# CVE-2021-21317: uap-core regular expression denial of service

Source: https://junglewise.ai/threats/cve-2021-21317-uap-core-regular-expression-denial-of-service  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2021-02-02. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2021-21317 · Severity: low · CVSS 3.1 · Published 2021-02-02

uap-core is a library used to parse and identify user agent strings in web requests. A regular expression vulnerability allows attackers to craft malicious User-Agent headers that cause the parser to consume excessive CPU and hang, disrupting service availability for all users of affected applications.

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

## Quick answers

**Is CVE-2021-21317 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-21317 being exploited in the wild.

**How severe is CVE-2021-21317?** Low severity, with a CVSS score of 3.1, published 2 February 2021.

**Which products and versions are affected?** uap-core (npm) SEMVER:introduced=0,fixed=0.11.0.

**Is there a fix for CVE-2021-21317?** Yes. uap-core (npm): upgrade to 0.11.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2021-21317 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2021-21317?** Check whether you run uap-core (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.

## How to fix

- uap-core (npm): upgrade to 0.11.0 or later

  ```sh
  npm install uap-core@0.11.0
  ```


## Technical details

The vulnerability is a regular expression denial of service (REDoS) attack caused by overlapping capture groups in regex patterns used to parse User-Agent headers. An attacker can send HTTP requests with specially crafted long User-Agent strings to trigger catastrophic backtracking in the regex engine. This is a network-accessible vulnerability requiring no authentication or user interaction. Successful exploitation results in CPU exhaustion and denial of service. The fix is available in uap-core version 0.11.0 and later.

## Affected products

- ua-parser uap-core <0.11.0

## Identifiers

- CVE-2021-21317
- GHSA-p4pj-mg4r-x6v4

## Timeline

- 2021-01-30: disclosed
- 2021-02-02: patched: Version 0.11.0 released

## References

- [https://github.com/ua-parser/uap-core/security/advisories/GHSA-p4pj-mg4r-x6v4](https://github.com/ua-parser/uap-core/security/advisories/GHSA-p4pj-mg4r-x6v4)

## Related threats

- [uap-core ReDoS in User-Agent parsing](https://junglewise.ai/threats/cve-2020-5243-uap-core-redos-in-user-agent-parsing)
- [UA-Parser uap-core regular expression denial of service](https://junglewise.ai/threats/cve-2018-20164-ua-parser-uap-core-regular-expression-denial-of-service)