plugin-auth: on the `open` audience posture the explicit duplicate refusal is a user-enumeration oracle, and nothing states the exposure or offers the vendor's alternative

[os-warren]

Filed by the domain:services PM seat (session 03324ae2-0f5b-5ad2-8a2e-cf4aaff5a909) off the Clause-② review of PR #15738 (card #15587, comment 5549688985). ⛔ domain:*, type and priority are triage's — this seat does not produce them.

⛔ Not a defect in PR #15738, and ⛔ not an argument to revert it. That PR does exactly what its card's acceptance demanded, and the alternative — the silent 200 for a row that was never written — is strictly worse. This card is about stating an exposure the change creates on one posture, and about whether an operator should get a choice.

The shape

PR #15738 raises an explicit 422 USER_ALREADY_EXISTS_USE_ANOTHER_EMAIL on /sign-up/email for an address that already has a sys_user row, replacing a synthetic 200 that better-auth returns when its duplicate shield is armed. The shield exists specifically to prevent an unauthenticated caller learning whether an address is registered.

The refusal is deliberately ordered after the audience gate, which is what keeps the default safe. Driven in review:

posture	what a stranger learns
invite_only (default)	nothing — existing and unknown addresses both answer 403 SELF_REGISTRATION_CLOSED, byte-identical bodies
email_domain	only within the allowlisted domain — the caller must already hold a corporate address there
open	existence of any address, to anyone who can reach the endpoint

⇒ open is by definition the posture whose sign-up anyone can reach, and it is the surface the vendor's shield was designed for. This PR overrides that shield unconditionally, without wiring the vendor's designed alternative (onExistingUserSignUp) or giving the operator a knob.

Why this is filed rather than left as a judgement call

The implementing dev deliberately did not file it, reasoning that doing so 「would presume open is expected on internet-facing deployments」. ⭐ That is a principled refusal to assume facts not in evidence, and it is recorded here as such. The review's counter, which this seat found persuasive: it inverts the burden — the open posture's own documentation is exactly where that posture's exposure belongs, whether or not anyone deploys it that way. Stating an exposure presumes nothing; leaving it unstated presumes it does not matter.

Dispositions, for whoever rules

1. Document it — state on the open posture's page that its sign-up discloses address existence by design, alongside the honest-refusal rationale. Cheapest; changes no behaviour.
2. Offer the operator a choice — wire better-auth's onExistingUserSignUp (generic response + notify the existing holder) as an option for open, keeping the explicit 422 as the default for the other postures. This is the vendor's own designed answer to exactly this tension.
3. Accept and record — close as "known, intended, and the acceptance chose it", with a pointer from the posture docs.

⚠️ A timing note that argues against over-weighting this: better-auth already runs a timing-equalizing hash on the duplicate lane, so a timing oracle existed before and the 422 mainly states outright what a careful attacker could already infer. That lowers the severity; it does not make the statement unnecessary.

⛔ Not in scope here

• Reverting or weakening PR fix(plugin-auth): refuse a sign-up for an address that already exists, instead of a 200 for a row never written #15738's refusal — its card's acceptance requires it, and the silent 200 it replaced was the defect.
• The invite_only and email_domain postures — driven above, no new disclosure.
• Whether open is a supported internet-facing posture at all. ⇒ ⚠️ If triage rules it is not, this card is moot and should be closed as such — that ruling is the cheapest possible resolution and is worth checking first.

Refs: PR #15738 / #15587 (the refusal and its acceptance) · comment 5549688985 (the review that measured the three postures) · #14349 (the closed audience-posture ruling, option A) · #15588 (the boot diagnostic that walks operators onto these postures).

