This month brings us yet another critical RCE (Remote Code Execution) bug found in the RDP (Remote Desktop Protocol) Client which has also been ported to the Hyper-V Manager “Enhanced Session Mode” feature. User interaction is a prerequisite since the vulnerability lies within the RDP client, requiring a victim to connect to a malicious RDP server.
Vulnerability Analysis: CVE-2021-38666
This RCE bug is very closely related to CVE-2021-34535 and to CVE-2020-1374 , where there is a heap-based buffer overflow in mstscax.dll due to an attacker-controlled payload size field. The vulnerability can be triggered via the RDP Smart Card Virtual Channel Extension feature [MS-RDPESC], by leveraging the existing local RDPDR static virtual channel setup between the client and server. The RDP Smart Card Virtual Channel Extension feature [MS-RDPESC] functionality was leveraged in the “EsteemAudit” Exploit released by the “Shadow Brokers,” but that vulnerability targeted the RDP server and not the client. The functionality being exploited here is the ability to share a smart card reader between the client and server. The destination buffer intended for the IOCTL (I/O control) call to locate each host smart card reader is a fixed size, but the user-controlled size field can be altered to cause the client to perform an OOB (Out of Bounds) write. Seeing how simple it is to trigger this vulnerability, our team decided to mutate the test case to verify whether any other IOCTLs within the [MS-RDPESC] specification are vulnerable. Enumerating through the 60 other IOCTL calls tied to the smart card reader, ..
Support the originator by clicking the read the rest link below.
