CVE-2021-43843

jsx-slack insufficient patch for CVE-2021-43838 ReDoS

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.

References

• https://github.com/yhatt/jsx-slack/commit/46bc88391d89d5fda4ce689e18ca080bcdd29ecc

Credits

Thanks to @hieki for finding out this vulnerability.

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

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2021-43843 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.5.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

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.

1. What is CVE-2021-43843? CVE-2021-43843 is a medium-severity uncontrolled resource consumption vulnerability in jsx-slack (npm), affecting versions < 4.5.2. It is fixed in 4.5.2. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2021-43843? CVE-2021-43843 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of jsx-slack are affected by CVE-2021-43843? jsx-slack (npm) versions < 4.5.2 is affected.
4. Is there a fix for CVE-2021-43843? Yes. CVE-2021-43843 is fixed in 4.5.2. Upgrade to this version or later.
5. Is CVE-2021-43843 exploitable, and should I be worried? Whether CVE-2021-43843 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-43843 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-43843? Upgrade jsx-slack to 4.5.2 or later.

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