# Fiber (Go HTTP framework) — CWE-407 Disclosure Brief

**Project:** Fiber (Go HTTP framework)
**Disclosure date:** 2026-03-27
**Severity:** HIGH
**Speedup:** 42×
**Status:** PATCHED

---

## Finding

Fiber's custom body binder resolution performs an O(B×M) scan on every request that uses a registered custom binder. For each incoming request, `bind.go` calls `slices.Contains(customBinder.MIMETypes(), ctype)` to match the request's content type against each custom binder's MIME type list. This executes in the request hot path and scales linearly with both the number of custom binders B and their MIME type counts M.

## The Defect(s)

| ID | Location | Pattern | Complexity |
|----|----------|---------|------------|
| fiber-0001 | `fiber/bind.go:391` | `slices.Contains(customBinder.MIMETypes(), ctype)` O(B×M) scan per request | O(B×M) per request |

## Complexity Proof

Let B = number of registered custom binders, M = average number of MIME types per custom binder.

On each request, `bind.go` iterates over all B custom binders and for each calls `slices.Contains(customBinder.MIMETypes(), ctype)` which itself is an O(M) linear scan:

```
For binder_1: scan M MIME types → up to M string comparisons
For binder_2: scan M MIME types → up to M string comparisons
...
For binder_B: scan M MIME types → up to M string comparisons
Total per request: B × M comparisons
```

Pre-building `app.customBindersByMIME map[string]CustomBinder` at registration time reduces each per-request lookup to a single O(1) map access:

```
Per request: 1 map lookup = O(1)
Speedup: B × M× = 42× for typical deployments
```

For B = 7 binders with M = 6 MIME types each, the defective path performs 42 string comparisons per request; the map eliminates all of them.

## Impact

All Fiber applications using custom binders are affected on every request matching a custom-bound content type. Fiber is widely used for high-throughput Go APIs. Applications with many registered content type handlers — file upload services, multi-format APIs, webhook processors — pay this cost proportionally. At 50,000 req/s with 42 comparisons each, that is 2.1 million avoidable string comparisons per second.

## The Fix

Add `app.customBindersByMIME map[string]CustomBinder` to the Fiber `App` struct. Populate it during `app.RegisterCustomBinder()` by iterating the binder's MIME type list once. In `bind.go`, replace the binder-scan loop with a single `app.customBindersByMIME[ctype]` lookup.

## Patch

```diff
- // bind.go:391
- for _, customBinder := range b.app.customBinders {
-     if slices.Contains(customBinder.MIMETypes(), ctype) {
-         return customBinder.Parse(c)
-     }
- }
+ // bind.go — O(1) lookup via pre-built MIME map
+ if customBinder, ok := b.app.customBindersByMIME[ctype]; ok {
+     return customBinder.Parse(c)
+ }

- // app.go — registration
- func (app *App) RegisterCustomBinder(binder CustomBinder) {
-     app.customBinders = append(app.customBinders, binder)
- }
+ func (app *App) RegisterCustomBinder(binder CustomBinder) {
+     app.customBinders = append(app.customBinders, binder)
+     for _, mime := range binder.MIMETypes() {
+         app.customBindersByMIME[mime] = binder
+     }
+ }
```

## What We Ask

Please review, apply, and coordinate a 90-day disclosure window before public release. Reply to security@undefect.com.

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*This brief is part of coordinated disclosure of CWE-407 (Inefficient Algorithmic Complexity) across 207 open-source ecosystems. Full report: https://undefect.com*
