CVE-2020-4031: 
NixOS vulnerability analysis and mitigation

CVE-2020-4031 affects FreeRDP versions before 2.1.2, where there is a use-after-free vulnerability in gdi_SelectObject. This vulnerability specifically impacts FreeRDP clients using compatibility mode with /relax-order-checks. The issue was discovered and disclosed in June 2020, and was fixed in version 2.1.2 (FreeRDP Advisory).

The vulnerability is classified as a use-after-free issue in the gdi_SelectObject function. The CVSS v3.1 base score is 7.5 (HIGH) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, indicating network accessibility, low attack complexity, and high impact on availability (NVD).

The vulnerability can lead to a crash of the affected FreeRDP client when exploited, resulting in a denial of service condition. The impact is primarily on availability, with no direct impact on confidentiality or integrity (FreeRDP Advisory).

The vulnerability is exploitable when FreeRDP clients are using compatibility mode with the /relax-order-checks option enabled. The attack vector is network-accessible and requires no special privileges or user interaction (NVD).

The primary mitigation is to upgrade to FreeRDP version 2.1.2 or later which contains the fix for this vulnerability. For users unable to upgrade immediately, a workaround is to avoid using the /relax-order-checks compatibility mode option (FreeRDP Release).