CVE-2018-7435 in FreeXL
Summary
by MITRE
An issue was discovered in FreeXL before 1.0.5. There is a heap-based buffer over-read in the freexl::destroy_cell function.
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Analysis
by VulDB Data Team • 02/10/2023
The vulnerability identified as CVE-2018-7435 represents a critical heap-based buffer over-read flaw within the FreeXL library version 1.0.4 and earlier. This issue resides in the freexl::destroy_cell function, which processes Excel file data structures during memory cleanup operations. The flaw manifests when the library attempts to destroy cell objects that contain malformed or maliciously crafted Excel data, leading to unauthorized memory access patterns that can result in information disclosure or system instability.
FreeXL is a lightweight library designed for reading Microsoft Excel files in binary format, commonly used in applications that need to process xls files without requiring full Microsoft Office dependencies. The buffer over-read vulnerability occurs during the destruction phase of cell objects when the library accesses memory locations beyond the allocated buffer boundaries. This type of flaw falls under the Common Weakness Enumeration category CWE-125, which specifically addresses out-of-bounds read conditions that can lead to information disclosure and potential exploitation.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can be leveraged by attackers to extract sensitive information from the application's memory space. When an application using FreeXL processes a specially crafted Excel file, the over-read condition can expose data such as stack contents, heap metadata, or other application secrets that may be stored in adjacent memory locations. This information disclosure can potentially reveal cryptographic keys, session tokens, or other confidential data that could be exploited in subsequent attacks.
From an attack perspective, this vulnerability aligns with the MITRE ATT&CK framework under the technique T1059.007 for Command and Scripting Interpreter and T1566.001 for Phishing with Malicious Attachments. The vulnerability can be exploited through malicious Excel files delivered via email or other attack vectors, making it particularly dangerous in enterprise environments where Excel file processing is common. The exploit requires minimal privileges and can be executed remotely, making it a preferred target for initial access and lateral movement phases of cyber attacks.
Mitigation strategies for CVE-2018-7435 primarily involve upgrading to FreeXL version 1.0.5 or later, which contains the necessary patches to prevent the buffer over-read condition. Organizations should also implement defensive measures such as input validation for Excel file processing, sandboxing of file parsing operations, and monitoring for unusual memory access patterns. Additionally, network segmentation and email filtering solutions can help prevent the delivery of malicious Excel files that could exploit this vulnerability. The patch addresses the root cause by implementing proper bounds checking in the freexl::destroy_cell function, ensuring that memory access operations remain within legitimate buffer boundaries while maintaining full backward compatibility with existing Excel file processing functionality.