Summary

The LoginLink signature used for password reset URLs covers only the user's id — it does not include the password hash. After a user clicks a reset link and successfully changes their password, the same link remains valid for up to 2 additional uses within a 1-hour window. Anyone who intercepts or caches the original link can log in as the user even after the password has been changed.

Details

config/packages/security.yaml:73-78 configures the login link:

login_link:
    check_route: link_login_check
    signature_properties: ['id']
    lifetime: 900
    max_uses: 3

The HMAC signature covers only id, not password, username, or email. The effective lifetime for password reset links is extended to 3600 seconds by PasswordResetController.php:106-119 via getPasswordResetRetryLifetime().

After the first successful use (where the user sets a new password), the __pw_reset__ session flag is cleared by WizardController.php:105. Subsequent uses of the same link bypass the forced-password-reset wizard and grant a normal authenticated session.

Symfony's LoginLinkHandler::consumeLoginLink decrements a per-link counter but allows up to 3 total uses. Re-issuing a new link does not invalidate the old one.

The same vulnerability affects UserLoginLinkCommand.php:77, which generates admin-on-demand login links with the same signature scheme.

PoC

Step 1 — Generate a password reset link:

docker exec <kimai_container> bin/console kimai:user:login-link <username>

Step 2 — Use the link to log in and change the password.

Step 3 — Open the same link in a different browser or incognito session.

The link logs the attacker in as a fully authenticated user, despite the password having been changed in Step 2.

Step 4 — The link works a third time (3 total uses within 1 hour).

Impact

A password reset link that leaks through any of the following channels remains a valid authentication credential even after the legitimate user has changed their password: corporate mail-relay scanners that pre-click links (Microsoft Defender ATP, Mimecast, Barracuda), shared inboxes, browser history sync across devices, HTTP referrer headers, or proxy/WAF logs. The user believes they have secured their account by setting a new password, but the leaked link still grants full access for up to 2 additional uses within the hour.