I was unable to identify specific vulnerable functions for CVE-2026-86478. The provided reference URL https://www.jetbrains.com/privacy-security/issues-fixed/ is a generic security page and does not contain details about this specific vulnerability or links to any patches. Attempts to find more information about 'JetBrains YouTrack' and the CVE ID through web searches did not yield any relevant source code, commit information, or detailed patch descriptions. JetBrains YouTrack is a proprietary product, and its source code is not publicly available, which prevents direct analysis of the code changes. Without access to the specific security patch or detailed technical information about the fix, it is impossible to pinpoint the exact functions that were vulnerable or modified to address the 'improper authentication' leading to 'unauthenticated account takeover via a self-asserted email address'.