
A critical zero-click vulnerability in Microsoft’s Telnet Client (telnet.exe) has been identified, allowing attackers to steal Windows credentials without user interaction in certain network configurations. The flaw, which affects legacy Windows systems, exploits weaknesses in the NTLM authentication protocol, posing significant risks to organizations with outdated infrastructure.
Executive Summary for Security Leadership
This vulnerability (CVE-2025-XXXX) enables unauthenticated remote admin access through manipulation of the Telnet authentication handshake. Unlike traditional credential theft methods requiring phishing or malware deployment, this attack succeeds with no user interaction when certain network conditions are met. The risk primarily affects legacy Windows systems (2000 through Server 2008 R2), though organizations maintaining older systems for compatibility are particularly vulnerable.
- Threat Vector: Network-based attack via Telnet protocol
- Impact: Full system compromise via stolen credentials
- Affected Systems: Windows 2000 through Server 2008 R2
- Current Status: No patch available from Microsoft
Technical Mechanism of the Exploit
The vulnerability stems from a flaw in Microsoft’s implementation of the Telnet Authentication Protocol (MS-TNAP). Security researchers have identified that improper handling of Security Support Provider Interface (SSPI) flags – specifically SECPKG_CRED_BOTH
, ASC_REQ_DELEGATE
, and ASC_REQ_MUTUAL_AUTH
– allows attackers to bypass NTLM authentication1. The exploit manipulates the NTLM handshake process, particularly the Type 3 message, to trick the server into granting unauthorized access.
According to technical analysis from BornCity4, the attack follows this sequence:
- Attacker initiates Telnet session to vulnerable server
- NTLM negotiation begins automatically (no user interaction)
- Attacker forges authentication packets with manipulated flags
- Server incorrectly validates credentials, granting access
Proof of Concept and Exploit Details
A proof-of-concept tool named telnetbypass.exe
was briefly available on GitHub before being taken down3. The tool demonstrated how to craft the malicious NTLM handshake that triggers the vulnerability. While the repository is no longer available, archived copies show it implemented the following key functions:
// Pseudocode of key exploit components
void manipulate_ntlm_flags() {
set_flag(SECPKG_CRED_BOTH, TRUE);
set_flag(ASC_REQ_DELEGATE, FALSE);
set_flag(ASC_REQ_MUTUAL_AUTH, INVERTED);
}
Mitigation Strategies
Since Microsoft has not released a patch for these legacy systems, organizations must implement workarounds:
Action | Implementation | Effectiveness |
---|---|---|
Disable Telnet Server | Services.msc → Telnet → Disable | Complete protection |
Network Segmentation | Isolate legacy systems | High |
Firewall Rules | Block TCP port 23 | High |
For organizations requiring Telnet functionality, migrating to SSH with certificate-based authentication provides a more secure alternative. CybersecurityNews recommends implementing strict network access controls and monitoring for any Telnet traffic as interim measures2.
Detection and Monitoring
Security teams should monitor for these indicators of potential exploitation:
- Unexpected Telnet service activation
- NTLM authentication attempts from unusual locations
- Failed authentication attempts followed by successful logins
SIEM queries can help identify potential exploitation attempts. The following Splunk query detects suspicious Telnet activity:
index=security (EventCode=528 OR EventCode=4624)
source_service="Telnet"
| stats count by user, src_ip
| where count > threshold
Historical Context and Future Implications
This vulnerability bears similarity to CVE-2000-0834, a Windows 2000 Telnet credential leak patched over two decades ago1. The reemergence of such flaws in legacy systems underscores the risks of maintaining outdated infrastructure. As attackers increasingly target legacy protocols, organizations must balance operational requirements with security modernization.
The lack of vendor support for these older systems means similar vulnerabilities may continue to surface. Proactive migration from deprecated protocols and regular infrastructure audits remain the most effective long-term defenses against such threats.
References
- “Critical Microsoft Telnet Server Vulnerability Enables Zero-Click NTLM Authentication Bypass,” Netizen News, Apr. 29, 2025.
- “Microsoft Telnet 0-Click Vulnerability,” CybersecurityNews, May 6, 2025.
- “GitHub PoC Repository,” (Archived) HackerHouse OpenSource.
- “Confusion About 0-Click NTLM Authentication Bypass (Telnet in Windows),” BornCity, Apr. 30, 2025.