CVE-2020-13396 in FreeRDP
Summary
by MITRE
An issue was discovered in FreeRDP before 2.1.1. An out-of-bounds (OOB) read vulnerability has been detected in ntlm_read_ChallengeMessage in winpr/libwinpr/sspi/NTLM/ntlm_message.c.
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Analysis
by VulDB Data Team • 05/19/2025
The vulnerability CVE-2020-13396 represents a critical out-of-bounds read flaw in the FreeRDP remote desktop protocol implementation that affects versions prior to 2.1.1. This issue resides within the NTLM authentication module, specifically in the ntlm_read_ChallengeMessage function located at winpr/libwinpr/sspi/NTLM/ntlm_message.c. The vulnerability manifests when processing NTLM challenge messages during the authentication handshake process, which is fundamental to establishing secure remote desktop connections using the FreeRDP library.
The technical exploitation of this out-of-bounds read vulnerability occurs when the ntlm_read_ChallengeMessage function processes malformed or crafted NTLM challenge messages without proper bounds checking on array accesses. This flaw allows an attacker to provide specially crafted input data that causes the function to read memory beyond the allocated buffer boundaries. The underlying cause aligns with CWE-129, which describes improper validation of array indices, and CWE-787, which addresses out-of-bounds write operations that can lead to information disclosure or code execution. When the vulnerable function attempts to parse NTLM challenge messages, it fails to validate the length of incoming data structures, leading to memory access violations that can be leveraged for information disclosure or potentially remote code execution.
The operational impact of this vulnerability extends significantly within environments that rely on FreeRDP for remote desktop connections, particularly in enterprise settings where Windows Remote Desktop Protocol (RDP) connectivity is prevalent. Attackers can exploit this vulnerability by establishing a connection to a target system that uses FreeRDP for authentication, sending a crafted NTLM challenge message that triggers the out-of-bounds read condition. This could result in sensitive information disclosure from memory, including authentication credentials, session tokens, or other confidential data stored in the process memory space. The vulnerability represents a significant risk in environments where FreeRDP is used for RDP gateway functionality or as part of larger remote access solutions, as it can be exploited without requiring authentication to the target system itself.
Mitigation strategies for CVE-2020-13396 should prioritize immediate patching of FreeRDP installations to version 2.1.1 or later, which contains the necessary fixes for the out-of-bounds read condition. Organizations should also implement network segmentation and access controls to limit exposure of systems running FreeRDP to trusted networks only. Additional defensive measures include monitoring network traffic for unusual NTLM authentication patterns and implementing intrusion detection systems that can identify potential exploitation attempts. Security teams should also consider disabling NTLM authentication where possible and migrating to more secure authentication mechanisms such as Kerberos or certificate-based authentication. The vulnerability's classification under the ATT&CK framework falls under T1075 Remote Services and T1566 Credential Access techniques, as it enables unauthorized access to authentication systems and can lead to privilege escalation through credential theft. Organizations using FreeRDP should conduct comprehensive vulnerability assessments to identify all systems running affected versions and ensure proper patch management protocols are in place to prevent similar issues from arising in the future.