curl would wrongly reuse an existing HTTP proxy connection doing CONNECT to a server, even if the new request uses different credentials for the HTTP proxy. The proper behavior is to create or use a separate connection.
Exploitability
AV:NAC:LPR:NUI:NScope
S:UImpact
C:LI:LA:N6.5/CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N