When reusing a libcurl handle for sequential transfers driven by
environment-variable proxy configuration, libcurl fails to clear the proxy
authentication state between requests. Specifically, if the initial transfer
authenticates against proxyA using Digest auth, a subsequent transfer routed
through proxyB erroneously leaks the Proxy-Authorization: header intended
solely for proxyA.
This critical-severity flaw causes libcurl to disclose proxy credentials when proxy discovery relies on OS environment variables and Digest authentication is negotiated with the first proxy hop. This product routes all proxy configuration through its own dedicated proxy discovery component, using only explicit proxy addressing — it does not honour OS proxy environment variables — and restricts proxy authentication exclusively to Basic Authentication, with Digest, NTLM, and Kerberos explicitly unsupported. Neither precondition can be satisfied, so this product is NOT_EXPLOITABLE.