CVE-2026-49859: Deno Network Access Control Bypass via DNS Resolution

Technical summary

The vulnerability stems from incomplete DNS resolution validation in Deno's fetch() implementation. The runtime enforced --deny-net rules at the hostname layer during the initial permission check, but did not re-validate against the resolved IP addresses before establishing the connection. This creates a time-of-check-time-of-use (TOCTTOU) style gap where an attacker-controlled script can specify a domain that passes hostname-level validation but resolves to an IP in the denied network range. The issue is classified under CWE-693 (Protection Mechanism Failure) and CWE-918 (Server-Side Request Forgery), reflecting both the failed access control mechanism and the SSRF implications when combined with script execution.

Business impact

For organizations running Deno in sandboxed or multi-tenant environments, this vulnerability undermines a core security boundary. If administrators rely on --deny-net to prevent scripts from accessing internal services, databases, or sensitive network segments, a compromised or malicious script can circumvent these restrictions entirely. The impact is particularly acute in CI/CD pipelines, containerized deployments, and shared hosting scenarios where network isolation is a control layer. Remediation requires upgrading to 2.8.1 before the vulnerability can be considered closed in the environment.

Affected systems

Deno versions prior to 2.8.1 are affected. The vulnerability applies to any deployment where Deno scripts are executed with --deny-net permission restrictions intended to limit outbound network access. This includes development machines, build agents, cloud functions, and any environment where untrusted or potentially compromised Deno code runs under network access controls.

Exploitability

Exploitability is straightforward for an attacker with the ability to supply or influence Deno code execution. The attack requires only a specially crafted domain name and network access to resolve it; no additional complexity or privileged position is needed beyond script execution. However, the vulnerability is local in nature (AV:L per CVSS) and requires the attacker to have script execution capability (PR:L), meaning it is not remotely exploitable on its own. Once code execution is achieved, bypassing network restrictions is trivial.

Remediation

Upgrade Deno to version 2.8.1 or later. The patch corrects the fetch() implementation to re-validate resolved IP addresses against the --deny-net ruleset before establishing connections. Organizations should inventory all Deno deployments currently running versions prior to 2.8.1 and prioritize upgrades, particularly in environments where script execution from external or untrusted sources is possible.

Patch guidance

Update Deno to 2.8.1 or a later stable release. Verify the version post-update by running 'deno --version' and confirm that the fetch() behavior no longer permits hostname-to-denied-IP bypasses. If you maintain custom Deno builds, ensure the DNS validation fix is included in your compilation. For containerized deployments, update base images and rebuild containers accordingly. For development environments, update the Deno toolchain through your package manager or by downloading the latest release from the official Deno repository.

Detection guidance

Monitor Deno runtime versions in use across your infrastructure using software inventory tools or container scanning. Look for deno binary versions prior to 2.8.1. Additionally, if you run Deno with network monitoring enabled, examine fetch() calls for patterns where hostnames pass your deny-net rules but resolve to IP addresses you intended to block. Correlate DNS query logs with network flow data to identify anomalous resolution patterns that might indicate exploitation attempts. Runtime observability tools that log hostname-to-IP mappings during fetch() calls can help detect suspicious behavior.

Why prioritize this

Although the CVSS score is 5.2 (Medium), the vulnerability directly defeats a security boundary in environments where Deno's network restrictions are a control layer. Organizations using --deny-net to isolate scripts from internal networks should treat this as higher priority than the base score suggests. If your threat model includes compromised or malicious scripts, or if you rely on Deno's permission model for defense-in-depth, prioritize this upgrade. For general-purpose Deno users without strict network isolation requirements, the patch should still be applied during normal update cycles.

Risk score, explained

CVSS 5.2 reflects a local attack vector, low privilege requirement, and consequential but not catastrophic impact (partial confidentiality and integrity loss, no availability impact). The score appropriately captures the fact that an attacker must already have script execution capability. However, context matters: in environments where Deno's --deny-net is a primary control, the practical risk is higher. The CWE classifications (CWE-693 and CWE-918) underscore that this is an access control failure with SSRF characteristics, not a memory corruption or arbitrary code execution vulnerability.

Frequently asked questions