Nimble is a package manager for the Nim programming language. In Nim release versions before versions 1.2.10 and 1.4.4, "nimble refresh" fetches a list of Nimble packages over HTTPS without full verification of the SSL/TLS certificate due to the default setting of httpClient. An attacker able to perform MitM can deliver a modified package list containing malicious software packages. If the packages are installed and used the attack escalates to untrusted code execution.
https://github.com/nim-lang/security/security/advisories/GHSA-c2wm-v66h-xhxx
https://github.com/nim-lang/nimble/blob/master/changelog.markdown#0130
https://github.com/nim-lang/Nim/pull/16940
https://consensys.net/diligence/vulnerabilities/nim-insecure-ssl-tls-defaults-remote-code-execution/