# CVE-2021-44686: Denial of service in n/a

> An attacker can cause a denial of service in Calibre via a Regular Expression Denial of Service vulnerability in the ebook conversion module.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 Dec 2021
- **Updated:** 4 Nov 2025
- **Also known as:** DEBIAN-CVE-2021-44686, GHSA-m6m7-mf2p-99p5, MGASA-2021-0593, UBUNTU-CVE-2021-44686

## What is CVE-2021-44686?

CVE-2021-44686 is a high-severity denial of service vulnerability in Fedoraproject Fedora, published on 6 Dec 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular Expression Denial of Service) in html_preprocess_rules in ebooks/conversion/preprocess.py.

Weakness: CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2021-44686?

It affects 2 products.


Products: Calibre-Ebook Calibre, Fedoraproject Fedora.

## How do I fix CVE-2021-44686?

Upgrade to a fixed version.

## Is CVE-2021-44686 being exploited?

There is no known exploitation of CVE-2021-44686 in the wild and no public exploit code.

## Other vulnerabilities in Fedoraproject Fedora

- [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-2021-44686 and fix it if it is affected.

What it is: CVE-2021-44686 is a high-severity denial of service vulnerability in Fedoraproject Fedora, published on 6 Dec 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-44686 in the wild and no public exploit code.

Affected product: Fedoraproject Fedora. 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-2021-44686/ (Markdown: https://insights.sbomify.com/CVE-2021-44686/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-44686
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-44686
- OSV: https://osv.dev/vulnerability/CVE-2021-44686

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-44686/
Summaries are generated and may contain mistakes; the official description above is the record.