CVE-2021-43843 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in jsx-slack. 2 public exploit references exist, so weaponization risk is real. A fix is available for jsx-slack — see the affected versions and patch details below.
jsx-slack insufficient patch for CVE-2021-43838 ReDoS
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-43843 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 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
We found the patch for CVE-2021-43838 in jsx-slack v4.5.1 is insufficient to save from Regular Expression Denial of Service (ReDoS) attack.
This vulnerability affects to jsx-slack v4.5.1 and earlier versions.
Impact
If attacker can put a lot of JSX elements into <blockquote> tag with including multibyte characters, 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(() => (
<p>亜</p>
))}
</blockquote>
</Section>
)
v4.5.1 has released by passing the test against ASCII characters but missed the case of multibyte characters. https://github.com/yhatt/jsx-slack/security/advisories/GHSA-55xv-f85c-248q
Patches
jsx-slack v4.5.2 has updated regular expressions for escaping blockquote characters to prevent catastrophic backtracking. It is also including an updated test case to confirm rendering multiple tags in <blockquote> with multibyte characters.
References
Credits
Thanks to @hieki for finding out this vulnerability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | jsx-slack | all versions | 4.5.2npm install jsx-slack@4.5.2 |
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 4.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43843 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-2021-43843 in your dependencies?
Find it across npm, including transitive dependencies.