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CVE-2022-37966: Zombie Crypto: How RC4 Returned from the Grave to Kill Your Domain (CVE-2022-37966)

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Zombie Crypto: How RC4 Returned from the Grave to Kill Your Domain (CVE-2022-37966)

Vulnerability ID: CVE-2022-37966
CVSS Score: 8.1
Published: 2022-11-09

A critical Elevation of Privilege vulnerability in the Windows Kerberos protocol that exploits the legacy RC4-HMAC encryption type. Despite RC4 being 'deprecated', its continued presence for backward compatibility allowed attackers to forge Privilege Attribute Certificate (PAC) signatures. By manipulating the weak crypto implementation, an attacker can modify the PAC to include arbitrary SIDs (like Domain Admins), effectively granting themselves total control over the Active Directory environment. The vulnerability was discovered by Tom Tervoort of Secura and forced Microsoft to fundamentally change how session keys are negotiated.

TL;DR

Kerberos RC4-HMAC implementation flaw allows PAC signature forgery. Attackers can elevate to Domain Admin by forcing RC4 usage and spoofing the ticket's privilege attributes. Fix requires forcing AES session keys.

⚠️ Exploit Status: WEAPONIZED

Technical Details

  • CWE ID: CWE-327 (Use of a Broken or Risky Cryptographic Algorithm)
  • Attack Vector: Network (Authenticated)
  • CVSS: 8.1 (High)
  • EPSS Score: 1.38%
  • Impact: Privilege Escalation (Domain Admin)
  • Exploit Status: PoC Available / Weaponized
  • Cipher: RC4-HMAC-MD5 (etype 23)

Affected Systems

  • Windows Server 2008 through 2022 (Domain Controllers)
  • Samba AD DC (versions prior to 4.15.13)
  • Legacy NAS devices integrated with AD via RC4
  • Any Active Directory environment with RC4 enabled
  • Windows Server 2019: < Nov 2022 Update (Fixed in: KB5021131)
  • Windows Server 2016: < Nov 2022 Update (Fixed in: KB5021131)
  • Samba: < 4.17.4 (Fixed in: 4.17.4)

Code Analysis

Commit: e301211

Samba patch implementing the fix for CVE-2022-37966, forcing AES for PAC signatures.

if (kdc_options & KDC_OPT_REQUEST_PAC_SUPPORT) { ... use_aes_key ... }
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Exploit Details

  • Black Hat Europe 2022: Original research and methodology by Tom Tervoort detailing the PAC spoofing technique.
  • GitHub: Python scripts and PoC tools related to the RC4 Kerberos attack.

Mitigation Strategies

  • Disable RC4-HMAC system-wide via Group Policy.
  • Enforce AES session keys for all Kerberos tickets.
  • Audit Active Directory for accounts explicitly configured to require RC4.

Remediation Steps:

  1. Install KB5021131 (and subsequent cumulative updates) on all Domain Controllers.
  2. Set the DefaultDomainSupportedEncTypes registry key to remove RC4 support (Value: 0x3C or higher, effectively).
  3. Identify legacy devices relying on RC4 using Event Logs (Event ID 4769) and upgrade or replace them.
  4. Wait for the 'Enforcement Phase' or manually trigger it to block vulnerable connections.

References

Read the full report for CVE-2022-37966 on our website for more details including interactive diagrams and full exploit analysis.

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