mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding (CVE-2026-81092) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-81092

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

Answer in brief

CVE-2026-81092 records a Unknown severity vulnerability in mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding. The current sources do not mark it as known exploited. The current feed maps mark3labs/mcp-go (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Aug 27, 2026Updated Aug 27, 2026Source checked Aug 27, 2026First seen by HOL Aug 27, 2026Material review Aug 27, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps mark3labs/mcp-go (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
mark3labs/mcp-gogeneric	>=0 <0.56.0	0.56.0

Recommended response

Check inventory. Check lockfiles and deployed manifests for mark3labs/mcp-go.
Review the reported fix. Update mark3labs/mcp-go to 0.56.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Aug 27, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Aug 27, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Aug 27, 2026

Sources and claim methodology

Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.