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.

Solution

Login links now also use the password hash for generating the signature, which will immediately expire the link after the user changed the password.

Read https://www.kimai.org/en/security/ghsa-m492-gv72-xvxj for more information.