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

- // 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.

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

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