What is CVE-2021-43838?

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 ` ` 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. Users are advised to upgrade as soon as possible.

How severe is CVE-2021-43838?

CVE-2021-43838 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-43838?

CVE-2021-43838 affects the following packages: jsx-slack (npm). Ecosystems affected: npm.

How do I fix CVE-2021-43838?

Update jsx-slack to 4.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43838 is resolved across your whole dependency graph.

How do I detect CVE-2021-43838 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for jsx-slack. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2021-43838 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2021-43838?

O3 pinpoints whether CVE-2021-43838 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2021-43838 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-43838, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-43838?

CVE-2021-43838 has an EPSS (Exploit Prediction Scoring System) score of 1.4%, placing it in the 68th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

When was CVE-2021-43838 published, and has it been updated?

CVE-2021-43838 was published on December 17, 2021 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

CVE-2021-43838

HIGH

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

Also known asGHSA-55xv-f85c-248q

Published

Dec 17, 2021

Updated

Apr 10, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

EPSS Exploitation Probability

via FIRST.org ↗

1.4%probability of exploitation in next 30 days

Lower Risk68th percentile+1.04%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

1 pkg affected

📦jsx-slack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.

Description

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. Users are advised to upgrade as soon as possible.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`jsx-slack`	all versions	4.5.1

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

jsx-slack is a library for building JSON objects for Slack Block Kit sur…

github.comNVDJan 2022

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for jsx-slack. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update jsx-slack to 4.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43838 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-43838 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2021-43838. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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. Users are advised to upgrade as soon as possible.

O3 Security · Impact-Aware SCA

Is CVE-2021-43838 in your dependencies?

O3 detects CVE-2021-43838 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works