Details
## Summary
Traefik's BasicAuth middleware coalesces concurrent credential checks through a `singleflight.Group` to avoid hashing the same password many times at once. Since v3.6.11 the deduplication key was built from the submitted password plus the stored secret, so it depended on server state: a non-existent username collapsed onto one shared key while each configured username produced its own. Under attacker-controlled concurrency, a probe request arriving inside a leader request's in-flight window is served the leader's fast coalesced result when the username does not exist, but computes its own hash (slow) when the username exists — reintroducing, only in the concurrent case, the unauthenticated username-enumeration timing oracle that GHSA-g3hg-j4jv-cwfr had hardened for sequential probing. The fix derives the singleflight key from the submitted credentials (username and password) only, so it no longer depends on whether the account exists or on any stored secret. Traefik v2 is not affected: the v2.11 BasicAuth middleware does not use singleflight coalescing, and Digest authentication is not affected. The impact is limited to username enumeration; no credential disclosure or authentication bypass is possible.
The vulnerability originates on the v3.6 line, which has reached end of life; users on v3.6 or earlier v3.x must upgrade to v3.7.13 to receive the fix.
## Patches
- https://github.com/traefik/traefik/releases/tag/v3.7.13
## For more information
If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).
<details>
<summary>Original Description</summary>
## Summary
Confirmed. `checkPassword` derives the `singleflight` key from the *stored secret*, so the key encodes whether the submitted username exists:
- username absent, `secret == ""`, key `= len(P) + ":" + P`
- username present, key `= len(P) + ":" + P + secret_T`
Every non-existent username therefore lands on one shared key, while every configured username gets its own. `singleflight.Group.Do` makes a follower on an equal key block on the leader's in-flight computation and return the leader's result. So an attacker who sends a leader request with a junk username and password `P`, then sends the probe for target `T` with the same `P` late inside the leader's window, reads user existence directly off the probe's latency: coalesced (fast) means `T` does not exist, own hash (slow) means `T` does exist.
This is the exact information leak that the `notFoundSecret` dummy hash at line 127 exists to remove. Sequential probing is fully equalised (measured ratio 1.00x, so the fix for `cwfr` / `8j2h` does work); the leak reappears only under attacker-controlled concurrency. Confirmed on the current `v3.7` head, which already carries the `8mrf` singleflight fix, and on `master` (no later fix exists).
## Affected code
- `pkg/middlewares/auth/basic_auth.go:125` (`checkPassword`)
## Code analysis
`pkg/middlewares/auth/basic_auth.go:119-131` matches the finding verbatim, including the cited line 125:
```go
func (b *basicAuth) checkPassword(user, password string) bool { // :119
secret := b.auth.Secrets(user, b.auth.Realm) // "" when the user is absent
key := strconv.Itoa(len(password)) + ":" + password + secret // :124
match, _, _ := b.singleflightGroup.Do(key, func() (any, error) { // :125
if secret == "" {
_ = b.checkSecret(password, b.notFoundSecret) // :127 dummy hash, equal cost
return false, nil
}
return b.checkSecret(password, secret), nil
})
return match.(bool)
}
```
Two independent properties combine:
1. **The dummy hash equalises the cost of one lookup.** `notFoundSecret` is a configured user's real hash (`slices.Collect(maps.Values(users))[0]`), so absent and present users each perform exactly one hash of the same algorithm and cost. That is why the sequential control below is flat.
2. **The key partitions on existence, so coalescing is not equalised.** The dummy hash was placed *inside* the closure (commit `122175ac2`, PR #12803), which is what pulls absent users into `Do` at all. Before that refactor, `secret == ""` returned `false` before `Do` was ever called.
Git archaeology of the whole sequence in this one function:
| Commit | Date | Effect |
|---|---|---|
| `6f469ee1e` | 2024-10-10 | Introduces `singleflight` to dedupe concurrent hashes (`Only calculate basic auth hashes once for concurrent requests`). Absent users returned `false` before `Do`. |
| `122175ac2` (PR #12803) | 2026-03-17 | Fix for `cwfr`. Moves the empty-secret branch inside the closure, which makes the key existence-dependent for the first time. |
| `8c4fc8957` | 2026-04-13 | Fix for `8j2h`: `notFoundSecret` was resolving to `""`, so the dummy hash was a no-op. |
| `b5ace8eb5` (PR #13572) | 2026-07-28 | Fix for `8mrf`: adds the `len(password) + ":"` prefix so distinct (password, secret) pairs cannot alias. Keeps the secret in the key and keeps the empty-secret branch inside the closure. |
J15 is the residue of that last fix. It does **not** depend on the key collision `8mrf` closed: the oracle works precisely *because* the keys differ. The prior analysis of `8mrf` recommended keeping "the empty-secret (unconfigured) path out of any key that a configured user can share"; the shipped patch only delimited the key, so the existence dependency survived.
Affected range: `>= v3.6.11` (the `122175ac2` refactor) through current `v3.7` head and `master`. Not the v2 line, and not earlier v3 releases, which returned early for absent users. Digest auth does not use `singleflight` and is unaffected.
