CVE-2026-49406: Deno Path Traversal in BYONM Module Resolution

Technical summary

The vulnerability stems from insufficient path validation in Deno's module resolution logic when BYONM mode is enabled (nodeModulesDir: "manual"). When resolving a package's entrypoint, the resolver accepts the value from the package.json main field without verifying that the resolved path remains within the package's own node_modules/<pkg>/ directory. An attacker can include parent directory references (..) in the main field to traverse outside the package directory and point to arbitrary files on the filesystem. The resolver then reads the target file directly, bypassing the --allow-read permission allowlist that would normally govern file access. This creates a privilege escalation where code execution within a require() context gains unintended file access. The fix in 2.7.12 adds proper path normalization and validation to ensure resolved entrypoints cannot escape their designated package directories.

Business impact

For organizations relying on Deno in production, particularly in multi-tenant or sandboxed environments, this vulnerability undermines the security model. The permission allowlist system—a core trust boundary in Deno—can be bypassed, potentially exposing sensitive files (configuration, credentials, source code) to packages that should have limited access. In development environments where BYONM mode is used, the risk extends to supply chain attacks: a compromised or malicious package dependency can silently exfiltrate data that developers believed was protected. Teams shipping Deno applications must assume that any transitive dependency could leverage this flaw to read files outside its intended scope.

Affected systems

Deno versions prior to 2.7.12 are affected when explicitly operating in BYONM mode (nodeModulesDir: "manual" in deno.json or equivalent configuration). Systems using Deno's default module resolution strategy are not impacted. The vulnerability requires the use of a malicious or compromised npm package with a crafted package.json, making it relevant to any Deno project that consumes npm dependencies via node_modules.

Exploitability

Exploitability is straightforward: an attacker must publish or compromise an npm package and manipulate its package.json main field to include path traversal sequences. The attack requires local code execution (require() of the malicious package) and is not remotely exploitable on its own. However, the low barrier to entry—simply editing a package.json field—combined with the commonality of transitive dependencies means the risk is practically significant in supply chain scenarios. No user interaction is needed beyond the normal act of installing and using a package.

Remediation

Upgrade Deno to version 2.7.12 or later. Organizations should audit their Deno deployments to identify which systems are using BYONM mode and prioritize updates accordingly. As an interim mitigation (if immediate upgrading is not feasible), restrict the packages installed in node_modules to trusted sources only, implement dependency scanning to detect suspicious package.json configurations, and consider using lock files to pin exact versions and prevent unexpected package updates.

Patch guidance

Apply the update to Deno 2.7.12 as soon as feasible. The patch includes corrected path validation logic that normalizes and validates entrypoint paths before resolution. Verify the update has been applied by checking the Deno version (deno --version should report 2.7.12 or later). Test your existing Deno applications post-upgrade to ensure no regressions in module resolution behavior, particularly if your codebase has non-standard require() patterns.

Detection guidance

Audit your deno.json or deno.jsonc configuration files for the presence of nodeModulesDir: "manual" to identify systems in BYONM mode. Review installed npm packages for suspicious package.json main fields containing .. or unusual path patterns—tooling like npm audit can help, though focused inspection may be necessary. Monitor file access patterns in Deno processes; unexpected reads of sensitive files outside expected directories could indicate exploitation. Use software composition analysis tools to track Deno versions across your infrastructure and flag instances below 2.7.12.

Why prioritize this

Although classified as MEDIUM severity, this vulnerability merits prompt attention because it directly undermines a foundational security mechanism (the permission allowlist) that developers rely on to contain untrusted code. The barrier to exploitation is low, and the impact—unauthorized file disclosure—can compromise secrets and intellectual property. Prioritize patching systems that use BYONM mode and consume external npm packages, particularly in supply chain-sensitive contexts.

Risk score, explained

The CVSS 3.1 score of 5.5 (MEDIUM) reflects local attack vector, low privileges required, low complexity, and a confidentiality impact with no integrity or availability damage. The score appropriately captures that the vulnerability requires local code execution and delivers information disclosure rather than system compromise. However, the score does not fully account for the privilege escalation aspect (bypassing intended access controls) or the risk amplification in supply chain scenarios, so security teams should consider context-specific factors when prioritizing.

Frequently asked questions