VulnerabilityPublished 27 Feb 2026Updated 2 Mar 2026

CVE-2026-27810: kovidgoyal calibre · HTTP Request/Response Splitting

Affects kovidgoyal calibreSource CVE Database V5

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Not known to be exploited · CVSS 6.4 Medium : upgrade to 9.4.0 when convenient.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score

6.4Medium

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 15% 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:L/UI:N/S:C/C:L/I:L/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Changed
Confidentiality: Low
Integrity: Low
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: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
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: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on 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

Upgrade all calibre Content Server instances to version 9.4.0 or later, which contains the fix for this vulnerability.
If immediate upgrade is not possible, restrict access to the Content Server to trusted networks and users only, minimizing exposure to authenticated attackers.
Implement strict input validation and sanitization on the 'content_disposition' parameter at the application or proxy level to prevent CRLF injection.
Employ web application firewalls (WAFs) with rules to detect and block HTTP response splitting attempts targeting the affected endpoints.
Educate users about the risks of clicking unsolicited or suspicious links, especially when authenticated to the Content Server.
Monitor server logs for unusual or malformed HTTP header injection attempts and investigate any anomalies promptly.
Consider implementing multi-factor authentication (MFA) to reduce the risk of compromised credentials enabling exploitation.
Review and limit privileges of users who can authenticate to the Content Server to reduce potential attack vectors.

Description

calibre is a cross-platform e-book manager for viewing, converting, editing, and cataloging e-books. Prior to version 9.4.0, an HTTP Response Header Injection vulnerability in the calibre Content Server allows any authenticated user to inject arbitrary HTTP headers into server responses via an unsanitized `content_disposition` query parameter in the `/get/` and `/data-files/get/` endpoints. All users running the calibre Content Server with authentication enabled are affected. The vulnerability is exploitable by any authenticated user and can also be triggered by tricking an authenticated victim into clicking a crafted link. Version 9.4.0 contains a fix for the issue.

Impact

The vulnerability allows attackers with authenticated access to inject arbitrary HTTP headers, enabling HTTP response splitting attacks. This can compromise confidentiality by hijacking user sessions or stealing sensitive information through manipulated responses. Integrity may be affected by altering server responses, potentially leading to web cache poisoning or cross-site scripting attacks. Although availability is not directly impacted, the manipulation of HTTP headers can degrade user trust and service reliability. Organizations using calibre Content Server in environments where multiple users authenticate remotely are at risk of internal threat actors or compromised accounts exploiting this flaw. The ability to trick authenticated users into clicking crafted links increases the attack surface, potentially enabling phishing or social engineering attacks that leverage this vulnerability. The medium severity score reflects the need for authentication but also the significant impact on confidentiality and integrity. Failure to patch could lead to exploitation in environments where calibre is used for e-book distribution, including educational institutions, libraries, and enterprises managing digital content.

CWE-113Improper Neutralization of CRLF Sequences in HTTP Headers (HTTP Request Splitting) (opens in a new tab)
Variant weaknessOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product receives data from an HTTP agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers.
Implementation · Input Validation
Construct HTTP headers very carefully, avoiding the use of non-validated input data.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. If an input does not strictly conform to specifications, reject it or transform it into something that conforms.
Implementation · Output Encoding
Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
HTTP Request Splitting (opens in a new tab)
Accessing/Intercepting/Modifying HTTP Cookies (opens in a new tab)
HTTP Response Splitting (opens in a new tab)
AJAX Footprinting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/kovidgoyal/calibre/security/advisories/GHSA-5fpj-fxw7-8grw (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-113

Technical details

References

Community reviews

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