# CVE-2026-27824: calibre has IP Ban Bypass via X-Forwarded-For Header Spoofing

> Attackers can bypass IP-based brute-force protection in calibre Content Server by spoofing the X-Forwarded-For header, facilitating credential stuffing attacks.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Feb 2026
- **Updated:** 2 Mar 2026
- **Also known as:** DEBIAN-CVE-2026-27824, GHSA-vhxc-r7v8-2xrw, UBUNTU-CVE-2026-27824

## What is CVE-2026-27824?

CVE-2026-27824 is a medium-severity vulnerability in kovidgoyal calibre, published on 27 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-346 (Origin Validation Error), CWE-307 (Improper Restriction of Excessive Authentication Attempts)

## Which versions are affected by CVE-2026-27824?

It affects 1 product.


Products: Calibre-Ebook Calibre.

## How do I fix CVE-2026-27824?

Upgrade to a fixed version.

## Is CVE-2026-27824 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27824 exists.

## Other vulnerabilities in kovidgoyal calibre

- [CVE-2011-4124](https://insights.sbomify.com/CVE-2011-4124/): Command injection in Calibre (critical 9.8)
- [CVE-2011-4125](https://insights.sbomify.com/CVE-2011-4125/): Privilege escalation in Calibre (critical 9.8)
- [CVE-2026-26064](https://insights.sbomify.com/CVE-2026-26064/): calibre: Path Traversal Vulnerability Enables Arbitrary File Write and Remote Code Execution (critical 9.3)
- [CVE-2026-26065](https://insights.sbomify.com/CVE-2026-26065/): calibre: Path Traversal can Lead to Arbitrary File Write and Potential Code Execution (critical 9.3)
- [CVE-2026-25635](https://insights.sbomify.com/CVE-2026-25635/): calibre has a Path Traversal Leading to Arbitrary File Write and Potential Code Execution (high 8.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-27824 and fix it if it is affected.

What it is: CVE-2026-27824 is a medium-severity vulnerability in kovidgoyal calibre, published on 27 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27824 exists.

Affected product: kovidgoyal calibre. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-27824/ (Markdown: https://insights.sbomify.com/CVE-2026-27824/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-27824
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27824
- OSV: https://osv.dev/vulnerability/CVE-2026-27824

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-27824/
Summaries are generated and may contain mistakes; the official description above is the record.