CVE-2001-0335 in Internet Information Server
Summary
by MITRE
FTP service in IIS 5.0 and earlier allows remote attackers to enumerate Guest accounts in trusted domains by preceding the username with a special sequence of characters.
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Analysis
by VulDB Data Team • 10/07/2025
The vulnerability described in CVE-2001-0335 represents a significant authentication bypass flaw within the Internet Information Services (IIS) version 5.0 and earlier implementations. This issue specifically targets the File Transfer Protocol (FTP) service component of IIS, which was widely deployed in enterprise environments during the late 1990s and early 2000s. The flaw enables remote attackers to exploit the service's handling of username credentials, particularly when dealing with guest accounts in trusted domain environments. The vulnerability operates through a specific manipulation of the username field during the authentication process, allowing unauthorized enumeration of guest account information.
The technical mechanism behind this vulnerability stems from improper input validation within the FTP service authentication routine. When an attacker submits a specially crafted username string that begins with a specific sequence of characters, the IIS FTP service processes this input in a manner that reveals information about guest accounts in trusted domains. This behavior occurs because the service does not adequately sanitize or validate the username format before attempting authentication, creating an information disclosure channel. The vulnerability is particularly dangerous because it allows attackers to discover the existence of guest accounts, potentially enabling further exploitation attempts. According to CWE classification, this represents a weakness in the input validation process where insufficient sanitization of user-provided data leads to unintended information exposure.
The operational impact of CVE-2001-0335 extends beyond simple information disclosure, as it provides attackers with valuable reconnaissance data that can facilitate more sophisticated attacks. Once guest account enumeration is achieved, threat actors can use this information to conduct targeted brute force attacks against these accounts or leverage them as stepping stones for lateral movement within the network. The vulnerability affects organizations using IIS 5.0 and earlier versions, which were prevalent in enterprise environments during the early 2000s, making it a significant concern for legacy system administrators. The attack vector requires only remote access to the FTP service, making it particularly dangerous as it can be exploited from anywhere on the internet without requiring physical access or elevated privileges.
Security professionals should consider this vulnerability in the context of broader attack patterns documented in the MITRE ATT&CK framework, particularly within the credential access and reconnaissance phases. The enumeration capability directly maps to techniques involving account discovery and credential dumping, which are commonly used in initial compromise phases of cyber attacks. Organizations should implement immediate mitigations including patching IIS installations to versions that address this vulnerability, disabling unnecessary FTP services, and implementing proper network segmentation to limit access to critical systems. The vulnerability highlights the importance of input validation and proper authentication handling within web server implementations, serving as a reminder of the critical security implications of inadequate sanitization of user inputs in network services. Additionally, this vulnerability underscores the necessity of regular security assessments and patch management programs to address known weaknesses in legacy systems that remain in production environments.