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CVE-2026-64684— RMCP: Custom HTTP headers leak to cross-origin redirect targets

Quick assessment

Affected
modelcontextprotocol rust-sdk
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.1.0, the rmcp crate's StreamableHttpClientTransport in crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs builds its default_http_client with reqwest's automatic redirect p

CVSS 6.8 · Medium

Possible ATT&CK Techniques 1 AI

T1040 · Network Sniffing
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I. Basic Information for CVE-2026-64684

Vulnerability Information
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Vulnerability Title
RMCP: Custom HTTP headers leak to cross-origin redirect targets
Source: CVE Program / CVE List V5
Vulnerability Description
RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.1.0, the rmcp crate's StreamableHttpClientTransport in crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs builds its default_http_client with reqwest's automatic redirect policy and applies caller-supplied values from StreamableHttpClientTransportConfig.custom_headers without marking them as sensitive. When a malicious or compromised MCP endpoint returns a cross-origin 307 or 308 redirect, reqwest follows the redirect and apply_custom_headers causes custom API keys or authentication tokens to be replayed to the new origin, where an attacker can capture and reuse them. The separate auth_header path is not affected because it uses the standard Authorization header, which reqwest strips on cross-origin redirects. This issue is fixed in version 2.1.0.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
信息暴露
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
modelcontextprotocol rust-sdk < 2.1.0 -

II. Public POCs for CVE-2026-64684

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III. Intelligence Information for CVE-2026-64684

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Patches & Fixes for CVE-2026-64684 (2)

Other References for CVE-2026-64684 (2)

Same Patch Batch · modelcontextprotocol · 2026-09-16 · 3 CVEs total

CVE-2026-63127 8.2 HIGH RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery
CVE-2026-63128 7.5 HIGH RMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transpor

IV. Related Vulnerabilities

Same product: rust-sdk
Same vendor: modelcontextprotocol
Same weakness: CWE-200

V. Comments for CVE-2026-64684

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