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How CVE-2026-69153 Works
PostCSS is a widely used tool that processes CSS files by transforming them into an Abstract Syntax Tree (AST), enabling developers to analyze and modify CSS rules programmatically. The vulnerability exists in versions prior to 8.5.19 and resides in the `PreviousMap.loadFile()` method, which is responsible for loading source map files referenced within CSS input.
When PostCSS processes a CSS file, it looks for a `sourceMappingURL` comment that points to an associated `.map` file. This mechanism helps developers debug transformed CSS by linking back to the original source files. The `from` option in PostCSS specifies the base path used to resolve relative paths, including those found in source map references. However, if the `from` parameter is left unset or not properly configured, the resolver falls back to a default behavior that can be exploited.
An attacker can craft a malicious CSS file containing a `sourceMappingURL` that uses absolute paths or directory traversal sequences (such as ../). Because the `from` option is missing, PostCSS attempts to resolve this path relative to the current working directory or uses other unsafe resolution logic. This allows the attacker to force `PreviousMap.loadFile()` to read arbitrary files from the server’s filesystem that are outside the intended web root.
The consequences are severe: the contents of the read source map file are parsed, and its `sources` and `sourcesContent` fields become exposed to the application. If the source map references sensitive files (e.g., configuration files, source code, or environment files), the application may inadvertently leak their contents through error messages, logs, or API responses. This constitutes an information disclosure vulnerability that can lead to further compromise.
The issue is fixed in version 8.5.19 by implementing proper path sanitization and enforcing that all source map references are resolved safely relative to a defined base path, with fallback mechanisms that reject malicious traversal attempts.
DailyCVE Form:
Platform: Node.js
Version: <8.5.19
Vulnerability: Path Traversal
Severity: Medium
date: 2026-08-03
Prediction: 2026-08-05
What Undercode Say: Analytics
Check installed PostCSS version npm list postcss Identify vulnerable installations npm list postcss | grep -E "postcss@[0-7].|postcss@8.[0-4].|postcss@8.5.[0-9]|postcss@8.5.[0-1][0-8]" Simulate the attack with a malicious CSS file echo "/ sourceMappingURL=../../../../etc/passwd.map /" > exploit.css Run PostCSS without the 'from' option to trigger the vulnerability npx postcss exploit.css --output output.css 2>&1 | grep -i "map" Check if sensitive files are being accessed via strace (Linux) strace -e openat,open npx postcss exploit.css --output output.css 2>&1 | grep -E "etc/passwd|.map"
// PoC: Malicious CSS content
const maliciousCSS = <code>/ sourceMappingURL=../../../../etc/passwd.map /
.test { color: red; }</code>;
// PoC: Node.js script to trigger the vulnerability
const postcss = require('postcss');
const fs = require('fs');
fs.writeFileSync('poc.css', maliciousCSS);
postcss([]).process(fs.readFileSync('poc.css'), {
// 'from' is intentionally omitted
}).then(result => {
console.log(result.map); // May contain leaked sourcesContent
}).catch(err => console.error(err));
Exploit
An attacker can exploit this vulnerability by serving a crafted CSS file to an application that uses PostCSS without setting the `from` option. The attacker injects a `sourceMappingURL` pointing to an absolute path or a path with directory traversal sequences, such as `../../../../etc/passwd.map` or /root/.ssh/id_rsa.map. When PostCSS processes the file, `PreviousMap.loadFile()` attempts to read the specified file. If the file exists and contains valid source map data, the `sources` and `sourcesContent` fields are extracted and made available to the application, potentially exposing sensitive information through subsequent processing or error messages.
Protection
- Upgrade PostCSS to version 8.5.19 or later immediately.
- If upgrading is not possible, always set the `from` option to a trusted base directory when calling
postcss.process(). - Implement input validation to reject CSS files containing `sourceMappingURL` with absolute paths or `../` sequences.
- Run PostCSS in a sandboxed environment with restricted filesystem access.
- Use a Web Application Firewall (WAF) to block requests containing path traversal patterns in CSS uploads.
Impact
Successful exploitation allows an attacker to read arbitrary files from the server’s filesystem that are accessible to the Node.js process. This can lead to disclosure of source code, configuration files, environment variables, credentials, and other sensitive data. The exposed information may be used to pivot to other attacks, such as privilege escalation, lateral movement, or further compromise of the application and underlying infrastructure.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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