Executive Summary

The vulnerability CVE-2026-49859 affects the Deno runtime's fetch() API. When fetch() was called, Deno checked the destination hostname against --deny-net rules but did not verify the IP addresses that the hostname resolved to. This means an attacker could use a specially crafted domain name that passes the hostname check but resolves to a denied IP address, bypassing the network restrictions entirely.

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Impact Analysis

This vulnerability allows an attacker to bypass network restrictions set by the --deny-net flag in Deno. If you run untrusted or third-party code with network restrictions relying on --deny-net, an attacker-controlled script could access restricted IP addresses by using a domain name that passes hostname checks but resolves to blocked IPs. This could lead to unauthorized network access and potential data exposure or other security risks.

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Detection Guidance

This vulnerability involves a bypass of network restrictions in the Deno runtime's fetch() API due to missing IP address checks after DNS resolution. Detection would involve monitoring or analyzing network requests made by Deno scripts, especially those using fetch() with --deny-net rules.

Since the issue is related to DNS resolution and IP address checks, one approach is to verify if any fetch() calls are reaching IP addresses that should be blocked by --deny-net but are still accessed. This could be done by capturing network traffic or logging DNS resolutions and comparing them against deny-net rules.

No specific detection commands are provided in the available resources. However, general network monitoring tools like tcpdump or Wireshark can be used to capture outgoing connections from Deno processes. Additionally, inspecting Deno logs or running scripts with verbose network output might help identify suspicious connections.

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Mitigation Strategies

The primary and immediate mitigation step is to upgrade the Deno runtime to version 2.8.1 or later, where the vulnerability is fixed.

No workaround exists other than upgrading. Users relying on --deny-net to restrict host access should ensure they are not running vulnerable versions.

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Compliance Impact

The vulnerability allows an attacker-controlled script to bypass network restrictions by resolving a domain name to a denied IP address, potentially enabling unauthorized network access.

Such unauthorized access could lead to data exposure or unauthorized communication, which may impact compliance with standards like GDPR or HIPAA that require strict controls over data access and transmission.

However, the provided information does not explicitly discuss the direct impact on compliance with these regulations.

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