CVE-2018-20450 in libxls
Summary
by MITRE
The read_MSAT function in ole.c in libxls 1.4.0 has a double free that allows attackers to cause a denial of service (application crash) via a crafted file, a different vulnerability than CVE-2017-2897.
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Analysis
by VulDB Data Team • 06/22/2023
The vulnerability identified as CVE-2018-20450 represents a critical double free error within the read_MSAT function of libxls version 1.4.0, specifically located in the ole.c file. This flaw constitutes a memory corruption vulnerability that fundamentally compromises the integrity of memory management operations within the affected library. The double free condition occurs when the same memory block is freed twice during the processing of malformed spreadsheet files, creating a scenario where the heap allocator's internal data structures become corrupted. This type of vulnerability falls under the Common Weakness Enumeration category CWE-415, which specifically addresses double free conditions in memory management operations. The vulnerability is particularly concerning because it allows attackers to manipulate the heap state through crafted input files, potentially leading to unpredictable behavior in applications that rely on libxls for spreadsheet processing.
The operational impact of this vulnerability extends beyond simple denial of service, as it creates a potential vector for more sophisticated attacks that could leverage heap corruption to achieve arbitrary code execution. When an application processes a maliciously crafted Excel file through libxls, the read_MSAT function fails to properly validate memory allocation states before attempting to free memory blocks, resulting in a cascade of memory management failures. The vulnerability differs from CVE-2017-2897 in its specific location and manifestation, yet shares similar underlying principles of memory corruption that can be exploited to crash applications or potentially gain unauthorized access to system resources. This vulnerability affects any software that utilizes libxls 1.4.0 for processing Microsoft Office Excel files, including various office suites, document viewers, and enterprise applications that integrate spreadsheet parsing capabilities.
The exploitation of this vulnerability requires an attacker to craft a specially formatted Excel file that triggers the specific code path within the read_MSAT function where the double free occurs. The attack vector is particularly insidious because it can be delivered through standard file attachments in email systems, web applications, or file sharing platforms where Excel files are commonly processed. From an operational security perspective, this vulnerability aligns with ATT&CK technique T1203, which involves exploiting software vulnerabilities to gain access to systems. The impact on system availability is significant, as applications using libxls will crash when encountering the malformed file, potentially leading to complete service disruption for end users. Organizations that process large volumes of Excel files, such as financial institutions, government agencies, and enterprise document management systems, face substantial risk from this vulnerability due to the widespread use of libxls in various applications.
Mitigation strategies for CVE-2018-20450 primarily involve immediate patching of libxls to version 1.4.1 or later, which contains the necessary memory management fixes to prevent the double free condition. System administrators should prioritize updating all applications that depend on libxls, particularly those handling untrusted spreadsheet files from external sources. Additionally, implementing input validation controls and sandboxing mechanisms can provide defense-in-depth measures to prevent exploitation of this vulnerability. Organizations should also consider deploying network-based intrusion detection systems that can identify suspicious file processing patterns associated with this vulnerability. The fix implemented in the updated libxls version addresses the root cause by ensuring proper memory allocation state tracking and preventing the reuse of freed memory blocks during the MSAT processing phase. Security monitoring should include detection of application crashes or abnormal memory usage patterns that could indicate exploitation attempts, as these behaviors often precede more serious security incidents.