CVE-2018-7439 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 function read_mini_biff_next_record.
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Analysis
by VulDB Data Team • 02/10/2023
The vulnerability identified as CVE-2018-7439 represents a critical heap-based buffer over-read flaw within the FreeXL library version prior to 1.0.5. This issue manifests specifically within the read_mini_biff_next_record function, which processes Microsoft Excel file formats and their underlying binary structures. The FreeXL library serves as an open-source component designed to read excel files and extract data from them, making it a widely used dependency in various applications that handle spreadsheet data processing. The vulnerability stems from insufficient bounds checking during the parsing of BIFF (Binary Interchange File Format) records, which is the native format used by Microsoft Excel for storing spreadsheet data. When processing malformed or specially crafted excel files, the library fails to properly validate the size of incoming data structures, leading to memory access violations that can result in unpredictable behavior.
The technical exploitation of this vulnerability occurs when an attacker provides a malicious excel file that contains malformed BIFF records with incorrect size indicators or corrupted data structures. The read_mini_biff_next_record function attempts to read beyond the allocated memory boundaries while processing these records, causing a buffer over-read condition that can lead to information disclosure, application crashes, or potentially remote code execution depending on the surrounding system context. This flaw aligns with CWE-125, which specifically addresses out-of-bounds read vulnerabilities in software systems. The vulnerability's impact is particularly concerning given that FreeXL is commonly integrated into web applications, database connectors, and data processing pipelines that handle user-uploaded excel files. Attackers could leverage this weakness to extract sensitive information from memory, potentially accessing credentials, personal data, or other confidential information stored in adjacent memory locations. The vulnerability demonstrates a classic memory safety issue that has been prevalent in C/C++ applications where developers fail to validate input data against buffer boundaries.
From an operational perspective, this vulnerability creates significant risk for organizations that utilize FreeXL in their data processing workflows, particularly those handling untrusted excel file inputs from external sources. The impact extends beyond simple application instability to potential data breaches and system compromise, as buffer over-read conditions can be chained with other vulnerabilities to achieve more sophisticated attacks. The vulnerability's exploitation requires minimal skill level, making it attractive to attackers who may not possess advanced exploitation capabilities. Organizations should consider the ATT&CK framework's T1059.007 technique, which covers script-based attacks, as this vulnerability could enable attackers to execute malicious code through compromised applications that process excel files. The vulnerability affects a wide range of applications including database connectors, web applications, and data analysis tools that rely on FreeXL for excel file parsing. Given the widespread adoption of this library, the potential attack surface is extensive, particularly in environments where excel files are frequently processed and imported into databases or other systems. Security teams must prioritize this vulnerability due to its potential for information disclosure and system compromise.
Mitigation strategies for CVE-2018-7439 should focus on immediate patching of affected FreeXL versions to 1.0.5 or later, which contains the necessary bounds checking fixes. Organizations should implement input validation measures that sanitize excel file inputs before processing, including implementing strict file format validation and size limitations for uploaded files. Network segmentation and access controls should be enforced to limit exposure of applications that process excel files to untrusted inputs. Additionally, implementing intrusion detection systems that monitor for unusual file processing patterns or memory access violations can help detect potential exploitation attempts. Application-level defenses should include memory protection mechanisms such as stack canaries, address space layout randomization, and data execution prevention features to mitigate potential exploitation. Regular security assessments and penetration testing should be conducted to identify other potential vulnerabilities in applications that utilize FreeXL, particularly focusing on input validation and memory safety. Organizations should also consider implementing automated vulnerability scanning tools that can detect the presence of vulnerable FreeXL versions in their software dependencies and provide alerts for remediation. The remediation process should include comprehensive testing of patched applications to ensure that the fix does not introduce regressions in functionality while maintaining the security improvements.