How severe is CVE-2026-9358?

No CVSS score has been assigned to CVE-2026-9358 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-9358?

CVE-2026-9358 affects the following packages: postcss-selector-parser (npm), postcss-selector-parser (npm). Ecosystems affected: npm.

How do I fix CVE-2026-9358?

Update postcss-selector-parser to 6.1.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-9358 is resolved across your whole dependency graph.

How do I detect CVE-2026-9358 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for postcss-selector-parser, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-9358 if there is no patch (or I can't update yet)?

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.

Is CVE-2026-9358 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-9358 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-9358?

CVE-2026-9358 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 47th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-9358?

CVE-2026-9358 is classified as CWE-404 (CWE-404), CWE-674 (CWE-674). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-9358 published, and has it been updated?

CVE-2026-9358 was published on May 24, 2026 and was last updated on October 21, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2026-9358: postcss-selector DoS

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-9358.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs47th percentile — riskier than 47% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

2 pkgs affected

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.

2Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

postcss-selector-parsernpm

88.2Mdownloads / week

Description

A vulnerability was determined in postcss-selector-parser up to 6.1.2/7.1.2. Affected is the function toString of the file src/selectors/container.js of the component AST Serialization. Executing a manipulation can lead to uncontrolled recursion. It is possible to launch the attack remotely. The exploit has been publicly disclosed and may be utilized. Upgrading to version 6.1.3 and 7.1.3 is able to address this issue. This patch is called 5bc698cef66f8abd12610dc623e5d67cbc0f869d. It is suggested to upgrade the affected component. The vendor explains, that according to his definition "DoS on server-side on user-generated CSS is low risk for us (since most users compile own CSS with PostCSS)." The commits were backported to 6.x branch, which was the most downloaded version.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`postcss-selector-parser`	≥ 6.1.0&&< 6.1.3	6.1.3`npm install postcss-selector-parser@6.1.3`
📦npm	`postcss-selector-parser`	≥ 7.1.0&&< 7.1.3	7.1.3`npm install postcss-selector-parser@7.1.3`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for postcss-selector-parser, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update postcss-selector-parser to 6.1.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-9358 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.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Moderate: This flaw in Postcss can lead to a denial of service (DoS) in applications that process untrusted CSS. The vulnerability arises from uncontrolled recursion in the AST Serialization component when handling specially crafted CSS. Exploitation requires user interaction, such as submitting attacker-controlled…

Workaround published by Red Hat

Upgrade to PostCSS 8.5.6 or later. As a workaround, validate and sanitize CSS input before processing with PostCSS to prevent deeply nested or malicious structures from reaching the serializer.

Source: Red Hat security advisory for CVE-2026-9358 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat Hardened Images	`rust-main-1.96.0-1.hum1`	RHSA-2026:22934
Red Hat Openshift Data Foundation 4.18	`odf4/cephcsi-rhel9:1786705347`	RHSA-2026:56431
Red Hat Openshift Data Foundation 4.22	`odf4/cephcsi-rhel9:1787057250`	RHSA-2026:56928

Frequently Asked Questions

A vulnerability was determined in postcss-selector-parser up to 6.1.2/7.1.2. Affected is the function toString of the file src/selectors/container.js of the component AST Serialization. Executing a manipulation can lead to uncontrolled recursion. It is possible to launch the attack remotely. The exploit has been publicly disclosed and may be utilized. Upgrading to version 6.1.3 and 7.1.3 is able to address this issue. This patch is called 5bc698cef66f8abd12610dc623e5d67cbc0f869d. It is suggested to upgrade the affected component. The vendor explains, that according to his definition "DoS on se

O3 Security · Impact-Aware SCA

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CVE-2026-9358 — postcss-selector-parser