CVE-2026-49411: Deno Permission Bypass via Numeric IP Aliases in Node.js Compatibility Layer

Technical summary

The vulnerability resides in Deno's Node.js compatibility layer, specifically in how TCP socket connection permissions are evaluated. The flaw stems from a Time-of-Check-Time-of-Use (TOCTOU) logic error: permission validation occurs against the raw hostname string before DNS resolution happens, but no re-validation occurs after the numeric or symbolic hostname resolves to its actual IP address. An unprivileged process with restricted network permissions can craft a connect() call using a numeric IP representation (integer or hexadecimal form) that resolves to a blocked destination. The socket layer honors the pre-resolution permission check and allows the connection. Callers running within Deno's permission sandbox (common in embeddings or untrusted script environments) can escape intended network access controls.

Business impact

Organizations embedding or executing untrusted Deno code in sandbox environments face privilege escalation risk. While the attack requires an already-running process with local code execution capability, it allows that process to bypass network isolation policies that are meant to prevent lateral movement, data exfiltration, or unauthorized service access. In scenarios where Deno serves as a runtime for third-party scripts, plugins, or multi-tenant containerized workloads, this weakness could permit compromised code to reach backend services, databases, or external systems that security policies intended to block.

Affected systems

Deno versions prior to 2.8.0 are affected. The vulnerability is specific to the Node.js compatibility layer, so code using Deno's native network APIs (not the node:net or node:http polyfills) is not impacted. Affected deployments include those relying on Node.js compatibility mode for legacy code or third-party libraries that expect Node.js APIs, particularly where network permissions are configured to restrict access to specific hosts or ranges.

Exploitability

The attack requires local execution within a Deno process already subject to permission restrictions—not a remote network vulnerability. Exploitation is straightforward once code execution is achieved: simply call node:net.connect() or node:http.request() with a numeric IP representation (decimal, hexadecimal, or octal) instead of a hostname. The numeric form bypasses the intended hostname-based access control. No special timing, race conditions, or user interaction is required. However, the practical impact depends on the configuration of network permissions in the target Deno environment and whether the process actually runs with such restrictions enforced.

Remediation

Upgrade Deno to version 2.8.0 or later. This patch adds post-resolution permission re-validation, ensuring that regardless of how a hostname is represented, the resolved IP address is checked against the permission policy before the connection is allowed. Organizations should prioritize updates for any Deno instances executing untrusted code or running in multi-tenant environments where network isolation is a security boundary.

Patch guidance

Apply Deno 2.8.0 or any subsequent version. Verify against the official Deno release notes and advisory to confirm the fix is included in your deployment. If using Deno in containerized or embedded scenarios, ensure container/embedding host tooling pulls the patched version and does not cache older builds. Test network permission policies after upgrade to confirm they function as intended.

Detection guidance

Review Deno process invocations and scripts for use of numeric IP addresses in node:net.connect() or node:http.request() calls, especially in conjunction with configured network permission restrictions. Monitor process logs or audit trails for connection attempts using unexpected numeric address formats. In containerized environments, verify the Deno version running in images and confirm it matches 2.8.0 or later. If source code is available, audit permission configuration and check whether post-resolution validation is expected but absent in older versions.

Why prioritize this

While the CVSS score is MEDIUM (6.5), the effective risk in your environment depends on two factors: whether you run untrusted or third-party Deno code, and whether network permissions are meaningfully configured. Patches are straightforward and low-risk. Prioritize for any instance running code from untrusted sources or acting as a multi-tenant runtime; deprioritize if Deno executes only trusted internal code with minimal network restrictions.

Risk score, explained

The CVSS 3.1 score of 6.5 (MEDIUM) reflects a local attack vector (AV:L), low attack complexity (AC:L), low privilege requirement (PR:L), no user interaction (UI:N), cross-system scope (S:C), high confidentiality impact (C:H), no integrity or availability impact (I:N/A:N). The confidentiality impact stems from the ability to reach and query blocked services; the local/privileged requirement limits the attack surface compared to remote exploits. The score appropriately reflects a sandbox escape scenario rather than a critical vulnerability.

Frequently asked questions