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fix(vmcp): reject Authorization and Cookie in passthroughHeaders, as documented #6235

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Merged
jhrozek merged 1 commit into from
Aug 11, 2026
+26 −1
Merged

fix(vmcp): reject Authorization and Cookie in passthroughHeaders, as documented #6235

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12 changes: 11 additions & 1 deletion pkg/vmcp/config/validator.go
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Original file line number Diff line number Diff line change
Expand Up @@ -591,6 +591,16 @@ func (*DefaultValidator) validateCompositeToolRefs(refs []CompositeToolRef) erro
return nil
}

// The standalone header-forward middleware deliberately allows Authorization
// (an operator may legitimately forward it), but for vMCP passthrough the
// documented contract is stricter: Authorization and Cookie are rejected at
// startup because forwarding caller-supplied credentials verbatim to every
// backend is a credential-leak footgun, not a pass-through use case.
var vmcpRestrictedHeaders = map[string]bool{
"Authorization": true,
"Cookie": true,
}

func (*DefaultValidator) validatePassthroughHeaders(cfg *Config) error {
for i, name := range cfg.PassthroughHeaders {
if name == "" {
Expand All @@ -599,7 +609,7 @@ func (*DefaultValidator) validatePassthroughHeaders(cfg *Config) error {

canonical := http.CanonicalHeaderKey(name)

if middleware.RestrictedHeaders[canonical] {
if middleware.RestrictedHeaders[canonical] || vmcpRestrictedHeaders[canonical] {
return fmt.Errorf("passthroughHeaders[%d]: %q is a restricted header and cannot be forwarded", i, canonical)
}

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15 changes: 15 additions & 0 deletions pkg/vmcp/config/validator_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1495,6 +1495,21 @@ func TestValidator_ValidatePassthroughHeaders(t *testing.T) {
wantErr: true,
errMsg: "X-Forwarded-For",
},
{
// Documented contract (virtualmcpserver-api.md): Authorization is
// rejected at startup. Forwarding caller-supplied credentials
// verbatim to every backend is a credential-leak footgun.
name: "Authorization is restricted",
headers: []string{"authorization"},
wantErr: true,
errMsg: "Authorization",
},
{
name: "Cookie is restricted",
headers: []string{"Cookie"},
wantErr: true,
errMsg: "Cookie",
},
{
name: "empty string header name is rejected",
headers: []string{""},
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