CVE-2026-23986 is a vulnerability classified under CWE-61 (Improper Restriction of Symbolic Links in a File System) affecting the copier library and CLI tool used for rendering project templates. Prior to version 9.11.2, copier assumes that templates marked as safe (i.e., those not requiring the --UNSAFE or --trust flags) cannot perform unsafe operations. However, this assumption is flawed because a safe template can exploit the _preserve_symlinks: true feature to create symbolic links within the generated directory structure. By carefully crafting these symlinks, a malicious template author can cause copier to write files outside the intended destination directory, effectively overwriting arbitrary files on the filesystem where the user has write permissions. This can lead to unauthorized modification or destruction of files, potentially disrupting system integrity or causing data loss. The attack requires user interaction to run the malicious template but does not require elevated privileges or authentication. The vulnerability was addressed in copier version 9.11.2 by correcting how symlinks are handled during project generation to prevent directory traversal outside the target path. No known exploits are currently reported in the wild, but the medium CVSS score of 6.9 reflects the significant impact possible if exploited.

For European organizations, this vulnerability poses a risk primarily to development and DevOps environments that use copier for automated project scaffolding or templating. Successful exploitation can lead to unauthorized file overwrites, potentially compromising system stability, corrupting critical configuration files, or inserting malicious code into projects. This can disrupt software development workflows, cause downtime, or facilitate further attacks if attackers overwrite security-sensitive files. Since copier is often used in software development pipelines, the impact can cascade into production environments if compromised templates are propagated. The vulnerability does not require elevated privileges but does require user interaction, so social engineering or supply chain attacks involving malicious templates are plausible. Organizations relying on copier in regulated industries (e.g., finance, healthcare) must consider compliance risks due to potential data integrity violations. Prompt patching and template validation are critical to mitigate these impacts.

European organizations should immediately upgrade copier to version 9.11.2 or later to ensure the vulnerability is patched. They should audit all project templates in use, especially those sourced externally, to detect and remove any that use _preserve_symlinks: true or contain suspicious symlink structures. Implement strict controls on who can add or modify templates in internal repositories to prevent introduction of malicious templates. Employ static analysis or sandbox testing of templates before deployment to detect unsafe symlink usage. Educate developers and DevOps teams about the risks of running untrusted templates and the importance of verifying template sources. Consider restricting copier usage to isolated environments with limited filesystem permissions to minimize potential damage from exploitation. Monitor file integrity on critical systems to detect unexpected changes that could indicate exploitation.