What you’re seeing
- System log events from source KDCSVC with IDs in the 201 to 209 range that mention RC4 encryption.
- Older services, a NAS, a scanner or a line-of-business app start failing to sign in after Windows updates on the domain controllers.
- Service accounts that have not changed their password in years begin getting Kerberos errors.
- Users report mapped drives or app connections that fail intermittently while others work.
Common causes
Every environment is different, but these are the causes our engineers find most often:
- Microsoft is phasing out RC4 as a default Kerberos encryption type, and recent domain controller updates warn about or block its use.
- Service accounts have old passwords that were set before AES keys existed, so they only hold RC4 keys.
- Devices such as older NAS units, copiers or Linux systems only support RC4 for Kerberos.
- The encryption types attribute on some accounts is explicitly limited to RC4.
- Keytab files on non-Windows systems were generated with RC4 and never refreshed.
Why it matters
RC4 is a weak cipher that attackers abuse to crack service account passwords offline, so Microsoft's changes are a real security improvement. The catch is that the same changes can take down file shares, scanners or business apps that quietly depended on RC4. Knowing which systems will break before enforcement arrives lets you plan replacements or updates instead of discovering them during a busy workday.
How AskTechGurus fixes it
Skip the trial and error. A certified engineer works on your actual system, finds the root cause, and fixes it properly.
Tell us what’s happening
Submit a ticket in about two minutes. Paste the error, describe what changed, and pick your tier. No charge to submit.
A certified engineer diagnoses it
A vetted, US-based systems engineer claims your ticket, usually within minutes during business hours, and investigates over a secure remote session you can watch and end at any time.
Fixed and verified with you
The engineer resolves the root cause and confirms with you that it’s working. You don’t just get a workaround, and you get a plain-English summary of what was wrong.
Pay only when it’s resolved
Fixed price from $99, invoiced only after you approve the fix. If it isn’t resolved, you don’t pay.
Frequently asked questions
Are these warnings urgent?
They are an early notice. Systems relying on RC4 will fail once enforcement is fully in place, so the warnings are the time to find and plan for them.
Can we just keep RC4 enabled?
Temporarily, perhaps, but it leaves service accounts open to password cracking, and Microsoft is removing the option over time.
Can AskTechGurus identify what will break?
Our certified US engineers can review the events and affected accounts with you over a remote secure session, from $99, and you pay only when it's resolved.