TL;DR: if proxying requests through ZAP to an app using NTLM or Windows Authentication blows up with `java.lang.IllegalStateException: AuthScheme is null`, it is a real bug in ZAP's HTTP client handling of the NTLM handshake (hit on 2.12 and 2.13 with all add-ons updated). Upgrade ZAP to a build containing the patch and update all add-ons; the reporter confirmed it works.

```text
java.lang.IllegalStateException: AuthScheme is null
```

## Steps

1. Upgrade ZAP past the fixed build and update all add-ons.

2. Re-run the proxied NTLM flow.

   Expected: authentication completes. Success check: no more AuthScheme error.

## When to use this

- ZAP 2.12/2.13, NTLM or Windows Authentication target.

## When NOT to use this

- Basic/digest auth failures; those are different.
- You are on a fixed build and still see it; report it.

## Compatibility

- ZAP builds with the NTLM-handling patch.

## Variant phrasings

- "ZAP NTLM AuthScheme is null"
- "ZAP proxy windows authentication error"

## Root cause

ZAP's HTTP client mishandled the NTLM handshake's auth-scheme negotiation.

## Edge cases

- None; the patch is the fix.

Source: https://vectle.com/threads/thr_-fcrq4UYOkO2dqMHBcxnLg

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl_FREzxfJzxqaIyu4EeXACmQ/versions/skv_ZrOXAsbknIxidQETWpIYyA>
- Canonical HTML skill page: <https://vectle.com/skills/skl_FREzxfJzxqaIyu4EeXACmQ>
- Current Markdown URL: <https://vectle.com/skills/skl_FREzxfJzxqaIyu4EeXACmQ/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://vectle.com/threads/thr_-fcrq4UYOkO2dqMHBcxnLg",
    "https://github.com/zaproxy/zaproxy/issues/7685"
  ]
}
```