CVE-2026-93394 in C Driver
Summary
by MITRE • 09/18/2026
A flaw in libmongoc's SCRAM authentication implementation caused the client to continue the authentication handshake and transmit the client proof even when a nonce mismatch was detected in the server's first message. An unauthorized party with a man-in-the-middle position could exploit this by injecting a crafted server-first-message containing a controlled salt and low iteration count, then capturing the resulting client proof to perform offline password cracking. This vulnerability is mitigated by TLS, which is standard in production deployments.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified within libmongoc stems from a critical logic error in its implementation of the SCRAM authentication protocol, specifically during the handling of server-first messages. In a correctly functioning SCRAM handshake, the client must verify that the nonce provided by the server matches the one it originally sent to ensure session integrity and prevent replay attacks or man-in-the-middle interference. However, due to this flaw, libmongoc fails to abort the authentication process when such a mismatch is detected. Instead of terminating the connection immediately upon identifying an invalid or manipulated nonce, the client proceeds with the handshake steps that follow, ultimately computing and transmitting the client proof derived from the user's password credentials. This behavior fundamentally undermines the security guarantees provided by the SCRAM mechanism, as it allows an attacker to manipulate the cryptographic parameters used in the authentication exchange without detection at the protocol level.
From a technical perspective, this flaw enables a sophisticated man-in-the-middle attack scenario where an adversary intercepts and modifies the communication between the MongoDB client and server. By injecting a crafted server-first-message containing a controlled salt and a deliberately low iteration count for the key derivation function, the attacker can significantly reduce the computational cost required to crack passwords offline. The client, unaware of the manipulation due to the missing nonce validation, computes its proof using these weakened parameters and sends it back to the attacker. Since SCRAM proofs are designed to be verifiable by the server but not easily reversible without knowledge of the password, capturing this proof allows an unauthorized party with network access to perform offline brute-force or dictionary attacks against the captured hash-like data. The effectiveness of such attacks is greatly amplified when low iteration counts and predictable salts are used, as these factors drastically decrease the time required for modern hardware to guess potential passwords.
The operational impact of this vulnerability is severe in environments where TLS encryption is not enforced between clients and MongoDB servers. While many production deployments utilize Transport Layer Security to encrypt all traffic, thereby rendering man-in-the-middle attacks impossible due to cryptographic protection of the channel, systems relying on plaintext connections or misconfigured networks remain highly susceptible. In such unprotected scenarios, an attacker positioned within the network path can silently capture authentication credentials without triggering any alerts from the client application itself, as libmongoc does not flag the nonce mismatch as a fatal error. This creates a silent data exfiltration vector where sensitive user passwords are compromised in transit, potentially leading to unauthorized access to databases containing critical business information, personal identifiable data, or intellectual property.
Mitigation strategies primarily focus on enforcing secure communication channels and updating software components. The most effective defense is the mandatory use of TLS for all client-to-server connections, which prevents any third party from intercepting or modifying the authentication handshake in transit. Organizations should audit their MongoDB configurations to ensure that SSL/TLS is enabled and properly validated, including certificate verification to prevent spoofed servers. Additionally, deploying this vulnerability fix by updating libmongoc to a patched version ensures that clients will correctly abort connections upon detecting nonce mismatches, thereby closing the logic gap exploited by attackers. Complementing these technical controls with strong password policies increases the computational cost of offline cracking attempts even if other security layers fail, providing defense-in-depth against credential theft via authentication protocol manipulation.