Medium severity · published Jul 29, 2026
CVE-2026-63119
MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)
6.2
CVSS
0.2%
EPSS
No
On KEV
Description
MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StdioTransport and MCP::Client::Stdio in the mcp gem use IO#gets without a byte limit, allowing a peer that sends data without a newline to exhaust process memory. This issue is fixed in version 0.23.0.
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Local
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 8% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
RU
ruby-sdk · modelcontextprotocol
< 0.23.0 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-63119
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-63119
github.com
https://github.com/modelcontextprotocol/ruby-sdk/commit/267b8fa6285453525c81ce43db6b7dcd7a8a8c2f
github.com
https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0
github.com
https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-7683-3w9x-ch42
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