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Fencer Vulns
medium 6.8

CVE-2026-81092 json

mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding

published 2026-08-27 2 mo ago modified 2026-09-23 46 d ago also EUVD-2026-67147

cvss
6.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 13%
public exploits
0
none known

Summary

mcp-go accepted requests on its HTTP transports without checking the Host header. StreamableHTTPServer.ServeHTTP in server/streamable_http.go and SSEServer.ServeHTTP in server/sse.go served any request arriving over a loopback connection regardless of the host it named, and the SSE transport's cross-origin default allowed any origin. A page in a browser could therefore point a name it controlled at the loopback address and reach a server listening there, invoking tools and reading resources that the server exposed on the assumption that only local software could connect. No release before 0.56.0 validated the header on either transport; 0.56.0 adds server/http_localhost.go, which rejects a loopback-bound request carrying a host that is not a loopback name, and wires it into both transports.

Patch / fix

  • mark3labs mcp-go 0.56.0
  • tempo3.0 hummingbird 1 3.0.3-0.2.hum1
  • tempo2.10 hummingbird 1 2.10.8-0.2.hum1
  • tempo3.0 hummingbird 1 3.0.3-0.1.hum1

red hat advisories RHSA-2026:62537RHSA-2026:62527RHSA-2026:54874

  • mcphost SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 0.34.0-160000.4.1

suse advisories SUSE-SU-2026:23496-1SUSE-SU-2026:23636-1

  • mcphost openSUSE Leap 16.0 0.34.0-160000.4.1
  • mcphost openSUSE Tumbleweed 0.34.0-10.1

opensuse advisories openSUSE-SU-2026:21771-1openSUSE-SU-2026:11670-1

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-2026-81092 updated cvss 6.8

github.com/mark3labs/mcp-go: mcp-go: DNS Rebinding vulnerability due to missing Host header validation

  • Red Hat Connectivity Link 1 rhcl-tech-preview/mcp-gateway-rhel9, rhcl-tech-preview/mcp-gateway-rhel9-operator Red Hat OpenShift AI (RHOAI) rhoai/rhai-cli-rhel9 Red Hat OpenShift distributed tracing 3 rhosdt/tempo-rhel9, rhosdt/tempo-query-rhel9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images tempo3.0 3.0.3-0.1.hum1, tempo2.10 2.10.8-0.2.hum1
    fixed: vendor fix RHSA-2026:62527

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
2 mo ago

Exploit prediction

0.22% chance of exploitation in the next 30 days, higher than 13% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 7.6 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
CNA 3.1 6.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
CNA 4.0 7.6 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected products

  • mark3labs mcp-go

Classification

Weakness

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.