CVE-2019-14868: 
NixOS vulnerability analysis and mitigation

In ksh version 20120801, a critical security flaw was discovered in the way it evaluates certain environment variables. The vulnerability, identified as CVE-2019-14868, allows attackers to override or bypass environment restrictions to execute shell commands. This issue is particularly concerning as services and applications that allow remote unauthenticated attackers to provide environment variables could be exploited remotely (NVD, CVE).

The vulnerability stems from KornShell's handling of environment variables during startup, where certain variables are interpreted as arithmetic expressions. Prior to the fix, these variables could contain arbitrary expressions, leading to potential code injection. The issue has a CVSS v3.1 base score of 7.8 (High), with vector CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H, indicating local access required but high impact on confidentiality, integrity, and availability (NVD).

The vulnerability allows attackers to execute arbitrary shell commands by bypassing environment restrictions. This is particularly dangerous in environments where ksh scripts are used by services or applications that accept remote user input for environment variables, as it could lead to remote code execution. The impact is heightened in scenarios where the shell is used in privileged contexts (Red Hat Bugzilla).

The vulnerability requires an attacker to be able to control the content of certain environment variables before a ksh shell is executed. This can be achieved either directly or through a script. The risk is particularly high in services and applications that allow remote users to provide environment variables, as it could enable remote exploitation (Red Hat Bugzilla).

The issue has been patched in various distributions and systems. The fix involves hardening the environment variable imports to only allow integer literals rather than arbitrary expressions. The patch was implemented in the AST project commit c7de8b641266bac7c77942239ac659edfee9ecd2, which improved validation of environment variables during shell initialization (GitHub Commit).

Multiple major vendors responded to this vulnerability by releasing security updates. Red Hat issued several security advisories (RHSA-2020:5351, RHSA-2020:2210) to address the vulnerability. Apple included fixes in their macOS security updates (Security Update 2020-003), and Debian released security patches through DLA-2284-1 (Debian Advisory, Apple Advisory).