Windows OLE RCE Vulnerability

Executive Summary

On January 14, 2025, a critical Remote Code Execution (RCE) vulnerability, CVE-2025-21298, affected the Windows Object Linking and Embedding (OLE). This vulnerability involves users opening or previewing a malicious Rich Text Format (RTF) file or email in Microsoft Outlook. This results in RCE, allowing the attacker to have full control of the user’s system. Microsoft has addressed the issue and strongly recommends that all users apply the latest security patches immediately, especially those systems that process RTF files or use Microsoft Outlook.

Background

Microsoft Outlook helps users organize and manage their calendars and emails, including sharing files and scheduling meetings. OLE allows for embedding and linking of documents and objects created using the Paste or Paste Link commands. However, it also poses security risks, as attackers can exploit vulnerabilities through malicious attachments.

The critical severity score for this vulnerability is 9.8 on the Common Vulnerability Scoring System (CVSS), indicating an extremely severe threat. The CVSS vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C represents the specific CVSS metric values that describe the vulnerability.

The weakness of this vulnerability comes from CWE-416: Use After Free. This flaw occurs when a program attempts to use a pointer referencing memory that has already been freed. Since the reallocated memory is referenced elsewhere in another pointer, the original pointer is no longer valid and points to a new allocated memory location. This leads to severe security risks, including denial of service (DoS), which crashes the application when invalid data is accessed. It also allows RCE by enabling attackers to overwrite a valid memory address with shellcode. It compromises integrity, confidentiality, and availability by allowing unauthorized modifications and the execution of malicious commands.

Exploitation

The vulnerability is located in “ole32.dll!UtOlePresStmToContentsStm,” a function responsible for converting data in an “OlePres” stream within an OLE storage into an appropriately formatted “CONTENTS” stream in the same storage. The issue is in the “pstmContents” variable, which initially stores a pointer to the “CONTENTS” stream object created at the beginning of the function. However, the stream is destroyed immediately after creation, causing the pointer to become invalid and stored in the “pstmContents” variable. The pointer is then freed in “coml2.dll!ExposedStream::~ExposedStream,” But since the “pstmContents” variable still holds the freed pointer, it may be reused to store a pointer in the “CONTENTS” stream again. Microsoft has resolved the issue by setting “pstmContents” to zero after its pointer is released, preventing reuse.

Significance and Impact

Securing Exchange servers alone isn’t enough, as the vulnerability resides in the OLE, making a large number of Windows endpoints a primary target. A single compromised endpoint could escalate into an entire network breach because of the broad user interaction between mail servers and desktop applications. If the vulnerability is exploited, attackers can run malware through arbitrary code execution and scripts, leading to unauthorized access to sensitive data. Once executed, attackers can bypass security controls and escalate privileges, gaining higher unauthorized access to critical data. This could result in the deletion and modification of files and configurations, ultimately leading to a full system compromise. 

Mitigation

To mitigate this vulnerability, users are recommended to enable plain text format for email messages and apply the latest security patches. The workarounds of email messages viewed in plain text format affects the preview pane and open messages like preventing pictures and other rich content. Implementing best practices can reduce compromise. This includes enhancing spam and phishing filters, blocking RTF file handling, enabling advanced threat detection, reviewing security architecture, and educating potential users on potential threats.

Conclusion

The Windows OLE RCE vulnerability poses a significant security risk, especially as a single compromised endpoint can lead to a full system breach. Attackers can gain unauthorized access to a user’s system through a malicious RTF file or email, potentially compromising a full system. Mitigating this threat requires not only applying the latest security patches but also implementing best practices, including user awareness training and enabling advanced threat detection tools to reduce the risk of exposure.

References 

[1] Microsoft Security Response Center. (2025, January 13). “CVE-2025-21298: Microsoft Outlook Vulnerability.” Msrc.microsoft.com. https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21298

[2] MITRE. (n.d.). “CWE-416: Use After Free.” Cwe.mitre.org. https://cwe.mitre.org/data/definitions/416.html

[3] Vulnu. (2025, January 23). “Zero-Click OLE RCE (CVE-2025-21298): Microsoft Outlook Impacted.” Vulnu.com. https://www.vulnu.com/p/zero-click-ole-rce-cve-2025-21298-microsoft-outlook-impacted

[4] Ynwarcs. (2025, January). “CVE-2025-21298 Proof of Concept.” Github.com. https://github.com/ynwarcs/CVE-2025-21298

[5] Microsoft. (2021, August 2). “OLE Background: Linking and Embedding” Learn.microsoft.com. https://learn.microsoft.com/en-us/cpp/mfc/ole-background-linking-and-embedding?view=msvc-170

[6] Microsoft Support. (n.d.). “What is Outlook?” Support.microsoft.com. https://support.microsoft.com/en-us/office/what-is-outlook-10f1fa35-f33a-4cb7-838c-a7f3e6228b20

[7] National Vulnerability Database. (2025, January 14). “CVE-2025-21298.” Nvd.nist.gov. https://nvd.nist.gov/vuln/detail/CVE-2025-21298