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Fencer Vulns
medium 6.8

CVE-2026-64684 json

RMCP: Custom HTTP headers leak to cross-origin redirect targets

published 2026-09-16 53 d ago modified 2026-09-23 46 d ago also GHSA-9g45-5xwm-f3wc , EUVD-2026-81266

cvss
6.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 43%
public exploits
0
none known

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.

Patch / fix

  • crates.io rmcp 2.1.0
  • openai-codex openSUSE Tumbleweed 0.155.1-1.1

opensuse advisories openSUSE-SU-2026:11849-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-64684 updated cvss 6.8

rmcp: RMCP: Sensitive HTTP headers leaked during cross-origin redirects

  • Red Hat Enterprise Linux 9 goose, goose 1.38.0-1.el10_2 Red Hat Enterprise Linux 10 goose 1.38.0-1.el10_2
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
51 d ago

Exploit prediction

0.50% chance of exploitation in the next 30 days, higher than 43% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 6.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
GitHub 3.1 6.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

Affected packages

ecosystem package affected fixed in
crates.io rmcp
<2.1.0
2.1.0

Affected products

  • modelcontextprotocol rust-sdk

Classification

Weakness
  • CWE-200 Exposure of Sensitive Information to an Unauthorized Actor

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.