5.2

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-49860

CVE-2026-49860: Deno: WebSocket API sandbox bypass via missing post-DNS check

Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.8.1, when a WebSocket connection was opened, Deno checked the destination hostname against --deny-net rules but did not re-check the IP addresses that hostname resolved to. An attacker-controlled script could use a specially crafted domain name that passes the hostname check yet resolves to a denied IP, bypassing the network restriction entirely. This vulnerability is fixed in 2.8.1.

(GitHub Advisory)

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CVE-2026-49860

CVE-2026-49860:

5.2

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation NoneAutomatable NoTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
deno	rust	<= 2.8.0	2.8.1

Technical Details

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Miggo AI

Root Cause Analysis

The vulnerability is a Server-Side Request Forgery (SSRF) caused by a Time-of-check to time-of-use (TOCTOU) flaw in Deno's network permission enforcement for the WebSocket API. When a script initiated a WebSocket connection, Deno would check the provided hostname against the --deny-net list. However, it failed to perform a second check on the IP address(es) that the hostname resolved to.

An attacker could exploit this by using a specially crafted domain name that is permitted by the --deny-net rules but resolves to a denied IP address, such as 127.0.0.1 or an internal service. This would bypass the intended network sandbox.

The patch addresses this by introducing a check on the resolved IP addresses. This is achieved by:

Plumbing the PermissionsContainer down to the DNS resolver used by fetch() and WebSocket connections.
In the DNS resolver (deno::ext::fetch::dns::Resolver::call), after the hostname is resolved, a new function check_resolved_permissions is called to validate each resulting IP address against the --deny-net list.
Introducing a PermissionedHttpConnector to ensure that the correct destination port is available to the resolver, making port-specific deny rules effective.

The vulnerable functions are those in the connection setup path that failed to ensure this post-resolution check was performed, primarily deno::ext::websocket::op_ws_create and the underlying DNS resolver deno::ext::fetch::dns::Resolver::call.

Vulnerable functions

deno::ext::fetch::dns::Resolver::call

ext/fetch/dns.rs

This function is responsible for resolving a hostname to IP addresses. The vulnerability lies in its original implementation, which did not perform any security checks on the resolved IP addresses. An attacker could use a hostname that passes the initial `--deny-net` check but resolves to a denied IP address (like localhost), thus bypassing the sandbox. The patch adds a call to `check_resolved_permissions` to verify the resolved IPs against the deny list.

deno::ext::websocket::op_ws_create

ext/websocket/lib.rs

This is the runtime operation that initiates a WebSocket connection from JavaScript. It was vulnerable because it failed to propagate the necessary security permissions to the underlying client creation logic, particularly when custom certificate options were used. This resulted in the DNS resolver not performing the required IP address check. An exploit triggering this vulnerability would have this function in its stack trace.

deno::ext::websocket::create_client_from_websocket_options

ext/websocket/lib.rs

This function creates the HTTP client used for WebSocket connections when custom options like `ca_certs` are provided. It was vulnerable because it configured the client's DNS resolver without attaching the `PermissionsContainer`. This omission meant the crucial post-DNS-resolution check against the `--deny-net` list was skipped, allowing the sandbox bypass.

deno::ext::fetch::create_http_client

ext/fetch/lib.rs

This function constructs the HTTP client for `fetch()` and standard WebSocket connections. The vulnerability was present because it did not wrap the standard HTTP connector with the `PermissionedHttpConnector`. This wrapper is essential for passing the correct destination port to the DNS resolver, ensuring that port-specific `--deny-net` rules are enforced after DNS resolution. Without it, even if the IP was checked, a port-specific deny rule could be bypassed.

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WAF Rule

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Reasoning

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