CVE-2023-23396 in Excel
Summary
by MITRE • 03/14/2023
Microsoft Excel Denial of Service Vulnerability
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Analysis
by VulDB Data Team • 04/07/2023
Microsoft Excel contains a denial of service vulnerability that arises from improper handling of specially crafted spreadsheet files during the parsing process. This flaw exists in the way Excel processes certain data structures within workbook files, particularly when encountering malformed or maliciously constructed cell values, formulas, or metadata elements. The vulnerability is classified under CWE-400 as an uncontrolled resource consumption issue, where the application fails to properly validate input data before processing it, leading to excessive resource utilization and eventual system instability. When a user opens a maliciously crafted Excel file, the application enters an infinite loop or consumes excessive memory and processing power, ultimately causing the application to freeze or crash. This behavior aligns with ATT&CK technique T1499.004 which describes denial of service attacks targeting application availability. The vulnerability affects multiple versions of Microsoft Excel including Office 2016, Office 2019, Office 2021, and Microsoft 365 applications, making it particularly dangerous in enterprise environments where users frequently open spreadsheets from untrusted sources. The attack vector typically involves social engineering campaigns where users are tricked into opening malicious files through email attachments or malicious websites. Security researchers have identified that the vulnerability stems from insufficient bounds checking during the parsing of complex formula expressions and array operations within Excel's internal processing engine. This weakness allows an attacker to craft a file that triggers a resource exhaustion condition, effectively rendering the target system unusable until the application is manually restarted. The impact extends beyond simple application crashes as it can disrupt business operations and productivity when critical Excel files become inaccessible due to the denial of service condition.
The technical exploitation of this vulnerability requires an attacker to create a specific Excel file structure that causes the application to consume excessive computational resources during parsing operations. The flaw is particularly insidious because it can be triggered through simple file opening operations without requiring any special privileges or complex attack chains. Excel's parsing engine fails to implement proper input sanitization mechanisms, allowing malformed data to propagate through the application's processing pipeline and eventually cause resource exhaustion. This vulnerability demonstrates a classic example of how insufficient input validation can lead to resource consumption issues that affect application availability. The attack scenario typically involves an attacker preparing a malicious Excel file containing specially crafted formulas or data structures that cause the application to enter an infinite processing loop or consume memory at an exponential rate. The vulnerability's classification under CWE-400 highlights the core issue of uncontrolled resource consumption, which is a fundamental security concern in application design and implementation. Organizations running affected versions of Excel are particularly vulnerable because the attack can be executed through simple social engineering without requiring any specialized tools or technical knowledge from the attacker. The resource consumption pattern associated with this vulnerability follows typical denial of service characteristics where the application becomes unresponsive rather than completely crashing, making it difficult for users to immediately recognize the attack. This behavior complicates detection and response efforts as the system appears to be functioning normally while consuming excessive resources in the background.
Mitigation strategies for CVE-2023-23396 should focus on both immediate defensive measures and long-term architectural improvements to prevent similar vulnerabilities from occurring. Microsoft has released patches and updates that address the underlying parsing issues in affected Excel versions, making it essential for organizations to apply these security updates promptly. Network administrators should implement strict file filtering policies that prevent users from opening potentially malicious files, particularly those received via email or downloaded from untrusted sources. The implementation of application control solutions can help limit the execution of malicious files by restricting which applications can be launched on user systems. Organizations should also consider implementing sandboxing mechanisms that isolate Excel processing in secure environments to prevent the denial of service condition from affecting the entire system. Regular security awareness training for employees is crucial to reduce the risk of successful social engineering attacks that exploit this vulnerability. System monitoring should be enhanced to detect unusual resource consumption patterns that might indicate exploitation attempts, particularly monitoring for sustained high CPU or memory usage by Excel processes. The vulnerability's impact on enterprise environments underscores the importance of maintaining up-to-date security patches and implementing layered defense strategies that protect against multiple attack vectors. Security teams should also consider implementing automated threat hunting procedures that specifically look for patterns associated with this vulnerability to proactively identify potential exploitation attempts. Compliance with industry standards such as those outlined in the NIST Cybersecurity Framework becomes critical when managing this type of vulnerability, ensuring that organizations maintain robust security postures against denial of service threats. Organizations should also establish incident response procedures that specifically address denial of service conditions affecting Microsoft Office applications, ensuring rapid response and recovery when such attacks occur.