CVE-2026-49860: Deno WebSocket Network Bypass Vulnerability (v2.8.1 Fix)

Technical summary

The vulnerability stems from incomplete validation logic in Deno's WebSocket connection handler. The runtime performs hostname-based filtering against --deny-net policies but does not implement secondary validation of resolved IP addresses. An attacker-controlled Deno script can exploit this by leveraging DNS resolution to route connections to restricted IPs. The vulnerability exists in CWE-918 (Server-Side Request Forgery), where DNS resolution becomes an attack vector for bypassing network boundaries. The issue is resolved in Deno 2.8.1 through implementation of post-resolution IP address validation.

Business impact

Organizations using Deno for sandboxed script execution or multi-tenant environments face a meaningful security degradation. Network isolation boundaries—a primary defense mechanism in restricted execution contexts—become unreliable for WebSocket traffic. An attacker with script execution rights could potentially exfiltrate data to blocked external servers, access internal services, or probe non-routable IP ranges. This is particularly concerning in CI/CD pipelines, containerized deployments, or environments processing untrusted scripts where network deny rules are relied upon as a containment measure.

Affected systems

Deno runtime versions prior to 2.8.1 are affected. The vulnerability only manifests when --deny-net rules are active and a malicious or compromised script can open WebSocket connections. Systems running Deno 2.8.1 or later are patched. The vulnerability requires local code execution context; remote unauthenticated attack is not possible without prior script injection or supply-chain compromise.

Exploitability

Exploitability requires an attacker to control or inject a Deno script that can execute in an environment where network deny rules are configured. The attack is straightforward once code execution is achieved—no complex techniques or race conditions are needed. However, the attack surface is limited to scenarios where Deno is used as a sandboxed runtime with explicit network restrictions. Exploitation does not require user interaction and succeeds reliably against vulnerable versions. This is not currently tracked in CISA's Known Exploited Vulnerabilities catalog.

Remediation

Update Deno to version 2.8.1 or later, which includes corrected IP address validation for WebSocket connections. Organizations should verify their Deno deployment version and apply the patch during the next maintenance window. Until patching is possible, restrict Deno script execution to trusted sources only and implement network-level controls (firewalls, network policies) as defense-in-depth measures alongside Deno's built-in deny rules.

Patch guidance

Deno 2.8.1 addresses this issue. Users should review release notes at the official Deno repository to confirm fix inclusion and any breaking changes. Apply the update to all systems running vulnerable versions. Test patched deployments with existing --deny-net configurations to ensure policies behave as expected post-update. If Deno is managed through a container image or package manager, ensure the updated version is pulled before redeployment.

Detection guidance

Monitor Deno process startup for --deny-net flag usage; environments without active deny rules are not vulnerable. Review script audit logs or source control for WebSocket connection attempts to unusual destinations. Network monitoring can detect outbound WebSocket connections to unexpected IP ranges, though this may require deep packet inspection. Consider querying running Deno instances for version information (deno --version) to identify systems still on pre-2.8.1 releases. SIEM rules should flag WebSocket operations combined with DNS lookups that resolve hostnames to private or restricted IP ranges within monitored Deno processes.

Why prioritize this

This vulnerability merits prompt but non-emergency attention. The CVSS score of 5.2 (MEDIUM) reflects limited severity due to local execution requirements and the specificity of the attack context (active --deny-net rules). However, organizations with strict network isolation policies or those using Deno in sandbox execution models should prioritize patching. The attack is reliable and requires minimal attacker effort once code execution is present, making prevention (timely patching) preferable to detection in security-critical environments.

Risk score, explained

The CVSS 3.1 score of 5.2 is driven by: Attack Vector (Local—code execution required), Attack Complexity (Low—straightforward bypass once inside), Privileges Required (Low—attacker needs local script execution rights), User Interaction (None), Scope (Changed—impacts the broader Deno process and potentially the system), Confidentiality (Low), Integrity (Low), and Availability (None). The score reflects that while the vulnerability enables real harm, exploitation requires pre-existing code execution and targeting of specific network configurations, limiting widespread risk. Organizations with high-sensitivity network controls should treat this as higher-priority than the base score suggests.

Frequently asked questions