CVE-2026-27824 Details
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.
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Exploit prediction scoring system (EPSS) score for CVE-2026-27824
EPSS (Exploit Prediction Scoring System): EPSS predicts the likelihood that a vulnerability will be exploited in the wild. A higher percentage means a greater chance of an exploit occurring. The EPSS model produces a probability score between 0 and 1 (0 and 100%).
0.15 Probability of exploitation activity being observed over the next 30 days (5th percentile)
CVSS score for CVE-2026-27824
CVSS (Common Vulnerability Scoring System): An open framework owned and managed by FIRST.Org, Inc. that assigns a numerical score from 0 to 10 to software vulnerabilities to indicate their severity.
5.3 Medium
Products affected by CVE-2026-27824
Calibre 1.48.0
Calibre 2.83.0
Calibre 3.39.1
Calibre 3.48.0
Calibre 4.15.0
DVE Score by Bitsight AI
DVE scores complement traditional static scoring systems like CVSS with Bitsight AI powered real-time threat intelligence and underground cyber criminal activity so that security teams can reduce alert fatigue and focus on vulnerabilities most likely to be exploited.
Low
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CVE-2026-27824 Global Footprint

Top 10 Identified Countries

Country Observations Percentage
US 303 35.77%
CN 165 19.48%
SG 86 10.15%
DE 63 7.44%
GB 48 5.67%
JP 24 2.83%
ID 14 1.65%
ES 14 1.65%
PH 10 1.18%
CA 10 1.18%

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CVE-2026-27824 Industry Footprint

Top 10 Identified Industries

*Service provider organizations (typically Technology and Telecommunications) are disproportionally represented in the results given their upstream ownership of end-user infrastructure. See our FAQs.

Industry* Observations Percentage
Technology 503 87.63%
Telecommunications 66 11.50%
Education 2 0.35%
Government/Politics 1 0.17%
Utilities 1 0.17%
Business Services 1 0.17%

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