Fiber: Username Enumeration via Timing Oracle in BasicAuth Default Authorizer

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Fiber is an Express inspired web framework written in Go. Prior to 3.3.0, the default Authorizer function in the BasicAuth middleware in middleware/basicauth/config.go uses short-circuit evaluation that skips password hash comparison for non-existent usernames, enabling reliable remote username enumeration through response timing differences. This issue is fixed in version 3.3.0.

Source published

8 Jul 2026 19:32:15 UTC

Source modified

9 Jul 2026 14:33:29 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Fiber is an Express inspired web framework written in Go. Prior to 3.3.0, the default Authorizer function in the BasicAuth middleware in middleware/basicauth/config.go uses short-circuit evaluation that skips password hash comparison for non-existent usernames, enabling reliable remote username enumeration through response timing differences. This issue is fixed in version 3.3.0.

Source published

Jul 8, 2026, 7:32:15 PM (timezone not stated)

Source modified

Jul 9, 2026, 2:33:29 PM (timezone not stated)

Observed here

7 Nov 2026 10:30:18 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Fiber is an Express inspired web framework written in Go. Prior to 3.3.0, the default Authorizer function in the BasicAuth middleware in middleware/basicauth/config.go uses short-circuit evaluation that skips password hash comparison for non-existent usernames, enabling reliable remote username enumeration through response timing differences. This issue is fixed in version 3.3.0.

Source published

8 Jul 2026 20:16:49 UTC

Source modified

25 Oct 2026 17:10:00 UTC

Observed here

27 Oct 2026 11:12:09 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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CVSS 3.1 · [email protected] · Secondary · 5.3 CVSS:3.1/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 ```go 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

Source published

2 Jul 2026 13:46:25 UTC

Source modified

24 Jul 2026 19:11:49 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Publisher rating · MODERATE

GoFiber Vulnerable to Username Enumeration via Timing Oracle in BasicAuth Default Authorizer in github.com/gofiber/fiber

Source published

24 Jul 2026 18:35:55 UTC

Source modified

24 Jul 2026 19:00:26 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.