Impact scope: username enumeration only. No credential disclosure, no authentication bypass, no result sharing across identities.
## Reproduction
Go tests written directly in `package auth`, driving the real `NewBasic` handler over a real HTTP server (`httptest`), with real `bcrypt` / `apr1` hashes and no instrumentation of the vulnerable logic. Files: `pkg/middlewares/auth/zz_scanpoc_J15{,b,c}_test.go`, deleted after the run.
**Commands**
```
cd /Users/emile/go/src/github.com/traefik/traefik
go test -count=1 -run TestZZScanPoCJ15 -v ./pkg/middlewares/auth/...
go test -count=1 -run TestZZScanPoCJ15Costs -v ./pkg/middlewares/auth/...
go test -count=1 -run TestZZScanPoCJ15H2 -v ./pkg/middlewares/auth/...
```
Probe shape, exactly the claimed scenario: calibrate `D` with one request, launch a leader with a junk username and password `P`, sleep `0.9 * D`, then send the probe with password `P` and either a configured (`alice`) or an absent (`bob`) username, and time the probe. `classify=OK` means `present > 2 * absent`, i.e. the oracle answered correctly.
**Observed, main PoC (test 1)**
```
[bcrypt-cost12] SEQUENTIAL control: absent=249.568275ms present=249.10215ms ratio=1.00x
[bcrypt-cost12] CONCURRENT D=245.521625ms frac=0.90 round 0: absent=21.750833ms present=242.081792ms ratio=11.1x classify=OK
[bcrypt-cost12] CONCURRENT D=245.521625ms frac=0.90 round 1: absent=20.197667ms present=245.234958ms ratio=12.1x classify=OK
[bcrypt-cost12] CONCURRENT D=245.521625ms frac=0.90 round 2: absent=25.675958ms present=243.981375ms ratio=9.5x classify=OK
[bcrypt-cost10] SEQUENTIAL control: absent=61.4374ms present=61.049608ms ratio=0.99x
[bcrypt-cost10] CONCURRENT D=60.54675ms frac=0.90 round 0: absent=5.572875ms present=60.8345ms ratio=10.9x classify=OK
[bcrypt-cost10] CONCURRENT D=60.54675ms frac=0.90 round 1: absent=4.042292ms present=61.988792ms ratio=15.3x classify=OK
[bcrypt-cost10] CONCURRENT D=60.54675ms frac=0.90 round 2: absent=4.53825ms present=61.557791ms ratio=13.6x classify=OK
[apr1-short-pw] SEQUENTIAL control: absent=360.7µs present=373.116µs ratio=1.03x
[apr1-short-pw] CONCURRENT D=370.792µs frac=0.90 round 0: absent=378.167µs present=409.458µs ratio=1.1x classify=FAIL
[apr1-short-pw] CONCURRENT D=370.792µs frac=0.90 round 1: absent=352.416µs present=333.875µs ratio=0.9x classify=FAIL
[apr1-short-pw] CONCURRENT D=370.792µs frac=0.90 round 2: absent=374.209µs present=349.125µs ratio=0.9x classify=FAIL
[apr1-8000B-pw] SEQUENTIAL control: absent=22.159325ms present=21.999433ms ratio=0.99x
[apr1-8000B-pw] CONCURRENT D=22.457583ms frac=0.90 round 0: absent=22.089458ms present=22.516708ms ratio=1.0x classify=FAIL
[apr1-8000B-pw] CONCURRENT D=22.457583ms frac=0.90 round 1: absent=698.042µs present=21.877ms ratio=31.3x classify=OK
[apr1-8000B-pw] CONCURRENT D=22.457583ms frac=0.90 round 2: absent=1.570875ms present=21.945417ms ratio=14.0x classify=OK
```
The **sequential control is the decisive part**: 1.00x / 0.99x / 1.03x / 0.99x on every algorithm. The constant-time countermeasure is intact for one-request-at-a-time probing, so the 10x to 15x concurrent separation is attributable to the coalescing and to nothing else. That rules out the alternative explanation that this is just `cwfr` / `8j2h` still unfixed.
**Observed, per-algorithm hash cost (test 2)**
```
bcrypt cost10 / short pw -> 60.5919ms per hash
bcrypt cost12 / short pw -> 242.384558ms per hash
apr1 / short pw -> 128.333µs per hash
apr1 / 8000-byte pw -> 42.575816ms per hash
```
**Observed, single HTTP/2 connection (test 3)**
Both probes multiplexed as two streams over one TLS connection, which pins them to a single Traefik process even behind an L4 load balancer fronting several replicas:
```
h2 single-connection: D=63.048ms
h2 round 0: absent=7.931459ms present=65.87375ms ratio=8.3x classify=OK
h2 round 1: absent=4.255667ms present=64.193291ms ratio=15.1x classify=OK
h2 round 2: absent=6.696417ms present=64.695208ms ratio=9.7x classify=OK
```
**Conclusion: REPRODUCED**, 9/9 correct classifications on bcrypt, single-shot, no statistics.
Claim-by-claim audit of the finding text:
_(truncated ; full analysis in the linked internal report)_
## Documentation grounding
**Not working-as-intended. The governing project document puts this class explicitly in scope.**
- `docs/content/security/` (header-underscores, request-path, content-length, http2-header-memory, multi-tenant-kubernetes) has no page covering BasicAuth, the timing posture or the singleflight dedup. Grep for `timing|basicauth|basic auth|enumerat|singleflight` across that directory: no match. No governing security doc, hence no WAI signal from there.
