PT-2026-55274 · Go · Github.Com/Gofiber/Fiber/V3

Publicado 2026-07-02 · Atualizado 2026-07-02 · CVE-2026-44332

OSV

CVSS v3.1

5.3

Média

Vetor base

AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Summary

The default Authorizer function in GoFiber's BasicAuth middleware uses short-circuit evaluation that skips password hash comparison for non-existent usernames. With bcrypt-hashed passwords (the primary use case), the timing difference between a valid and invalid username is approximately 1,000,000:1 (~100ms vs ~100ns), enabling reliable remote username enumeration.

Vulnerable Code

File: middleware/basicauth/config.go, lines 126-138

if cfg.Authorizer == nil {
  verifiers := make(map[string]func(string) bool, len(cfg.Users))
  for u, hpw := range cfg.Users {
    v, err := parseHashedPassword(hpw)
    if err != nil {
      panic(err)
    }
    verifiers[u] = v
  }
  cfg.Authorizer = func(user, pass string,  fiber.Ctx) bool {
    verify, ok := verifiers[user]
    return ok && verify(pass)  // line 137: short-circuit skips verify() if user unknown
  }
}

Data Flow

1. Attacker sends Authorization: Basic <base64(candidate:wrongpass)>
2. BasicAuth middleware decodes credentials and calls cfg.Authorizer(user, pass, c)
3. Map lookup verifiers[user] returns ok=false for non-existent users
4. Go && short-circuit: false && verify(pass) returns immediately without calling verify()
5. For valid users, verify(pass) executes bcrypt.CompareHashAndPassword() (line 167: ~100ms at default cost 10)
6. Timing difference: ~100ns (invalid user) vs ~100ms (valid user) = 1,000,000:1 ratio

Timing comparison by hash type:

Hash Type	Valid User	Invalid User	Ratio
bcrypt ($2)	~100 ms	~100 ns	1,000,000:1
SHA-512	~1-5 us	~100 ns	10-50:1
SHA-256	~1-5 us	~100 ns	10-50:1

Impact

• Username enumeration: Attacker reliably determines which usernames exist by measuring response latency
• Targeted brute force: After enumerating valid usernames, password brute force is focused only on real accounts
• Account discovery: In applications where usernames are sensitive (internal tools, admin panels), leaking their existence is itself a security issue

Notes

• Password hash comparisons themselves are timing-safe: subtle.ConstantTimeCompare is used correctly for SHA-256 (line 185), SHA-512 (line 176), and bcrypt uses its own constant-time comparison
• The fix is to always execute a dummy hash comparison for unknown users: bcrypt.CompareHashAndPassword(dummyHash, []byte(pass)) and discard the result
• This pattern matches CVE-2023-36456 (Authentik timing oracle) and similar findings in other auth libraries

Correção

Side Channel Attack

Enumeração de Fraquezas

CWE-203

Identificadores relacionados

CVE-2026-44332

GHSA-G5VH-55HW-RXM8

Produtos afetados

Github.Com/Gofiber/Fiber/V3

Referências · 3

• https://github.com/gofiber/fiber/security/advisories/GHSA-g5vh-55hw-rxm8 · Nota
• https://github.com/gofiber/fiber · Nota
• https://osv.dev/vulnerability/GHSA-g5vh-55hw-rxm8 · Aviso do Fabricante