentially elevated privileges before the authorization decision is finalized. The race condition occurs because the process that initially requested the operation may have terminated and had its pid reassigned to another process, creating a scenario where the authorization decision is based on incorrect process ownership information.
This vulnerability directly maps to CWE-362, which describes a race condition in security-critical operations, and aligns with ATT&CK technique T1068, which covers local privilege escalation through process manipulation. The operational impact of this flaw is significant as it allows malicious actors to potentially bypass authorization checks and execute privileged operations without proper authentication. An attacker could exploit this by creating a process that requests a privileged operation, then quickly replacing that process with another that has higher privileges, effectively tricking the authorization system into granting elevated access.
The security implications extend beyond simple privilege escalation, as this vulnerability could enable attackers to perform unauthorized system modifications, access restricted files, or manipulate backup operations to gain deeper system access. The deprecated unix-process authorization method fails to provide the necessary atomicity required for secure privilege checking, making it susceptible to process replacement attacks that have been well-documented in Unix-like systems. Organizations using Back In Time versions affected by this vulnerability face potential exposure to attackers who could leverage this race condition to execute unauthorized operations with elevated privileges, particularly in multi-user environments where process manipulation is more feasible.
Mitigation strategies for CVE-2017-7572 should focus on upgrading to Back In Time versions that have addressed this specific vulnerability by implementing more secure authorization mechanisms. The recommended approach involves replacing the deprecated unix-process authorization method with modern alternatives that provide atomic privilege checking or implementing proper process ownership validation that does not rely on /proc filesystem queries. System administrators should also consider implementing additional monitoring to detect unauthorized process manipulation attempts and ensure that the polkit configuration files are properly secured to prevent modification by unprivileged users. The fix should also include ensuring that the application properly validates process ownership throughout the entire authorization lifecycle rather than relying on potentially stale process information retrieved at a specific point in time.