CVE-2021-43838 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in jsx-slack. 1 public exploit reference exists, so weaponization risk is real. A fix is available for jsx-slack — see the affected versions and patch details below.
Regular Expression Denial of Service (ReDoS) in jsx-slack
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-43838 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
jsx-slacknpmDescription
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.
/** @jsxImportSource jsx-slack */
import { Section } from 'jsx-slack'
console.log(
<Section>
<blockquote>
{[...Array(40)].map((_, i) => (
<p>{i + 1}</p>
))}
</blockquote>
</Section>
)
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | jsx-slack | all versions | 4.5.1npm install jsx-slack@4.5.1 |
Affected Products
jsx-slackjsx-slack_projectResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for jsx-slack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2021-43838 in your dependencies?
Find it across npm, including transitive dependencies.