CVE-2026-6231

A numeric-type conversion flaw in the BSON handling library of the MongoDB C# Driver allows an out-of-range binary subtype value supplied through untrusted textual input to bypass the library's binary-element integrity checks. When such a value is later encoded, it is narrowed to a smaller wire representation, producing binary elements that misdeclare their type or length. An unauthenticated party able to supply text parsed by an affected application can cause it to emit and persist malformed binary data, which subsequent readers either reject with an error or silently interpret as shorter than the data that was validated.

A client-side encryption schema construction component in the MongoDB C# Driver places caller-supplied field identifiers into a position that is interpreted as a pattern expression, without normalizing or validating them. As a result, an application that designates certain fields for client-side field-level encryption may have the protection silently fail to apply, so data intended to be encrypted is stored and returned in cleartext; conversely, the rule may apply more broadly than intended. No user action is required to cause the condition, and any party with ordinary read access to the stored data — without possession of the encryption keys — can observe the unprotected values.

A data-query-logic neutralization weakness in the update-statement construction component of the MongoDB C# Driver can cause application-supplied values to be interpreted as query/update logic rather than as literal data. An application user who can influence the content of values that the application passes into the affected builder surface, without any special privilege beyond ordinary read-write application access, may cause stored data to be modified or removed in ways the application never intended. The result is unauthorized integrity impact on the affected application data; there is no direct loss of confidentiality or availability of the database service.

A memory-handling weakness in the BSON deserialization component of the MongoDB C# Driver may cause data left over from previously processed content to be returned when specially crafted serialized input is parsed. An unauthenticated party able to supply serialized data to an application that maps certain primitive collection types can cause residual bytes from unrelated data processed earlier on the same execution thread to be surfaced as legitimate values. This can result in disclosure of a limited amount of information belonging to other requests or principals of the consuming application. No authentication or special privileges within the application are required.

The MongoDB C# Driver compares TLS client certificates by issuer and serial number only when deciding whether two clients can share cached connections. In an application that creates clients from certificates supplied by different users, a user who provides a certificate copying another user's issuer and serial number, without that user's private key, can be given connections authenticated with the other user's certificate. Database operations can then run under the other user's identity, affecting the confidentiality and integrity of data that identity can access.

The MongoDB C# Driver does not neutralize dot and dollar characters in dictionary keys, field names and array-filter identifiers used in LINQ queries before placing them into the field paths it sends to the server. An application user whose input reaches these positions can redirect a query or update to fields the application did not name, which can expose other users' documents and widen updates beyond the intended data. Certain values can also write malformed fields that leave stored documents unreadable by the application until they are repaired.

The MongoDB C# Driver does not reject replacement documents whose top-level field names begin with a dollar sign in FindOneAndReplace, unlike other replace operations. An application that builds such a document from user-supplied field names can have the server apply update operators instead of a full replacement. The effect is limited to the document matched by the application's own filter, whose contents can be modified in ways the application did not intend.

A data-validation flaw exists in the BSON handling and binary serialization path of the MongoDB C# Driver. When an application constructs a raw or lazy BSON document from externally supplied binary data, the driver does not verify that the declared size matches the actual data provided and copies it unchanged. An unauthenticated party supplying binary content can insert or remove elements in the database command without database credentials.

A 32-bit integer wraparound in the MongoDB C Driver's Windows-only Kerberos/SSPI authentication code can cause an undersized heap buffer allocation when an excessively long credential value is supplied in the client's connection configuration. A party controlling that configuration can cause the driver to write outside the allocated buffer. This may corrupt client process memory and cause denial of service of the client application.

The MongoDB C# Driver's BSON library contains an uncontrolled resource consumption weakness when deserializing one BSON value type. An application deserializing untrusted content retains derived data for the process lifetime with no limit. An unauthenticated remote party can drive unbounded memory growth, exhausting available memory and denying service.