PatchSiren cyber security CVE debrief
CVE-2026-9358 postcss CVE debrief
A vulnerability in postcss up to version 7.1.1 allows uncontrolled recursion in the `toString` function of `src/selectors/container.js` during AST serialization. An attacker can trigger this remotely by manipulating user-generated CSS input, leading to denial of service through stack exhaustion. The vendor has publicly stated that server-side DoS on user-generated CSS is considered low risk, as most users compile their own CSS with PostCSS. The CVSS 4.0 score of 2.1 (LOW) reflects limited availability impact with user interaction required. The exploit has been publicly disclosed via a GitHub Gist.
- Vendor
- postcss
- Product
- postcss
- CVSS
- LOW 2.1
- CISA KEV
- Not listed in stored evidence
- Original CVE published
- 2026-05-24
- Original CVE updated
- 2026-05-26
- Advisory published
- 2026-05-24
- Advisory updated
- 2026-05-26
Who should care
Organizations running server-side CSS processing services with postcss ≤7.1.1 on untrusted user input; development teams using postcss in CI/CD pipelines processing external CSS; security teams monitoring for DoS conditions in JavaScript-based build tools.
Technical summary
The `toString` method in `src/selectors/container.js` fails to limit recursion depth during AST serialization. Maliciously crafted CSS with deeply nested structures can trigger stack exhaustion. Attack vector is network-based with low attack complexity, requiring user interaction (UI:P) but no privileges. Availability impact is low (VA:L) per CVSS 4.0 scoring. The vulnerability is exploitable remotely against services that process untrusted user-generated CSS through postcss.
Defensive priority
low
Recommended defensive actions
- Upgrade postcss to a version newer than 7.1.1 when available
- Implement input size limits and recursion depth guards for CSS parsing in production environments
- Monitor for excessive CPU consumption or stack overflow errors in CSS processing pipelines
- Consider sandboxing or resource-constrained execution environments for untrusted CSS compilation
- Review vendor security advisories for postcss for patch availability
Evidence notes
Vulnerability affects postcss ≤7.1.1 in AST serialization component. CWE-404 (Improper Resource Shutdown or Release) and CWE-674 (Uncontrolled Recursion) identified. CVSS 4.0 vector: AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:L. Vendor risk assessment downplays server-side DoS impact.
Official resources
public