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Merged
SamMorrowDrums merged 3 commits into from
Sep 1, 2026
Merged

fix(http): allow projected MCP headers in preflights #3167

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a160278
fix(http): allow projected MCP headers in preflights
SamMorrowDrums Aug 27, 2026
c6be6e0
chore: regenerate license files
github-actions[bot] Aug 27, 2026
7ddfad7
fix(http): bound projected preflight headers
SamMorrowDrums Sep 1, 2026
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fix(http): bound projected preflight headers 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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@SamMorrowDrums
SamMorrowDrums and Copilot committed Sep 1, 2026
commit 7ddfad7295b5074ec748b350a5a2029b6ba5e14e
8 changes: 7 additions & 1 deletion pkg/http/middleware/cors.go
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,8 @@ import (
"golang.org/x/net/http/httpguts"
)

const maxCORSProjectedRequestHeaders = 64

var corsAllowedRequestHeaders = []string{
headers.ContentTypeHeader,
"Mcp-Session-Id",
Expand Down Expand Up @@ -62,7 +64,8 @@ func corsPreflightAllowedRequestHeaders(requestHeaders http.Header) string {
}

prefix := strings.ToLower(headers.MCPParamHeaderPrefix)
var projected []string
projected := make([]string, 0, maxCORSProjectedRequestHeaders)
requestedHeaders:
for _, value := range requestHeaders.Values("Access-Control-Request-Headers") {
for header := range strings.SplitSeq(value, ",") {
header = strings.TrimSpace(header)
Expand All @@ -80,6 +83,9 @@ func corsPreflightAllowedRequestHeaders(requestHeaders http.Header) string {

seen[key] = struct{}{}
projected = append(projected, key)
if len(projected) == maxCORSProjectedRequestHeaders {
break requestedHeaders
}
}
}
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33 changes: 33 additions & 0 deletions pkg/http/middleware/cors_test.go
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
package middleware_test

import (
"fmt"
"net/http"
"net/http/httptest"
"strings"
Expand Down Expand Up @@ -77,6 +78,38 @@ func TestSetCorsHeadersPreflight(t *testing.T) {
}
}

func TestSetCorsHeadersPreflightBoundsProjectedHeaders(t *testing.T) {
const (
maxProjectedHeaders = 64
requestedHeaders = 1024
)

requested := make([]string, 0, requestedHeaders*2)
expected := strings.Split(fixedAllowedRequestHeaders, ", ")
for i := range requestedHeaders {
header := fmt.Sprintf("mcp-param-%04d", i)
requested = append(requested, header, strings.ToUpper(header))
if i < maxProjectedHeaders {
expected = append(expected, http.CanonicalHeaderKey(header))
}
}
requestedValue := strings.Join(requested, ", ")

handler := middleware.SetCorsHeaders(http.HandlerFunc(func(http.ResponseWriter, *http.Request) {
t.Fatal("preflight reached the inner handler")
}))
req := httptest.NewRequest(http.MethodOptions, "/", nil)
req.Header.Set("Access-Control-Request-Headers", requestedValue)
rr := httptest.NewRecorder()

handler.ServeHTTP(rr, req)

allowedValue := rr.Header().Get("Access-Control-Allow-Headers")
assert.Equal(t, expected, strings.Split(allowedValue, ", "))
assert.NotContains(t, allowedValue, "Mcp-Param-0064")
assert.Less(t, len(allowedValue), len(requestedValue))
}

func TestSetCorsHeadersPostPreservesBehavior(t *testing.T) {
innerCalled := false
inner := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
Expand Down
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