Source: cve@mitre.org
One-Time Passwords In Everything (a.k.a OPIE) 2.32 and 2.4 allows remote attackers to determine the existence of user accounts by printing random passphrases if the user account does not exist and static passphrases if the user account does exist.
OPIE 2.32 and 2.4 suffers from a critical vulnerability allowing remote attackers to enumerate user accounts. By observing the passphrase output, attackers can determine if a user account exists, enabling them to target specific users for further attacks and potentially gain unauthorized access to systems.
Step 1: Target Identification: The attacker identifies a system running OPIE 2.32 or 2.4.
Step 2: Username Enumeration: The attacker submits a series of username attempts to the OPIE authentication service.
Step 3: Response Analysis: The attacker observes the output from each authentication attempt.
Step 4: Account Detection: If the output is a random passphrase, the attacker infers the username does not exist. If the output is a static passphrase, the attacker infers the username exists.
Step 5: Target Selection: The attacker uses the enumerated usernames to target specific users for further attacks, such as password guessing, phishing, or brute-force attempts.
The vulnerability stems from a flawed implementation in OPIE's authentication process. When a user attempts to authenticate with a non-existent account, the system prints random passphrases. Conversely, for existing accounts, it prints static passphrases. This difference in output allows attackers to distinguish between valid and invalid usernames. The root cause is a lack of proper input validation and output sanitization, leading to information leakage. The core issue is the predictable behavior of the authentication process based on account existence, which violates the principle of least privilege and information hiding. The flaw is not a buffer overflow or race condition, but rather a logic error in how the system handles authentication failures.
While no specific APTs are definitively linked to this specific CVE, the ease of exploitation makes it attractive to a wide range of attackers. This vulnerability could be used as a pre-attack reconnaissance step before deploying more sophisticated attacks. Not on CISA KEV.
Monitor authentication logs for a high volume of failed login attempts with varying usernames.
Analyze network traffic for repeated attempts to authenticate to OPIE services.
Examine system logs for unusual activity related to OPIE processes.
Look for the generation of static passphrases in authentication logs, indicating a successful authentication attempt (and thus, a valid username).
Upgrade to a patched version of OPIE or a more secure two-factor authentication solution.
Disable OPIE if it is not required.
Implement rate limiting to restrict the number of authentication attempts from a single source.
Review and harden the system's authentication configuration.
Implement robust logging and monitoring to detect and alert on suspicious activity.