CVE-2009-0537 in OpenBSD
Summary
by MITRE
Integer overflow in the fts_build function in fts.c in libc in (1) OpenBSD 4.4 and earlier and (2) Microsoft Interix 6.0 build 10.0.6030.0 allows context-dependent attackers to cause a denial of service (application crash) via a deep directory tree, related to the fts_level structure member, as demonstrated by (a) du, (b) rm, (c) chmod, and (d) chgrp on OpenBSD; and (e) SearchIndexer.exe on Vista Enterprise.
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Analysis
by VulDB Data Team • 11/24/2024
The vulnerability described in CVE-2009-0537 represents a critical integer overflow condition within the filesystem traversal functionality of operating systems, specifically affecting the fts_build function in libc libraries. This flaw manifests in both OpenBSD versions 4.4 and earlier, as well as Microsoft Interix 6.0 build 10.0.6030.0, where the integer overflow occurs in the fts_level structure member during directory tree traversal operations. The vulnerability is particularly concerning because it can be exploited by context-dependent attackers to trigger application crashes and subsequent denial of service conditions. The flaw operates by manipulating directory tree depth to cause integer overflow during the calculation of the fts_level structure member, which is responsible for tracking directory levels during filesystem traversal. This type of vulnerability falls under CWE-190, which specifically addresses integer overflow conditions that can lead to unpredictable behavior and system instability.
The operational impact of this vulnerability extends across multiple system utilities and applications that rely on filesystem traversal functions. On OpenBSD systems, the attack surface includes fundamental utilities such as du, rm, chmod, and chgrp commands, all of which can be exploited to cause system crashes when processing deeply nested directory structures. Additionally, Microsoft Windows Vista Enterprise systems are affected through the SearchIndexer.exe process, which performs filesystem indexing operations that can trigger the same vulnerability. The exploitation technique involves creating or accessing deeply nested directory trees that exceed the maximum representable value for the integer type used to track directory levels. This integer overflow condition results in memory corruption that manifests as application crashes, effectively causing denial of service for legitimate system operations.
From an attack perspective, this vulnerability aligns with ATT&CK technique T1499.004, which involves network denial of service attacks through exploitation of system resources. The vulnerability demonstrates how seemingly benign filesystem operations can be weaponized to cause system instability, particularly affecting system utilities that are essential for normal operation. The integer overflow occurs at the boundary conditions of the fts_level structure, where the system attempts to track directory nesting levels beyond what the integer data type can represent. This flaw represents a classic example of a buffer overflow vulnerability in the context of filesystem traversal, where the system's inability to properly validate directory depth leads to predictable memory corruption. The attack requires minimal privileges and can be executed remotely in certain contexts, making it particularly dangerous for system administrators who rely on these utilities for routine maintenance operations.
The mitigation strategies for this vulnerability encompass both immediate patching and operational security measures. System administrators should prioritize updating affected OpenBSD installations to versions 4.5 or later, where the integer overflow has been corrected through proper bounds checking in the fts_build function. For Microsoft Interix systems, upgrading to patched versions or applying the appropriate security updates is essential to prevent exploitation. Operational mitigations include implementing directory depth restrictions in scripts and applications that perform filesystem traversal, monitoring for unusual directory nesting patterns, and establishing proper input validation for filesystem operations. The vulnerability also highlights the importance of regular security audits of core system libraries and the need for proper integer overflow protection mechanisms in system-level functions. Organizations should implement monitoring solutions that can detect abnormal filesystem traversal patterns and alert administrators to potential exploitation attempts. The fix typically involves adding proper integer overflow checks before incrementing the fts_level counter and ensuring that directory traversal operations gracefully handle maximum nesting levels without crashing the application.