Executive Summary

This vulnerability affects the calibre Content Server's brute-force protection mechanism prior to version 9.4.0. The protection uses a ban key derived from both the client's IP address (remote_addr) and the X-Forwarded-For HTTP header. However, because the X-Forwarded-For header is taken directly from the HTTP request without validation or trusted-proxy configuration, an attacker can manipulate this header to bypass IP-based bans.

As a result, the brute-force protection can be completely circumvented by simply changing or adding the X-Forwarded-For header, allowing attackers to continue password guessing or credential stuffing attacks without being blocked.

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Impact Analysis

If you run a calibre Content Server exposed to the internet and use its brute-force protection to defend against unauthorized access attempts, this vulnerability can allow attackers to bypass those protections.

This means attackers can perform credential stuffing or password guessing attacks without being blocked by IP bans, increasing the risk of unauthorized access to your e-book management system.

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Compliance Impact

I don't know

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Detection Guidance

I don't know

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Mitigation Strategies

To mitigate this vulnerability, upgrade calibre to version 9.4.0 or later, where the issue with the brute-force protection mechanism has been fixed.

Additionally, avoid exposing the calibre Content Server directly to the internet without proper protections, as the vulnerability allows attackers to bypass IP-based bans by manipulating the X-Forwarded-For header.

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