Is CVE-2026-63118 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-63118 being exploited in the wild.

How severe is CVE-2026-63118?

Medium severity, with a CVSS score of 4, published 29 July 2026.

Which products and versions are affected?

mcp (RubyGems) ECOSYSTEM:introduced=0,fixed=0.23.0.

Is there a fix for CVE-2026-63118?

Yes. mcp (RubyGems): upgrade to 0.23.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-63118 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-63118?

Check whether you run mcp (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-63118: modelcontextprotocol ruby-sdk DNS rebinding in StreamableHTTPTransport

CVE-2026-63118 · Severity: medium · CVSS 4 · Published 2026-07-29

The Model Context Protocol (MCP) Ruby SDK contains a security flaw in its HTTP transport component, which is used to build servers that expose local tools and data to AI clients. Because the software fails to verify where incoming web requests are coming from, an attacker can trick a user's browser into connecting to a locally running MCP server. This allows a malicious website to silently execute commands on the user's computer, steal local files, or access sensitive credentials through the server's exposed tools.

Technologies: mcp (RubyGems). Vendors: RubyGems.

Quick answers

Is CVE-2026-63118 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-63118 being exploited in the wild.
How severe is CVE-2026-63118?: Medium severity, with a CVSS score of 4, published 29 July 2026.
Which products and versions are affected?: mcp (RubyGems) ECOSYSTEM:introduced=0,fixed=0.23.0.
Is there a fix for CVE-2026-63118?: Yes. mcp (RubyGems): upgrade to 0.23.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-63118 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-63118?: Check whether you run mcp (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

mcp (RubyGems): upgrade to 0.23.0 or later
```
bundle update mcp
```

Technical details

The `MCP::Server::Transports::StreamableHTTPTransport` class in the `mcp` gem fails to validate the HTTP `Host` and `Origin` headers of incoming JSON-RPC requests. This omission makes local MCP servers vulnerable to DNS-rebinding attacks. An attacker can lure a victim to a malicious webpage that rebinds its hostname to `127.0.0.1`, bypassing the Same-Origin Policy. Since the transport does not implement an allowlist for hosts or origins, the attacker can then issue `tools/list` and `tools/call` requests to the local server, effectively exfiltrating sensitive data or executing local actions. The vulnerability is addressed in version 0.23.0 by introducing a secure-by-default allowlist that restricts access to loopback addresses.

Affected products

modelcontextprotocol mcp <= 0.22.0

Identifiers

CVE-2026-63118
GHSA-rjr6-rcgv-9m7m

Timeline

2026-07-08: advisory: Initial GitHub Advisory published
2026-07-30: patched: Updated with patched version 0.23.0

References

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