Source: cve@mitre.org
An issue was discovered in Mellium mellium.im/sasl before 0.3.1. When performing SCRAM-based SASL authentication, if the remote end advertises support for channel binding, no random nonce is generated (instead, the nonce is empty). This causes authentication to fail in the best case, but (if paired with a remote end that does not validate the length of the nonce) could lead to insufficient randomness being used during authentication.
Mellium mellium.im/sasl versions prior to 0.3.1 are vulnerable to a critical authentication bypass. This flaw allows an attacker to potentially weaken the SCRAM-based SASL authentication process by manipulating the nonce generation, leading to insufficient randomness and potentially enabling credential theft or unauthorized access. Successful exploitation could compromise sensitive communication channels and user accounts.
Step 1: Negotiation: The attacker initiates a SASL authentication attempt with a vulnerable server, advertising support for channel binding.
Step 2: Channel Binding Advertisement: The server, due to the vulnerability, advertises support for channel binding.
Step 3: Nonce Generation Failure: The mellium.im/sasl library, due to the bug, fails to generate a proper random nonce, instead using an empty string.
Step 4: Authentication Attempt: The attacker sends their authentication credentials, potentially using the empty or predictable nonce.
Step 5: Authentication Weakening: Because of the weak or missing nonce, the authentication process is significantly weakened. This could lead to authentication failure in the best case, or in a worst-case scenario, allow for a brute-force attack or other credential-cracking methods if the server does not properly validate the nonce length or randomness.
Step 6: Credential Compromise (Potential): If the attacker can successfully bypass the authentication, they gain access to the protected resource.
The vulnerability stems from a logic error in the mellium.im/sasl library's handling of SCRAM authentication when channel binding is advertised. Specifically, the code fails to generate a random nonce when channel binding is enabled, instead using an empty string. This is a critical flaw because the nonce is a crucial component of the SCRAM authentication process, providing entropy and preventing replay attacks. The lack of a random nonce, or the use of an empty one, drastically reduces the security of the authentication. This can be exploited if the server does not validate the length of the nonce, allowing for a predictable or easily guessable nonce to be used. This allows for a potential brute-force attack or other methods to crack the authentication credentials. The root cause is a missing or incorrect implementation of the nonce generation logic when channel binding is enabled, resulting in a weakened cryptographic key exchange.
There is no specific APT or malware known to be actively exploiting this vulnerability. However, the nature of the vulnerability (authentication bypass) makes it attractive to any threat actor seeking to gain unauthorized access to systems. The potential for credential theft makes it a high-value target. This vulnerability is not currently listed on the CISA KEV (Known Exploited Vulnerabilities) catalog.
Monitor network traffic for SCRAM authentication attempts with channel binding enabled, especially if the nonce length is unusually short or empty.
Analyze server logs for failed authentication attempts originating from suspicious IP addresses or user accounts.
Implement intrusion detection rules to identify anomalous SCRAM authentication patterns, such as repeated attempts with the same credentials or unusual nonce values.
Review the source code of the mellium.im/sasl library (if used) for the specific nonce generation logic and look for any deviations from expected behavior.
Monitor for updates to the mellium.im/sasl library and apply them promptly.
Upgrade the mellium.im/sasl library to version 0.3.1 or later.
If upgrading is not immediately possible, disable channel binding in the SASL configuration as a temporary workaround. This will prevent the vulnerable code path from being triggered.
Implement robust input validation on the server-side to ensure that the nonce length and format meet the requirements of the SCRAM authentication protocol.
Regularly audit the authentication configuration and logs for any suspicious activity.
Implement multi-factor authentication (MFA) to provide an additional layer of security, even if the primary authentication mechanism is compromised.
Conduct penetration testing and vulnerability assessments to identify and address any other potential security weaknesses.