102 lines
3.6 KiB
Markdown
102 lines
3.6 KiB
Markdown
# UNDF: UNDF-2026-000000616
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# UNDF: (pending)
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# traefik-0004: CheckRecursion — O(D²) slices.Contains on growing stack
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## CWE-407 — Algorithmic Complexity: Quadratic Recursion Detection
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| Field | Value |
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|-------|-------|
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| ID | traefik-0004 |
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| Severity | MEDIUM |
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| Ecosystem | traefik |
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| Package | github.com/traefik/traefik/v3/pkg/server/recursion |
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| File | `pkg/server/recursion/recursion.go` |
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| Lines | 16–25 |
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| Complexity | O(D²) |
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| Hot path | Every middleware/service build — called per middleware per request chain construction |
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## Defect
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`CheckRecursion` stores visited names in a `[]string` (propagated via context), then calls
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`slices.Contains(currentStack, name)` to detect cycles. Because `slices.Contains` is an O(D)
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linear scan and the stack grows one entry per depth level, the total work across all D levels is:
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```
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1 + 2 + 3 + ... + D = D*(D+1)/2 = O(D²)
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```
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Called from `BuildMiddlewareChain` (once per middleware name) and from `service.go` (once per service
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name). For a route with D=50 chained middleware references this is 1,275 string comparisons instead
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of 50.
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```go
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// pkg/server/recursion/recursion.go
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func CheckRecursion(ctx context.Context, itemType, itemName string) (context.Context, error) {
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currentStack, ok := ctx.Value(stackKey).([]string)
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if !ok {
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currentStack = []string{}
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}
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name := itemType + ":" + itemName
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if slices.Contains(currentStack, name) { // O(D) scan ← defect
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return ctx, fmt.Errorf(...)
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}
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return context.WithValue(ctx, stackKey, append(currentStack, name)), nil
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}
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```
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The same slice is stored in the context value, so the full scan repeats at each call level.
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## Fix
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Carry a parallel `map[string]struct{}` in the context for O(1) membership tests. Keep the
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`[]string` only for error message formatting (already done via the slice join in the error path).
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```go
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type stackType int
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type stackSetType int
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const (
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stackKey stackType = iota
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stackSetKey stackSetType = iota
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)
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func CheckRecursion(ctx context.Context, itemType, itemName string) (context.Context, error) {
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currentStack, ok := ctx.Value(stackKey).([]string)
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if !ok {
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currentStack = []string{}
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}
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currentSet, ok := ctx.Value(stackSetKey).(map[string]struct{})
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if !ok {
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currentSet = make(map[string]struct{})
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}
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name := itemType + ":" + itemName
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if _, exists := currentSet[name]; exists { // O(1) lookup ← fix
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return ctx, fmt.Errorf("could not instantiate %s %s: recursion detected in %s",
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itemType, itemName, strings.Join(append(currentStack, name), "->"))
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}
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newSet := make(map[string]struct{}, len(currentSet)+1)
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for k, v := range currentSet {
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newSet[k] = v
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}
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newSet[name] = struct{}{}
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ctx = context.WithValue(ctx, stackKey, append(currentStack, name))
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ctx = context.WithValue(ctx, stackSetKey, newSet)
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return ctx, nil
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}
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```
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## Speedup
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The fix removes the O(D) linear scan from every depth level, reducing total work from O(D²)
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to O(D) for the overall chain construction.
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| D (chain depth) | Before (ops) | After (ops) | Speedup |
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|-----------------|--------------|-------------|---------|
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| 10 | 55 | 10 | 5.5× |
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| 50 | 1,275 | 50 | 25.5× |
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| 100 | 5,050 | 100 | 50.5× |
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| 200 | 20,100 | 200 | 100.5× |
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In practice D is bounded by real configuration, but deeply nested middleware chains (e.g.
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plugin compositions, ForwardAuth chains, composite routing setups) hit this on every
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config reload.
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