RMCP: Custom HTTP headers leak to cross-origin redirect targets
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- High
- Impact type
- Information disclosure
- Confidentiality · integrity · availability
- High · None · None
- Fix status
- Partial Red Hat: 2 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-200.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| MLflow | rust-sdk | before 2.1.0 | 2.1.0 | Patch available | CNA |
| — | rmcp (crates.io) | before 2.1.0 | 2.1.0 | Patch available | GHSA-9g45-5xwm-f3wc |
- rust-sdk VendorMLflow Affected before 2.1.0 Fixed in 2.1.0 Patch available CNA
- rmcp (crates.io) Vendor— Affected before 2.1.0 Fixed in 2.1.0 Patch available GHSA-9g45-5xwm-f3wc
Remediation
- Fix · CNA rust-sdk (modelcontextprotocol) Fixed in 2.1.0
- Fix · GHSA-9g45-5xwm-f3wc rmcp (crates.io) Fixed in 2.1.0 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Red Hat Enterprise Linux 10 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 9 | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2026-64684
- 2026-10-03 Alias added: GHSA-9g45-5xwm-f3wc
References
Other 5
- https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9g45-5xwm-f3wc
- https://github.com/modelcontextprotocol/rust-sdk/pull/936
- https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b
- https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0
- https://github.com/modelcontextprotocol/rust-sdk
More MLflow vulnerabilities
- TI-2026-46366 Server-side request forgery in mlflow KEVCVSS 3.1CRITICAL 9.3
- TI-2023-03518 Absolute Path Traversal in mlflow/mlflow CVSS 3.1CRITICAL 10.0
- TI-2023-01113 Path Traversal: '\..\filename' in mlflow/mlflow CVSS 3.1CRITICAL 9.8
- TI-2025-31759 Redis Lua Use-After-Free may lead to remote code execution CVSS 3.1CRITICAL 10.0
- TI-2026-81527 Code execution via scorer deserialization in MLflow servers CVSS 3.1CRITICAL 9.8
- TI-2025-14028 Command Injection in mlflow/mlflow CVSS 3.1CRITICAL 10.0
Sources
- CVE-2026-64684 · CVE Program
- CVE-2026-64684 · NVD
- FIRST EPSS
- GHSA-9g45-5xwm-f3wc · GitHub Advisory Database · CC BY 4.0
- CVE-2026-64684 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences