CVE-2026-81529 in C# Driver
Summary
by MITRE • 08/27/2026
Improper neutralization of delimiters in connection-URL construction allows connection-option injection in the MongoDB C# Driver. When an application passes untrusted text into the driver's connection-URL builder and round-trips the builder back into a client configuration, the untrusted text is serialized without neutralizing the URL/option delimiters and is then re-parsed as authoritative connection options. A low-privileged user of such an application can thereby introduce or suppress security-relevant connection settings.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability described constitutes a critical injection flaw within the MongoDB C# Driver, specifically categorized under CWE-74 Improper Neutralization of Special Elements used in an XML Document (CWE-74) and more accurately as CWE-91 Injection (CWE-91). This issue arises from a failure to properly sanitize or escape special characters when constructing connection URLs. In secure software design, any data derived from untrusted sources must be rigorously validated before being incorporated into command structures or configuration strings. The driver's connection-url builder fails to neutralize delimiters such as ampersands and equals signs, which are critical for separating distinct connection options in a MongoDB URI string. This lack of sanitization creates a pathway for attackers to manipulate the structure of the URL itself rather than just its content values.
The operational mechanism of this vulnerability relies on the round-tripping behavior of the driver's configuration objects. When an application accepts untrusted input and passes it into the connection-url builder, the system serializes these inputs directly into the URI string without escaping delimiter characters. Subsequently, when this serialized URL is parsed back to configure the MongoDB client, the parser treats the injected delimiters as legitimate separators between key-value pairs. This allows a low-privileged user or an external attacker who can influence the input data to inject arbitrary connection options. For instance, by injecting ampersands and equals signs, an actor can append new parameters to the end of the URL string that override existing settings or introduce entirely new ones.
The security impact of this flaw is severe because it allows for the manipulation of critical authentication and authorization mechanisms. By introducing specific connection options, a malicious user could potentially suppress required authentication fields such as username and password, effectively bypassing access controls if the server configuration permits anonymous connections in certain contexts. Alternatively, an attacker might inject parameters that alter the behavior of the database session or query execution environment. This aligns with ATT&CK technique T1078 Valid Accounts, where attackers leverage legitimate credentials to gain unauthorized access, although here the mechanism is through configuration manipulation rather than credential theft directly. The ability to suppress security-relevant settings undermines the integrity and confidentiality guarantees provided by MongoDB's authentication layer.
Mitigation strategies must focus on strict input validation and output encoding at multiple layers of the application stack. Developers should implement allow-listing for all connection parameters, ensuring that only predefined, safe values are accepted from user inputs. Furthermore, any dynamic construction of URIs must utilize established library functions designed to safely encode special characters rather than manual string concatenation. The MongoDB C# Driver maintainers have addressed this by implementing proper escaping mechanisms in subsequent versions, so upgrading to the latest patched version is the primary remediation step. Additionally, applications should enforce least privilege principles for database accounts and avoid relying solely on connection-string configurations for security-critical settings where possible, preferring explicit code-based configuration that does not involve parsing untrusted strings as authoritative directives.