# UNDF: UNDF-2026-000000825
# go-stdlib-0001 — net/http/internal/http2: rfc9218Priority allocates []string per header field

**Severity:** MEDIUM
**CWE:** CWE-407 (Algorithmic Complexity — Linear Membership Test in Loop)
**File:** `src/net/http/internal/http2/frame.go`
**Function:** `(*MetaHeadersFrame).rfc9218Priority`

## Defect

```go
// frame.go:1655-1662
func (mh *MetaHeadersFrame) rfc9218Priority(priorityAware bool) (p PriorityParam, ...) {
    for _, field := range mh.Fields {           // O(F) — F header fields
        if field.Name == "priority" { ... }
        if slices.Contains([]string{"via", "forwarded", "x-forwarded-for"}, field.Name) {
            // ^^^^ allocates a NEW 3-element []string on EVERY iteration
            hasIntermediary = true
        }
    }
```

On each iteration of the `mh.Fields` loop, `slices.Contains([]string{...}, ...)` creates a
fresh heap-allocated slice literal. For an HTTP/2 server under load, this function is called
once per HEADERS frame (i.e., per request), and the inner allocation fires once per header
field. A request with 30 headers performs 30 needless allocations and 3-element scans.

At high request rates the GC pressure compounds: a server handling 100 k req/s with avg 20
headers/request = 2 M unnecessary allocations per second.

## Fix

Pre-declare the intermediary-header set as a package-level `map[string]bool` (zero allocation
at call time, O(1) lookup):

```go
// package-level, evaluated once at init time
var rfc9218IntermediaryHeaders = map[string]bool{
    "via":             true,
    "forwarded":       true,
    "x-forwarded-for": true,
}

func (mh *MetaHeadersFrame) rfc9218Priority(priorityAware bool) (p PriorityParam, ...) {
    for _, field := range mh.Fields {
        if field.Name == "priority" { ... }
        if rfc9218IntermediaryHeaders[field.Name] {   // O(1), zero alloc
            hasIntermediary = true
        }
    }
```

## Patch

```diff
--- a/src/net/http/internal/http2/frame.go
+++ b/src/net/http/internal/http2/frame.go
@@ -1650,6 +1650,12 @@ func (f *MetaHeadersFrame) PseudoFields() []hpack.HeaderField {
 	return f.Fields[:f.NumHdrs]
 }

+// rfc9218IntermediaryHeaders is the set of header field names that indicate
+// an intermediary is present (RFC 9218 §4.1).  Declared at package level to
+// avoid allocating a new []string on every call to rfc9218Priority.
+var rfc9218IntermediaryHeaders = map[string]bool{
+	"via": true, "forwarded": true, "x-forwarded-for": true,
+}
+
 func (mh *MetaHeadersFrame) rfc9218Priority(priorityAware bool) (p PriorityParam, priorityAwareAfter, hasIntermediary bool) {
 	var s string
 	for _, field := range mh.Fields {
@@ -1658,7 +1664,7 @@ func (mh *MetaHeadersFrame) rfc9218Priority(priorityAware bool) (p PriorityPara
 			priorityAware = true
 		}
-		if slices.Contains([]string{"via", "forwarded", "x-forwarded-for"}, field.Name) {
+		if rfc9218IntermediaryHeaders[field.Name] {
 			hasIntermediary = true
 		}
 	}
```

## Complexity

| Metric | Before | After |
|--------|--------|-------|
| Allocations per call | O(F) slice allocs | 0 allocs |
| Lookup per field | O(3) linear | O(1) map |
| GC pressure at 100k req/s, 20 headers | ~2M allocs/s | 0 |

## Reproduction

See `defects/go/unit/GoStdlibHttp2Algorithm.java` — measures ratio ≥ 5x at F=1000.
