Denial of Service in uap-core when processing crafted User-Agent strings

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.7 · CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H

EPSS

• CVE-2020-5243: 2.29% estimated exploitation activity in the next 30 days; percentile 83.22 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact Some regexes are vulnerable to regular expression denial of service (REDoS) due to overlapping capture groups. This allows remote attackers to overload a server by setting the User-Agent header in an HTTP(S) request to maliciously crafted long strings. Patches Please update uap-core to >= v0.7.3 Downstream packages such as uap-python, uap-ruby etc which depend upon uap-core follow different version schemes. Details Each vulnerable regular expression reported here contains 3 overlapping capture groups. Backtracking has approximately cubic time complexity with respect to the length of the user-agent string. Regex 1: is vulnerable in portion ([^;]+) and can be attacked with e.g. Regex 2: is vulnerable in portion \d+[^\);]+[^\);] and can be attacked with Regex 3: is vulnerable in portion ([^;]) and can be attacked with Regex 4: is vulnerable in portions (?:CUS:([^;])|([^;]+)) and ([^;]) and can be attacked with Reported by Ben Caller @bcaller

Published by the source

20 Feb 2020, 23:26 UTC

Last modified by the source

8 Jul 2026, 06:27 UTC

Source last checked

26 Oct 2026, 12:30 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-cmcx-xhr8-3w9p

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-cmcx-xhr8-3w9p

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# Patch Next advisory brief: CVE-2020-5243

Advisory: Denial of Service in uap-core when processing crafted User-Agent strings
IDs: CVE-2020-5243, GHSA-cmcx-xhr8-3w9p
Record: https://patchnext.com/advisories/cve-2020-5243 (revision 2, source checked 26 Oct 2026, 12:30 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: uap-core 0.7.3; user_agent_parser 2.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm uap-core: affected < 0.7.3; fixed in 0.7.3
- RubyGems user_agent_parser: affected < 2.6.0; fixed in 2.6.0

## Suggested inspection (read-only)
- `npm ls uap-core --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- RubyGems user_agent_parser: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on uap-core, user_agent_parser.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-cmcx-xhr8-3w9p (OSV record GHSA-cmcx-xhr8-3w9p lists affected ranges: npm uap-core >=0 =0 <2.6.0 (fixed))
- Verified: https://github.com/advisories/GHSA-cmcx-xhr8-3w9p (GitHub-reviewed advisory GHSA-cmcx-xhr8-3w9p lists affected ranges: npm uap-core >=0 =0 <2.6.0)

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