CVE-2021-43838: jsx-slack is a library for building JSON objects for Slack Block Kit surfaces from JSX

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MEDIUM 5.3REMOTEUPGRADE

CVE-2021-43838 is a medium-severity flaw (CVSS 5.3) affecting jsx-slack. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

jsx-slack is a library for building JSON objects for Slack Block Kit surfaces from JSX. In versions prior to 4.5.1 users are vulnerable to a regular expression denial-of-service (ReDoS) attack. 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. jsx-slack v4.5.1 has patched to a regex for escaping blockquote characters. Us

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.4% (top 27%)
CISA exploited list	No
Weakness	CWE-400, CWE-1333
Published	2021-12-17
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: jsx-slack project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-12-17 19:15 UTCPublished CVE record published
2026-06-17 04:11 UTCUpdated Record last modified

jsx-slack project jsx-slack UPGRADE

Affected: < 4.5.2

Fixed in: 4.5.2

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls jsx-slack

2. UPGRADE to 4.5.2

npm install jsx-slack@">=4.5.2"

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-43838?

jsx-slack: upgrade to 4.5.2. Exact commands for each OS are on this page.

Is CVE-2021-43838 being exploited?

CVE-2021-43838 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-43838 affect?

CVE-2021-43838 affects jsx-slack. Severity is medium (CVSS 5.3). jsx-slack is a library for building JSON objects for Slack Block Kit surfaces from JSX. In versions prior to 4.5.1 users are vulnerable to a regular expression denial-of-service (ReDoS) attack. If attacker can put a lot of JSX elements into

How urgent is CVE-2021-43838?

Priority 47/100. Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Slack vulnerabilities

Related CVEs

CVE-2021-43843 MEDIUM 5.3 UPGRADE jsx-slack is a package for building JSON objects for Slack block kit surfaces from JSX

References

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