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Public proof of conceptFix available

CVE-2026-42559

A public proof of concept exists; no exploitation attested.

  • Published2026-05-14
  • Fixed in1.4.0
  • WeaknessCWE-346 · CWE-350

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

RMCP is an official Rust SDK for the Model Context Protocol. Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface. This vulnerability is fixed in 1.4.0.

Timeline

dates in UTC
  1. 2026-05-1415:16CVE publishedNVD
  2. 2026-10-2319:23First public proof of conceptPoC-in-GitHub

Versions per branch

OSV
rmcp
BranchAffectedFixed in
1.4≥ 0.0.0-0, < 1.4.01.4.0
1.4< 1.4.01.4.0
dynoxide
BranchAffectedFixed in
0.9≥ 0.9.3, < 0.9.130.9.13
dynoxide-rs
BranchAffectedFixed in
0.9≥ 0.9.3, < 0.9.130.9.13

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Products declared by the CNA1 product

As GitHub_M, the CNA that assigned this CVE, declared them in the CVE record; versions as written.

  • modelcontextprotocol rust-sdk
    Affected
    • = < 1.4.0no fix stated

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-05-14

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • nushellWolfiall versions · fixed in 0.112.2-r3
  • nushellChainguardall versions · fixed in 0.112.2-r3
  • sglang-cuda-13.0Chainguardall versions · fixed in 0
  • zedWolfiall versions · fixed in 1.2.6-r1
  • zedChainguardall versions · fixed in 1.2.6-r1

Full record on NVD

Fix

  1. Update rmcp to 1.4.0

Detection

read 2026-11-01 01:02 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • NVDCVE recordread 2026-10-27
  • CISA-ADPSSVC decisionnone2026-05-14
  • CNACVSS 3.18.8known since 2026-10-27
  • CNAweakness (CWE)CWE-346, CWE-350known since 2026-10-27
  • FIRST EPSSEPSS score0.3 %2026-11-08
  • PoC-in-GitHubproof of concept1known since 2026-11-01
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryHIGH2026-05-18

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Published
2026-05-14
Last Modified
2026-10-26
NVD status
Deferred
GitHub Advisory
GHSA-89vp-x53w-74fx GHSA-fvh2-gm75-j4j7

Frequently asked questions

What is CVE-2026-42559?

RMCP is an official Rust SDK for the Model Context Protocol. Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface. This vulnerability is fixed in 1.4.0.

Is CVE-2026-42559 actively exploited?

Active exploitation of CVE-2026-42559 has not been confirmed. Its EPSS score was 0.3% on 2026-11-08, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-42559?

CVE-2026-42559 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

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