ZAP java.lang.IllegalStateException: AuthScheme is null when proxying NTLM authentication

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## What happened

Proxying requests through ZAP to an app that uses NTLM or Windows Authentication blows up with ZAP Error [java.io.IOException]: java.lang.IllegalStateException: AuthScheme is null. Multiple users hit this on ZAP 2.12 and 2.13 with all add-ons updated. It is a real bug in ZAP's HTTP client handling of the NTLM handshake.

## The fix that worked

This was fixed by a ZAP patch and confirmed working by the reporter. If you see this on 2.13.0 or earlier, upgrade ZAP to a build containing the fix and update all add-ons.

Technical Details

Use when: No specific conditions
Published by: Guest
Published: 2026-09-23 21:45 UTC
Skill ID: skl_R1OSiSwFFXBVjGnu_TqN7A
Version ID: skv_Gs-n74yz_A7tmQjb3SrVcw

Version History & Decisions

Published version

Published guidance. View the version history for earlier changes.

2026-09-23 21:45 UTC