Regular Expression Denial of Service (ReDoS) in jsx-slack

Exploitation

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

CVSS

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

EPSS

• CVE-2021-43838: 1.42% estimated exploitation activity in the next 30 days; percentile 72.90 (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.

jsx-slack v4.5.1 and earlier versions are vulnerable to a regular expression denial-of-service (ReDoS) attack. Impact If attacker can put a lot of JSX elements into <blockquote tag, an internal regular expression for escaping characters may consume an excessive amount of computing resources. Patches See also: https://github.com/yhatt/jsx-slack/security/advisories/GHSA-hp68-xhvj-x6j6 jsx-slack v4.5.2 has updated regular expressions to prevent catastrophic backtracking. jsx-slack v4.5.1 also had patched a workaround. It has no problems to contents with ASCII characters, but still vulnerable to contents with multibyte characters. (https://github.com/yhatt/jsx-slack/commit/36e4a10405e4c7745333e245fcc5029c02c7065d) References - https://nvd.nist.gov/vuln/detail/CVE-2021-43838 - https://github.com/yhatt/jsx-slack/commit/36e4a10405e4c7745333e245fcc5029c02c7065d Credits Thanks to @hieki for finding out this vulnerability.

Published by the source

17 Dec 2021, 19:59 UTC

Last modified by the source

8 Jul 2026, 06:26 UTC

Source last checked

26 Oct 2026, 12:22 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-55xv-f85c-248q

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-55xv-f85c-248q

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# Patch Next advisory brief: CVE-2021-43838

Advisory: Regular Expression Denial of Service (ReDoS) in jsx-slack
IDs: CVE-2021-43838, GHSA-55xv-f85c-248q
Record: https://patchnext.com/advisories/cve-2021-43838 (revision 2, source checked 26 Oct 2026, 12:22 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: jsx-slack 4.5.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm jsx-slack: affected < 4.5.1; fixed in 4.5.1

## Suggested inspection (read-only)
- `npm ls jsx-slack --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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 jsx-slack.
- 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-55xv-f85c-248q (OSV record GHSA-55xv-f85c-248q lists affected ranges: npm jsx-slack >=0 <4.5.1 (fixed))
- Verified: https://github.com/advisories/GHSA-55xv-f85c-248q (GitHub-reviewed advisory GHSA-55xv-f85c-248q lists affected ranges: npm jsx-slack >=0 <4.5.1)

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