Security readout for executives and security teams
Plain-English summary
CVE-2026-27824 affects calibre Content Server before 9.4.0. Its login brute-force ban can be bypassed by spoofing an HTTP forwarding header, weakening protection against password guessing. The issue matters most for internet-exposed calibre servers. Version 9.4.0 contains the fix.
Executive priority
Treat as a timely patch for exposed calibre servers, not a broad emergency. Prioritize internet-facing instances because the flaw weakens the main defense against password guessing.
Technical view
The ban key uses both remote_addr and the unvalidated X-Forwarded-For request header. Because calibre trusts this header without trusted-proxy validation, changing it can avoid IP-based bans. CVSS is 5.3, network-accessible, low complexity, no privileges or user interaction required. CWE mappings include CWE-307 and CWE-346.
Likely exposure
Organizations running calibre Content Server versions earlier than 9.4.0 are affected. Risk is highest when the server is reachable from the internet or broadly reachable by untrusted networks.
Exploitation context
The issue enables bypass of brute-force protection; it does not by itself authenticate the attacker. It can support credential stuffing or password guessing against exposed servers. The source bundle does not indicate active exploitation, and CISA KEV status is false.
Researcher notes
Evidence is limited to the CVE record and GitHub advisory. No exploit-in-the-wild evidence is provided. The affected scope is calibre Content Server before 9.4.0; do not generalize to unrelated calibre features without confirmation.
Mitigation direction
- Upgrade calibre to version 9.4.0 or later.
- Inventory any exposed calibre Content Server instances.
- Restrict Content Server access to trusted networks where possible.
- Check vendor advisory for any additional operational guidance.
Validation and detection
- Confirm the installed calibre version on all Content Server hosts.
- Identify whether Content Server is reachable from the internet.
- Review access logs for repeated login failures with varied forwarding headers.
- Verify upgraded systems report calibre 9.4.0 or later.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-307: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCWE-346: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCredential and access behavior lookup
The CVE wording references authentication or credential exposure, so valid-account and credential-access review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2026-27824 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Medium
- CVSS
- 5.3 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N3.91.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.3MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/kovidgoyal/calibre/security/advisories/GHSA-vhxc-r7v8-2xrwCVE reference · x_refsource_CONFIRM
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Restriction of Excessive Authentication Attempts
Improper Restriction of Excessive Authentication Attempts represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Origin Validation Error
Origin Validation Error represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.