Vulnerability · Published Jul 29, 2026 · Assigned by GitHub_M

CVE-2026-67431MCP Ruby SDK: Ruby SSE Session Poisoning

High 8.3

In plain English

A high-severity improper access control in ruby-sdk from modelcontextprotocol. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 30% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)8.3High

Exploit odds (30 days)0.4%top 69.7% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. This issue is fixed in version 0.23.0.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
RubyGems	mcp	0.23.0	GHSA-5p9g-j988-pcwv

Affected products (as reported)

Vendor	Product	Versions
modelcontextprotocol	ruby-sdk	< 0.23.0

Weakness: CWE-284 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N

References

https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv x_refsource_CONFIRM
https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356 x_refsource_MISC
https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0 x_refsource_MISC