- `docs/content/contributing/security-decisions.md`, section **Authentication Middleware Correctness**, is the settled public position and it is directly on point:
> **Our position.** In scope: credential or identity handling that leaks across requests or users, **observable timing differences that disclose whether a principal exists**, and credentials forwarded to a destination the operator did not authorise.
>
> **Where the line is.** Choosing a weak authentication mechanism, or configuring it permissively, is the operator's decision. **The middleware failing to deliver what its documentation promises is ours.**
_(truncated ; full analysis in the linked internal report)_
## Precedent in comparable projects
Corpus refreshed 2026-08-24 for nginx, ingress-nginx, kong, caddy, apisix, nginx-plus; envoy (2026-06-29), envoy-gateway (2026-06-15), haproxy (2026-07-16) and istio (2026-05-12) are staler.
No exact analogue of a request-deduplication timing oracle. Adjacent prior art:
- **Envoy, CVE-2026-47775, medium**: "OAuth2 Filter: Padding Oracle via AES-256-CBC Cookie Decryption". A side-channel oracle inside an auth filter, published as a medium CVE. Framed on the observability of the discrepancy, not on the difficulty of measuring it.
- **Envoy Gateway, CVE-2026-53715 / GHSA-8fv2-88gg-hm7q, medium**: "Wasm cache ServeHTTP reads mappingPath2Cache without lock". A concurrency defect in a shared per-process cache in the request path, treated as a real medium CVE and fixed by correcting the shared-state handling.
- **APISIX, CVE-2025-62232, high**: basic-auth credential exposure. Same component, unrelated mechanism (logging).
Takeaway: the industry treats side-channel oracles in auth filters, and concurrency defects in shared per-process request-path caches, as genuine publishable CVEs of roughly this severity. Nothing in the corpus argues the class is by design. The strongest precedent, however, is not a competitor: it is Traefik's own two published CVEs on this exact guarantee.
## Recommended fix
Assign for fix. No fix exists: `gh search prs --repo traefik/traefik "singleflight"` returns only PR #13572 (the merged `8mrf` key-collision fix) and `"basic auth timing"` only PR #12803 / #12796; `git log --all` on the checkout shows `b5ace8eb5` as the last change to `pkg/middlewares/auth/` and `master` (`174e5d811`, a merge of `v3.7`) carries nothing later.
**Fix shape: remove the secret from the key and qualify it by username.** Something along the lines of
```go
key := strconv.Itoa(len(user)) + ":" + user + ":" + password
```
with the dummy-hash branch left inside the closure. This is strictly better than the current key on all four counts:
- **It closes J15**: the key no longer encodes existence, so two absent users no longer share a bucket that a present user is excluded from. Coalescing then happens only for an identical `(user, password)` pair, which leaks nothing an attacker did not already supply.
- **It closes `8mrf` structurally** rather than by delimiting: the stored hash never enters the key, so no choice of password can alias a configured user's key. The `len` prefix is still needed, now on `user`, to keep `("ab", "c")` and `("a", "bc")` apart.
- **It preserves the purpose of `6f469ee1e`**: the case that commit exists for is a burst of concurrent requests carrying the *same* credentials, which is exactly what a username-qualified key still dedupes.
- **It keeps the constant-time property**: one hash of `notFoundSecret` for absent users, one hash of `secret` for present ones, unchanged.
Do not fix this by making the dummy branch share the leader's timing envelope; as the finding correctly notes, that only helps if the key stops depending on `secret == ""`.
Secondary, low cost: restore the "Timing attacks" admonition that PR #12803 added to the BasicAuth page. It is the statement of the guarantee, it is the thing `security-decisions.md` holds the project to, and it is currently absent from the `v3.7` docs tree.
If filed: new cluster slug `basicauth-singleflight-existence-oracle`, sibling of `basicauth-singleflight-key-collision`, affected range `>= v3.6.11` through the current `v3.6` / `v3.7` heads, v2 unaffected, digest auth unaffected. Correct the 40x claim and the `$apr1$` "trivial" claim in the published description per the audit table above.
## Provenance
Found by an external automated code scan (`CLAUDE-SECURITY-20260824-122205`) of `pkg/middlewares`, `pkg/proxy`, `pkg/server`, `pkg/muxer` and `pkg/tls` on branch `v3.7` at commit `d5072ce7b8765c9574246072e05dd81d84950da7`, then triaged with the `advisory-check` process : mechanism-level duplicate check against the existing advisory corpus, CVE-policy gate, security-documentation grounding, comparable-project precedent, and a mandatory reproduction attempt.
Triage outcome : **Likely Valid**, confidence High, reproduced (yes). Expected publication likelihood at triage time : High.
Scanner finding id : F16. Internal report : `findings/scan-20260824/verdicts/J15.md` in the security-advisor repository.
</details>
---
