Skip to content
Fencer Intel
medium 5.3

CVE-2026-27824 json

calibre has IP Ban Bypass via X-Forwarded-For Header Spoofing

published 8mo ago modified 4mo ago also EUVD-2026-9057

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

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.

Patch / fix

  • calibre-ebook calibre 9.4.0
  • calibre Debian 12 6.13.0+repack-2+deb12u6
  • calibre Debian 13 8.5.0+ds-1+deb13u2
  • calibre Debian 14 9.4.0+ds+~0.10.5-1

debian advisories DEBIAN-CVE-2026-27824

  • calibre openSUSE Tumbleweed 9.7.0-1.1

opensuse advisories openSUSE-SU-2026:10587-1

Is CVE-2026-27824 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

Check with Fencer

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-27824 updated cvss 5.3

calibre: Calibre: Brute-force protection bypass via X-Forwarded-For header manipulation

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
8mo ago

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • github.com https://github.com/kovidgoyal/calibre/security/advisories/GHSA-vhxc-r7v8-2xrw referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Affected products

  • kovidgoyal calibre

Classification

Weakness
  • CWE-307 Improper Restriction of Excessive Authentication Attempts
  • CWE-346 Origin Validation Error

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.