# CVE-2021-34801: Valine denial of service via malformed User-Agent

Source: https://junglewise.ai/threats/cve-2021-34801-valine-denial-of-service-via-malformed-user-agent  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2021-06-21. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2021-34801 · Severity: low · CVSS 3.1 · Published 2021-06-21

Valine is a popular comment system embedded in websites. An attacker can crash the comment system on a page by submitting a comment with a specially crafted User-Agent header, preventing all other visitors from seeing or loading comments on that page until the malicious comment is removed.

Technologies: [valine (npm)](https://junglewise.ai/threats/technologies/valine). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2021-34801 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-34801 being exploited in the wild.

**How severe is CVE-2021-34801?** Low severity, with a CVSS score of 3.1, published 21 June 2021.

**Which products and versions are affected?** valine (npm) SEMVER:introduced=0,last_affected=1.4.14.

**Is there a fix for CVE-2021-34801?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2021-34801 attacked?** It is attacked over the network.

**Am I affected by CVE-2021-34801?** Check whether you run valine (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

Valine 1.4.14 contains a vulnerability in User-Agent (UA) parsing that causes an application crash when processing an incomplete or malformed UA string (e.g., "Mozilla/8.0"). The root cause is improper validation of the UA field during comment processing; the vulnerability allows remote, unauthenticated attackers to submit a comment with a crafted UA value that triggers a fatal error in the comment system. When exploited, the entire comment section on an affected page becomes non-functional until the malicious comment is deleted. The attack requires no authentication and can be triggered via a standard HTTP request to submit a comment.

## Affected products

- Valine Valine 1.4.14 and earlier

## Identifiers

- CVE-2021-34801
- GHSA-p2c4-gxp4-j3xp

## Timeline

- 2021-06-16: disclosed: Vulnerability reported on GitHub
- 2021-06-21: advisory: GHSA advisory published
- 2021-06-16: other: CVE-2021-34801 assigned

## References

- [https://github.com/xCss/Valine/issues/366](https://github.com/xCss/Valine/issues/366)

## Related threats

- [Valine code injection vulnerability](https://junglewise.ai/threats/cve-2022-38545-valine-code-injection-vulnerability)
- [Valine stored cross-site scripting in nick parameter](https://junglewise.ai/threats/cve-2020-28847-valine-stored-cross-site-scripting-in-nick-parameter)
- [Valine HTML injection via embedded PDF files](https://junglewise.ai/threats/cve-2018-19289-valine-html-injection-via-embedded-pdf-files)