The vulnerability resides in the expression engine used by heym when it processes workflow expressions. An authenticated user can construct expressions that exploit the DotList map/filter and fallback resolver to perform a sandbox escape. By using dunder attribute access within item expressions or through the fallback resolver, the attacker obtains a reference to os.system and can execute arbitrary shell commands as the backend process. This results in a remote code execution flaw that can compromise the host system. The flaw is classified as CWE-94: Improper Control of Generation of Code via Dynamic Web Content or Reflection.

All installations of the heym package from himyrun that are at a version earlier than 0.0.91 are affected. The vendor provides a fix in the 0.0.91 release. No specific minor releases are listed; users should upgrade to the latest available version and verify that the patched code includes the updated expression engine logic.

The CVSS score of 8.7 indicates a high severity. The EPSS score is not available, so current exploitation probability cannot be quantified. The vulnerability is not listed in the CISA KEV catalog. Attackers must possess authenticated access to the heym backend, likely via the web interface or API responsible for creating and executing workflow expressions. Once authenticated, the attacker can construct a malicious expression to trigger os.system, thereby gaining code execution ability. The lack of an available exploit for unauthenticated users limits the attack surface but still represents a significant risk for environments where user privileges are broad. The risk is therefore high for systems with exposed workflow features and broad user permissions.