CVE-2026-69153 in PostCSS
Summary
by MITRE • 08/03/2026
PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. Prior to 8.5.19, if from is unset, an attacker can cause PreviousMap.loadFile() to read an unintended source-map file by supplying an absolute or directory-traversal sourceMappingURL. The resulting map’s sources and sourcesContent may then be exposed to the application. This issue is fixed in version 8.5.19.
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Analysis
by VulDB Data Team • 08/03/2026
PostCSS serves as a powerful tool for processing CSS files through its ability to transform stylesheets into Abstract Syntax Trees enabling sophisticated analysis and modification operations. The vulnerability exists within the PreviousMap.loadFile() method which handles source map loading functionality when the from parameter is not explicitly set during processing. This creates a critical security gap where malicious actors can exploit directory traversal mechanisms by crafting specially formatted sourceMappingURL directives containing absolute paths or traversal sequences such as ../../etc/passwd. When PostCSS processes these crafted source maps, it inadvertently loads unintended files from the filesystem rather than the intended source map location. The exploitation occurs because the system fails to properly validate or sanitize file paths before attempting to load source map files, allowing attackers to access sensitive data that may be included in the loaded source maps. This vulnerability directly relates to CWE-22 known as "Improper Limitation of a Pathname to a Restricted Directory" and represents a classic path traversal attack vector within a build tool context.
The operational impact of this vulnerability extends beyond simple information disclosure as it can potentially expose sensitive source code components, configuration files, or other assets that might be included in source maps. When attackers successfully load unintended source map files, they gain access to the sources and sourcesContent properties which may contain proprietary code fragments, development artifacts, or other confidential information. The vulnerability affects all versions prior to 8.5.19 and represents a significant risk for applications that process untrusted CSS input through PostCSS, particularly in environments where source maps might be generated from user-supplied content or when the tool processes files from multiple sources without proper path validation. This issue demonstrates how seemingly innocuous build tool functionality can create security exposure points when proper input sanitization is not implemented.
The fix implemented in version 8.5.19 addresses this vulnerability by introducing proper validation and sanitization of file paths within the PreviousMap.loadFile() method to prevent directory traversal attacks. Security practitioners should ensure all PostCSS installations are updated to version 8.5.19 or later to mitigate this risk. Organizations using PostCSS in their build pipelines should also implement additional controls such as input validation for CSS files, monitoring for suspicious source map references, and ensuring that source maps are not generated from untrusted inputs. The vulnerability aligns with ATT&CK technique T1211 which involves the exploitation of vulnerabilities in software components to gain access to sensitive information, and represents a common pattern where path traversal flaws in build tools can be leveraged for information disclosure attacks against development environments.
This vulnerability serves as a reminder of the importance of proper input validation in all aspects of software development tooling, particularly when dealing with file system operations. The exposure of source map content through directory traversal represents a significant privacy and security concern for organizations that rely on PostCSS for their CSS processing workflows. Security teams should conduct audits of their build environments to ensure no other tools within their pipeline may be vulnerable to similar path traversal attacks, as the exploitation of such vulnerabilities can provide attackers with valuable reconnaissance data about application internals and development practices.