An attacker can embed a malicious python: template inside a custom column definition within the calibre:user_metadata field of an EPUB, OPF, or PDF file. When calibre reads the metadata, the template is passed unsanitized to the template formatter, allowing arbitrary Python code execution. This is a CWE‑94 code injection vulnerability that can compromise confidentiality, integrity, and availability of the system where calibre is running.

The issue exists in the KovidGoyal calibre e‑book manager prior to the release of version 9.10.0. Any installation running calibre 9.9 or earlier, or any other legacy releases that have not applied the fix, is vulnerable. Users that interact with untrusted EPUB, OPF, or PDF files in these versions are at risk.

The CVSS score is 8.5, indicating high severity. The EPSS score of less than 1% denotes a very low, yet non‑zero, probability of exploitation. The vulnerability is not listed in CISA KEV. The likely attack vector is the local processing of a malicious book file that an attacker supplies to a user; once the user opens or edits the file, arbitrary code runs on the system where calibre is installed.