CVE-2026-27824: calibre-ebook calibre

Vulnerability summary

CVE-2026-27824 is a medium (CVSS 5.3) vulnerability in calibre-ebook calibre. EPSS estimates a 0.19% chance of exploitation in the next 30 days, higher than 8.6% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 27-02-2026
   PublishedVulnerability first disclosed.
2. 02-03-2026
   Last modifiedVulnerability information updated.

Description

calibre is a cross-platform e-book manager for viewing, converting, editing, and cataloging e-books. Prior to version 9.4.0, the calibre Content Server's brute-force protection mechanism uses a ban key derived from both `remote_addr` and the `X-Forwarded-For` header. Since the `X-Forwarded-For` header is read directly from the HTTP request without any validation or trusted-proxy configuration, an attacker can bypass IP-based bans by simply changing or adding this header, rendering the brute-force protection completely ineffective. This is particularly dangerous for calibre servers exposed to the internet, where brute-force protection is the primary defense against credential stuffing and password guessing attacks. Version 9.4.0 contains a fix for the issue.

• CWE-307Improper Restriction of Excessive Authentication AttemptsThe product does not implement sufficient measures to prevent multiple failed authentication attempts within a short time frame.
• CWE-346Origin Validation ErrorThe product does not properly verify that the source of data or communication is valid.