CVE-2026-49356: Babel @babel/core Arbitrary Source Map File Read Vulnerability

Technical summary

The vulnerability exists in @babel/core versions prior to 8.0.0-rc.6 and 7.29.6. During compilation, Babel processes sourceMappingURL comments in JavaScript files. An attacker can inject a malicious sourceMappingURL that references an arbitrary file path on the system. If Babel attempts to resolve and read this file as a source map, an attacker with access to the compiler output can infer file contents through observable differences in behavior or error messages. The issue is rooted in insufficient validation of sourceMappingURL paths before file access (CWE-22: Improper Limitation of a Pathname to a Restricted Directory, also known as path traversal) and uncontrolled information disclosure (CWE-200).

Business impact

Organizations using Babel in build pipelines or development environments may inadvertently expose source map contents if untrusted code is compiled. Source maps typically contain unminified source code, variable names, and file structure information that can aid reverse engineering. The risk is primarily relevant in scenarios where external or partially-trusted code is processed through Babel during CI/CD pipelines, though the practical exploitation barrier is high due to the preconditions required.

Affected systems

Babel @babel/core versions before 8.0.0-rc.6 and before 7.29.6 are affected. Any development environment, build system, or continuous integration pipeline that uses these versions to compile JavaScript is in scope. Organizations should inventory their use of Babel across all projects, including transitive dependencies, as Babel is a foundational tool in many JavaScript toolchains.

Exploitability

Exploitation requires three simultaneous conditions: the attacker must control the input source code being compiled, must be able to observe or exfiltrate the compilation output, and must know the exact file system path of the source map to target. This combination of constraints significantly limits real-world exploitation. The vulnerability does not allow unauthenticated remote exploitation and requires local system access or prior compromise of the code supply chain.

Remediation

Update @babel/core to version 8.0.0-rc.6 or later (for the 8.x line) or 7.29.6 or later (for the 7.x line). Verify against the official Babel release notes to confirm patch versions. After patching, rebuild all artifacts to ensure Babel processes code with the fixed compiler.

Patch guidance

Verify your current Babel version using npm list @babel/core or yarn list @babel/core. For npm-based projects, update using npm install --save-dev @babel/core@latest or npm install --save-dev @babel/[email protected] (for v7 projects). For yarn, use yarn upgrade @babel/core. Test your build pipeline thoroughly after updating to ensure compatibility with your project configuration. If you maintain a monorepo or manage multiple projects, prioritize those that compile untrusted or external code.

Detection guidance

Review your package-lock.json or yarn.lock files to identify @babel/core versions in use. Automated dependency scanning tools (npm audit, yarn audit, Snyk) will flag outdated versions. In your build logs, check for any sourceMappingURL resolution errors or unexpected file access attempts during Babel compilation. If you have security monitoring in place, look for file access patterns where Babel reads files outside the expected source directory.

Why prioritize this

While Babel is a ubiquitous tool in the JavaScript ecosystem, the low CVSS score (3.2) reflects the high exploitation friction. Prioritize patching if your organization compiles externally-sourced or user-supplied code, or if source maps contain sensitive intellectual property. For typical projects that only compile internal, trusted code, this is a medium-priority update suitable for the next regular maintenance cycle.

Risk score, explained

The CVSS 3.1 score of 3.2 (LOW) reflects a local attack vector (AV:L), high complexity (AC:H) requiring multiple conditions to align, no privileges needed (PR:N), unchanged scope (S:C), and only confidentiality impact (C:L). The high complexity factor is the primary driver of the low score—exploitation requires attacker control over input, visibility of output, and advance knowledge of file paths, making opportunistic exploitation impractical.

Frequently asked questions