VulnerabilityPublished 27 Feb 2026Updated 2 Mar 2026

CVE-2026-27824: kovidgoyal calibre · Improper Restriction of Excessive Authentication Attempts

Affects kovidgoyal calibreSource CVE Database V5

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Not known to be exploited · CVSS 5.3 Medium : upgrade to 9.4.0 when convenient.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.1% chance of exploitation in 30 days – higher than 6% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 9.4.0

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 27 Feb 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade calibre and kovidgoyal/calibre to 9.4.0.

Affected software

Product	Affected	Fixed in
kovidgoyalcalibre	Affected <9.4.0	Fixed in 9.4.0
GitHubkovidgoyal/calibre`pkg:github/kovidgoyal/calibre`Other threats	Affected <9.4.0	Fixed in 9.4.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

To mitigate this vulnerability, organizations should immediately upgrade all calibre Content Server instances to version 9.4.0 or later, where the issue is fixed. Until upgrading is possible, administrators should restrict access to the calibre Content Server by implementing network-level controls such as IP whitelisting, VPN access, or firewall rules to limit exposure to trusted users only. Additionally, configuring reverse proxies or load balancers to properly validate and sanitize the X-Forwarded-For header can help prevent header spoofing. Enforcing strong, unique passwords and enabling multi-factor authentication (if supported) will reduce the risk of successful brute-force attacks. Monitoring authentication logs for repeated failed attempts and implementing alerting can provide early detection of attack attempts. Finally, disabling or limiting the Content Server's internet exposure when not necessary is recommended to reduce attack surface.

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.

Impact

The primary impact of this vulnerability is the potential compromise of user credentials on calibre Content Servers exposed to the internet. By bypassing brute-force protections, attackers can perform unlimited password guessing or credential stuffing attacks, increasing the likelihood of unauthorized access. This can lead to unauthorized disclosure of e-book libraries, user privacy violations, and potential further exploitation if credentials are reused elsewhere. Although the vulnerability does not directly affect system integrity or availability, successful unauthorized access could enable attackers to manipulate or delete content. Organizations relying on calibre servers for remote e-book access are at risk, especially if they use weak or reused passwords. The medium CVSS score (5.3) reflects the moderate confidentiality impact and ease of exploitation without authentication or user interaction. The lack of known exploits in the wild suggests limited active exploitation but does not diminish the risk for exposed servers.

Summary

Calibre is a widely used cross-platform e-book management application that includes a Content Server for remote access to e-book libraries. Prior to version 9.4.0, the Content Server implemented brute-force protection by banning IP addresses after excessive failed authentication attempts. This ban key was generated using the client's remote IP address combined with the X-Forwarded-For HTTP header. However, the server reads the X-Forwarded-For header directly from incoming HTTP requests without validating whether the header originates from a trusted proxy. Since HTTP headers can be easily spoofed by an attacker, this allows them to manipulate the X-Forwarded-For value to evade IP bans. Consequently, the brute-force protection mechanism can be bypassed, enabling attackers to perform unlimited password guessing or credential stuffing attacks against the server. This vulnerability is classified under CWE-307 (Improper Restriction of Excessive Authentication Attempts) and CWE-346 (Origin Validation Error). The vulnerability affects all calibre Content Server instances exposed to the internet running versions earlier than 9.4.0. The issue does not require any authentication or user interaction to exploit, and no known exploits are currently observed in the wild. The vendor addressed this flaw in version 9.4.0 by properly validating or restricting the use of the X-Forwarded-For header to trusted proxies only, restoring the effectiveness of brute-force protections.

CWE-307Improper Restriction of Excessive Authentication Attempts (opens in a new tab)
Base weaknessOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
The product does not implement sufficient measures to prevent multiple failed authentication attempts within a short time frame.
Architecture and Design
Common protection mechanisms include: Disconnecting the user after a small number of failed attempts; Implementing a timeout; Locking out a targeted account; Requiring a computational task on the user's part.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
OWASP Cheat Sheets
Authentication (opens in a new tab)
Transport Layer Protection (opens in a new tab)
TLS Cipher String (opens in a new tab)
OWASP ASVS 4.0.3
V2.2.1 (opens in a new tab)
V2.8.5 (opens in a new tab)
V2.8.4 (opens in a new tab)
Attack patterns
Dictionary-based Password Attack (opens in a new tab)
Password Brute Forcing (opens in a new tab)
Password Spraying (opens in a new tab)
Credential Stuffing (opens in a new tab)
Use of Known Kerberos Credentials (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-346Origin Validation Error (opens in a new tab)
Class weaknessOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
The product does not properly verify that the source of data or communication is valid.
OWASP Cheat Sheets
OWASP Authentication (opens in a new tab)
OWASP ASVS 4.0.3
V14.5.2 (opens in a new tab)
V14.5.3 (opens in a new tab)
V1.4.4 (opens in a new tab)
Attack patterns
JSON Hijacking (aka JavaScript Hijacking) (opens in a new tab)
Cache Poisoning (opens in a new tab)
DNS Cache Poisoning (opens in a new tab)
Exploit Script-Based APIs (opens in a new tab)
Application API Message Manipulation via Man-in-the-Middle (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/kovidgoyal/calibre/security/advisories/GHSA-vhxc-r7v8-2xrw (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 6 Mar 2026, 21:41 UTC

WeaknessesCWE-307, CWE-346

Technical details

References

Community reviews

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