CVE-2016-4296 in Office
Summary
by MITRE
When opening a Hangul Hcell Document (.cell) and processing a record that uses the CSSValFormat object, Hancom Office 2014 will search for an underscore ("_") character at the end of the string and write a null terminator after it. If the character is at the very end of the string, the application will mistakenly write the null-byte outside the bounds of its destination. This can result in heap corruption that can lead code execution under the context of the application
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Analysis
by VulDB Data Team • 04/20/2025
The vulnerability identified as CVE-2016-4296 represents a classic buffer overflow condition affecting Hancom Office 2014's handling of Hangul Hcell Document files with the .cell extension. This flaw manifests during the processing of records that utilize the CSSValFormat object, creating a scenario where memory management fails catastrophically. The vulnerability stems from improper bounds checking when the application searches for an underscore character at string termination, leading to memory corruption that can be exploited for arbitrary code execution.
The technical implementation of this vulnerability involves the application's string processing logic where it specifically looks for underscore characters at the end of strings to determine where to place a null terminator. When the underscore character appears at the very end of a string, the application's boundary checking mechanism fails, causing it to write a null byte beyond the allocated memory buffer. This heap corruption occurs because the application assumes that there is sufficient space for the null terminator after the underscore character, without proper validation of the string boundaries. The flaw directly maps to CWE-121, which describes unsafe use of stack-based buffers, and CWE-122, which covers heap-based buffer overflow conditions.
The operational impact of this vulnerability extends beyond simple memory corruption, as it provides attackers with a pathway for code execution within the context of the application running with elevated privileges. When an attacker crafts a malicious .cell file containing specially formatted CSSValFormat records, they can trigger the vulnerable code path and potentially execute arbitrary code with the same privileges as the Hancom Office application. This represents a significant security risk in environments where users might encounter malicious documents, particularly in targeted attack scenarios or when users open documents from untrusted sources. The vulnerability affects the application's memory management system and can lead to denial of service conditions or complete system compromise depending on the execution context and privileges of the target application.
Mitigation strategies for this vulnerability should focus on both immediate remediation and long-term architectural improvements. The most effective approach involves applying the vendor-provided security patches and updates that address the specific buffer overflow condition in the CSSValFormat processing code. Organizations should also implement defensive measures such as restricting user access to potentially malicious file types through application whitelisting, email filtering, and web application firewalls. Additionally, the implementation of memory safety features including stack canaries, address space layout randomization, and data execution prevention can significantly reduce the exploitability of similar buffer overflow conditions. From an ATT&CK perspective, this vulnerability aligns with techniques involving initial access through malicious documents and privilege escalation through application exploitation, making comprehensive endpoint protection and user education critical components of the overall security posture.