CVE-2021-1722 in Windows
Summary
by MITRE • 02/26/2021
Windows Fax Service Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-24077.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 06/05/2026
The Windows Fax Service remote code execution vulnerability represents a critical security flaw within Microsoft's faxing infrastructure that has significant implications for enterprise network security. This vulnerability specifically affects the Windows Fax Service component that handles incoming and outgoing fax communications, creating an attack surface that can be exploited by malicious actors to gain unauthorized access to systems. The flaw stems from improper input validation and handling within the fax service's processing mechanisms, allowing attackers to craft specially malformed fax messages that trigger buffer overflows or other memory corruption conditions. The vulnerability is particularly concerning because fax services often run with elevated privileges and may be accessible from external networks, making them attractive targets for initial compromise attempts.
The technical implementation of this vulnerability involves the fax service's failure to properly validate and sanitize incoming fax data before processing it through internal parsing routines. When a malicious fax message is received, the service attempts to parse the data without adequate boundary checks, leading to memory corruption that can be leveraged to execute arbitrary code within the context of the fax service process. This typically occurs during the handling of fax headers, document formatting information, or embedded binary data within fax transmissions. The vulnerability's exploitation requires the attacker to send a crafted fax message to a system running the Windows Fax Service, which then processes this malformed data in a way that allows code execution. The flaw is classified under CWE-121 as a stack-based buffer overflow, though it may also manifest as heap-based corruption depending on the specific implementation details and data structures involved in the fax processing pipeline.
The operational impact of this vulnerability extends far beyond simple remote code execution, as it can serve as a foothold for more extensive network compromise operations. Once an attacker successfully exploits this vulnerability, they can potentially escalate privileges, establish persistent access, or use the compromised system as a launch point for lateral movement within the network. The fax service typically operates with system-level privileges, meaning successful exploitation can result in complete system compromise without requiring additional privilege escalation techniques. This makes the vulnerability particularly dangerous in enterprise environments where fax services may be used for legitimate business communications but also serve as potential attack vectors for advanced persistent threats. The impact is further amplified by the fact that many organizations maintain fax services for legacy applications, making them less likely to be properly patched or monitored for security issues.
Organizations should implement immediate mitigations including disabling the Windows Fax Service if it is not actively required for business operations, applying the relevant Microsoft security updates, and implementing network segmentation to limit access to fax services from untrusted networks. The vulnerability aligns with several ATT&CK techniques including T1059 for remote code execution and T1071 for application layer protocol usage, making it a significant concern for threat hunting and incident response teams. Network monitoring should focus on unusual fax traffic patterns and suspicious communication with fax services, while endpoint protection solutions should be configured to detect and block malicious fax data processing activities. Regular security assessments should include enumeration of fax service installations and verification of patch status, as this vulnerability has been exploited in the wild by threat actors targeting enterprise networks. The remediation process should also include reviewing fax service configurations to ensure minimal functionality and proper access controls are in place to reduce the overall attack surface and limit potential exploitation avenues.