CVE-2026-9358 in postcss-selector-parser
Summary
by MITRE • 05/24/2026
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.
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Analysis
by VulDB Data Team • 06/17/2026
The vulnerability in postcss-selector-parser represents a critical security flaw affecting versions up to 6.1.2 and 7.1.2, specifically within the AST serialization component's toString function located in src/selectors/container.js. This issue manifests as uncontrolled recursion during the processing of manipulated input, creating a potential denial of service condition that can be exploited remotely. The vulnerability's impact extends beyond simple service disruption as it can be triggered through user-generated CSS content processed by PostCSS, making it particularly dangerous in server-side environments where user input is parsed and compiled. The flaw stems from inadequate input validation and recursive processing logic that fails to properly handle maliciously crafted selector sequences, allowing attackers to craft inputs that cause infinite recursion during the serialization process.
The technical nature of this vulnerability aligns with CWE-674, which describes "Uncontrolled Recursion" in software systems, and demonstrates characteristics consistent with CWE-400, "Uncontrolled Resource Consumption," as the recursive behavior consumes system resources without proper bounds checking. From an operational perspective, this vulnerability can be exploited through the ATT&CK technique T1499.004, "Utilities: File and Directory Permissions Modification," where attackers manipulate CSS inputs to trigger resource exhaustion. The vulnerability's exploitability is heightened by the fact that it has been publicly disclosed and may already be in use by threat actors, making it a pressing concern for organizations running PostCSS-based systems. The patch addressing this issue, identified by commit hash 5bc698cef66f8abd12610dc623e5d67cbc0f869d, specifically targets the recursive processing logic within the container.js file's toString method.
Organizations utilizing affected versions of postcss-selector-parser must prioritize upgrading to version 6.1.3 or 7.1.3 to mitigate this vulnerability, as the vendor has acknowledged that the patch has been backported to the 6.x branch, which represents the most widely downloaded version of the library. The vendor's risk assessment, categorizing server-side DoS on user-generated CSS as low risk, appears to underestimate the potential impact given that this vulnerability can be exploited remotely and may already be in active use by malicious actors. The remediation strategy should include not only upgrading the library but also implementing input validation measures to further reduce attack surface. The fact that this vulnerability affects the most commonly used version of the library makes it particularly concerning, as it suggests that a significant portion of the PostCSS ecosystem is potentially exposed to this recursive processing flaw. Security teams should monitor for exploitation attempts and consider implementing network-level protections against malicious CSS inputs, particularly in applications that process user-generated content through PostCSS.