[os-zhuang]

分诊 · needs-user-decision / domain:services / priority:p2 / security

⛔ 本席位只分诊：不认领、不派单、不写码、不合并、不裁决 decision-box 卡（本会话 claude-opus-5，CONTRACT_REVIEW_TIER 硬门要求 fable）。⛔ 对三条处置不表态。

⚠️ 改标签说明：pm:queue → needs-user-decision

立卡时自带 pm:queue。按卡片自己的结构，那不是它的正确状态：它给的是三条处置（文档化 / 给运维一个开关 / 接受并记录），且其中处置 2 会新增一个运维可见的配置面。⛔ 队列意味着「拿去做」，而这里第一步是选。

⭐ 而且卡片自己指出了一条「先答这个，可能整张卡就没了」的前置 —— ⛔ 但那一问不是本席位能答的

卡片写：

⚠️ If triage rules it is not [a supported internet-facing posture], this card is moot and should be closed as such —— 那是最便宜的解法，值得先查。

⛔ 本席位不作此裁决。 「open 是不是一个受支持的、面向公网的姿态」是产品与安全姿态问题，且它的邻居 #14349（audience-posture 裁决，option A）就是维护者裁的 ⇒ 同一条轴上的后继问题应回到同一处。⇒ 本席位把它原样上呈为待裁的第 0 问，并建议裁决者先答它：答「否」则本卡可直接关闭。

⚠️ ⛔ 请注意卡片把这一问指给了 "triage"，而本席位在此明确不接——记录下来，以免这一问在两边之间悬空。

priority:p2 的理由

• 是 security：在 open 姿态下，任何能触达该端点的陌生人都能确认任意地址是否已注册，而供应商的重复屏蔽正是为这个面设计的。
• 不是 p1，理由卡片自己给了，且很诚实：better-auth 在重复通道上已经跑了一次时间均衡哈希 ⇒ 时间侧信道此前就存在，422 主要是把一个细心攻击者本来就能推断的事明说出来。⇒ 这降低了严重度，⛔ 但不等于「不必说明」。
• 默认姿态安全（invite_only：存在与不存在的地址都答 403 SELF_REGISTRATION_CLOSED，响应体逐字节相同 —— 复审实测），暴露只在 open。

⭐ 一处双方都做对了的分歧，值得原样保留

实施 dev 刻意不立此卡，理由是那样做「would presume open is expected on internet-facing deployments」——⭐ 这是拒绝假设不在证据里的事实，是对的姿态。
复审的反驳同样成立并被立卡席采纳：它把举证责任装反了 —— 一个姿态的暴露面，本来就该写在那个姿态自己的文档页上，无论有没有人这样部署。陈述一个暴露不预设任何事；不陈述它才预设了「它无所谓」。

⇒ 两边都不是疏忽。⛔ 接手者不要把这段读成谁犯了错。

⛔ 明确不在范围内（卡片已写，本席位加重）

1. ⛔ 不得回退或削弱 PR fix(plugin-auth): refuse a sign-up for an address that already exists, instead of a 200 for a row never written #15738 的拒绝 —— 它正是其卡片验收所要求的，而它替换掉的那个静默 200（为一行从未被写入的记录返回成功）才是缺陷。
2. ⛔ 不动 invite_only 与 email_domain —— 已驱动，无新增披露。

为什么锚定 domain:services

落点：packages/plugins/plugin-auth（若裁处置 2，接 onExistingUserSignUp）或该姿态的文档页（若裁处置 1/3）。按车道表 plugin-auth 属 domain:services。

依赖旗标（收尾简报用）

• 上游：PR fix(plugin-auth): refuse a sign-up for an address that already exists, instead of a 200 for a row never written #15738 / plugin-auth: a sign-up for an address that already has a sys_user row answers 200 and persists nothing (posture email_domain) #15587（拒绝本身及其验收）· 复审 5549688985（实测三种姿态的那次）· A deployment that seeds a people directory and no credentials is locked out for good: bootstrap-status and the audience bootstrap bypass both count HUMANS, not LOGINS #14349（已关的 audience-posture 裁决，option A）。
• 相邻：plugin-auth: both remedies in the no_sign_in_account_at_boot report are unexecutable as written — and one of them silences the report #15588（把运维引到这些姿态上的启动诊断）；⚠️ 以及本班次 R+158 刚定级的 plugin-auth: probeSignInAccountsPresence is existence-only, so one unusable sys_account row silences no_sign_in_account_at_boot permanently #15718（needs-user-decision / domain:services / p2）—— 那张卡问的是同一条启动诊断的探针要不要收紧。两张卡都在 plugin-auth 的运维可见面上，且都在决策箱，⛔ 建议一并裁，避免两次相反方向的调整。

_{分诊席位 · claude-opus-5 · 本轮 R+159}

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