Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas
- Published
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- Fix
- Fixed in 1.30.0, 2.2.0
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: none
- Automatable: no
- Technical impact: partial
Summary
The Model Context Protocol Python SDK, used by clients that connect to MCP servers, is affected. A malicious server can make the client fetch arbitrary web or local file addresses while validating tool results, which can leak information from the client's network or read local files and stall other sessions running in the same process.
Summarized by anthropic/claude-haiku-5.5. Verify details against the original sources.
Read the original description
The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.
Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: none · Automatable: no · Technical impact: partial
- Record last updated
Affected versions
- modelcontextprotocol python-sdk
From 1.10.0 · fixed in 1.30.0; From 2.0.0 · fixed in 2.2.0
- mcpPyPI
From 2.0.0 · fixed in 2.2.0
- mcpPyPI
From 1.10.0 · fixed in 1.30.0