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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
2026-07-23
Advisory published
2026-05-24
Advisory updated
2026-07-23

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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-9358 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-9358

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-9358 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-9358

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.