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:

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