CVE-2026-106428 in C Driver
Summary
by MITRE • 10/08/2026
An out-of-bounds read in SCRAM authentication response parsing in the MongoDB C Driver can read one byte beyond a fixed-size buffer when processing a malformed server-final message. A server or network intermediary able to provide this message before server-signature verification can cause the application using the driver to terminate. The extra byte is not returned through the protocol.
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Analysis
by VulDB Data Team • 10/08/2026
The MongoDB C Driver contains an out-of-bounds read vulnerability within its implementation of the SCRAM authentication mechanism, specifically during the parsing phase of the server-final message. This flaw arises when the driver processes a malformed or maliciously crafted response from the database server. The technical root cause is located in the buffer handling logic used to extract data from the incoming network packet. Instead of strictly validating that the available payload length matches the expected size before accessing memory, the parsing routine allows for an access one byte beyond the boundaries of the fixed-size internal buffer. This specific off-by-one error occurs prior to the verification of the server signature, which is a critical security step intended to ensure the authenticity and integrity of the authentication exchange.
From an operational perspective, this vulnerability primarily manifests as a denial-of-service condition rather than a data exfiltration or remote code execution vector. Because the extra byte read is not returned through the protocol interface to the application logic, it does not leak sensitive information such as credentials or internal memory contents directly via the SCRAM channel. However, the act of reading beyond the allocated buffer boundary triggers undefined behavior in C-based applications. In most practical scenarios involving modern operating systems and compilers, this illegal memory access results in a segmentation fault, causing the application utilizing the MongoDB driver to crash abruptly. This termination disrupts service availability for any users or processes dependent on that specific instance, effectively allowing an attacker who can control the server's response to deny service to clients connecting through vulnerable drivers.
The threat model involves either a malicious database server configured by an insider with access to modify authentication responses, or a network intermediary capable of performing man-in-the-middle attacks and injecting crafted packets into the communication stream between the client driver and the MongoDB instance. Since the vulnerability is triggered before signature verification, the attacker does not need valid credentials to exploit this flaw; they only need to intercept or generate the server-final message during the authentication handshake. This makes it particularly dangerous in environments where TLS encryption might be misconfigured or absent, although even with encrypted channels, a compromised endpoint could still deliver such payloads if the driver is running on that host.
This vulnerability aligns with Common Weakness Enumeration (CWE) category CWE-125, which describes Out-of-bounds Read. It also relates to CWE-20, Improper Input Validation, as the driver fails to adequately validate the length of incoming data against buffer constraints before processing. In terms of the MITRE ATT&CK framework, this behavior is consistent with techniques used in Denial of Service attacks (T1499) and potentially Initial Access if combined with other exploits, though its primary impact here is availability disruption. The lack of signature verification prior to the crash means that standard authentication bypass protections do not mitigate this specific memory safety issue.
Mitigation strategies should focus on both immediate patching and long-term architectural improvements. Organizations using affected versions of the MongoDB C Driver must upgrade to a patched version where the buffer bounds are strictly enforced during SCRAM message parsing. Developers integrating the driver into their applications should ensure that their build environments utilize AddressSanitizer or similar memory error detection tools in staging environments to catch such issues early. Furthermore, network security teams should enforce strict TLS encryption for all MongoDB connections to prevent external intermediaries from injecting malformed packets. While this specific flaw does not lead directly to code execution due to the nature of the read operation and its isolation within the authentication handshake, treating it as a critical availability risk is essential to maintain robust service continuity in production environments relying on MongoDB infrastructure.