# CVE-2026-103646: Ultimate Multisite < 2.17.0 - Unauthenticated Authentication Bypass via 'email_address' Parameter

> Unauthenticated attackers can log in as any user, including admins, by exploiting email normalization flaws in the checkout process.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Oct 2026
- **Updated:** 8 Oct 2026
- **Also known as:** GHSA-3qw6-7g4w-jh2g

## What is CVE-2026-103646?

CVE-2026-103646 is a critical authentication bypass vulnerability in Unknown Ultimate Multisite, published on 8 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The Ultimate Multisite  WordPress plugin before 2.17.0 does not require authentication before a logged-out checkout is linked to, and logged in as, an existing WordPress account matching the submitted email address, and its duplicate-account check normalizes that address differently from the lookup used to create the customer, so an unauthenticated attacker can log in as any existing user, including a Network Super Admin, whose email address they know.
This bypass is not addressed by the 2.15.1 fix for CVE-2026-75957 and remains exploitable in all versions up to and including 2.16.1, the releases that fix was expected to cover. Exploitation requires a checkout form configured without a password field (auto-generated password) and a target account that has no existing customer record in the Ultimate Multisite  WordPress plugin before 2.17.0.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-103646?


## How do I fix CVE-2026-103646?

No fix has been published for CVE-2026-103646 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-103646 being exploited?

There is no known exploitation of CVE-2026-103646 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-103646 and fix it if it is affected.

What it is: CVE-2026-103646 is a critical authentication bypass vulnerability in Unknown Ultimate Multisite, published on 8 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-103646 in the wild and no public exploit code.

Affected product: Unknown Ultimate Multisite. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-103646/ (Markdown: https://insights.sbomify.com/CVE-2026-103646/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-103646
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-103646
- OSV: https://osv.dev/vulnerability/CVE-2026-103646

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-103646/
Summaries are generated and may contain mistakes; the official description above is the record.