CVE-2026-81521 in Go Driver
Summary
by MITRE • 08/27/2026
The MongoDB Go Driver's client-level bulk write operation may accept a caller-supplied database name containing a reserved separator character without escaping it before the name is used to build the target namespace for the operation. An application that passes untrusted input as a database name could therefore have the write directed at a database and collection other than the ones it intended. Only the Client.BulkWrite API is affected.
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Analysis
by VulDB Data Team • 08/27/2026
The MongoDB Go Driver contains a critical logic flaw within its client-level bulk write operation, specifically affecting only the BulkWrite API method. This vulnerability arises from an insufficient validation of user-supplied input when constructing database names for targeted operations. In standard MongoDB usage, database and collection identifiers are typically alphanumeric strings that do not contain special characters. However, the driver fails to properly escape or sanitize reserved separator characters embedded within a caller-provided database name before this string is concatenated with other components to form the target namespace. This lack of input sanitization creates a pathway for an attacker who can control the database name parameter to manipulate the destination of write operations.
From a technical perspective, MongoDB namespaces are constructed by joining the database name and collection name using a specific separator character, usually a dot or similar delimiter depending on the protocol version and driver implementation details. When the BulkWrite function accepts a database name containing this reserved separator without escaping it, the resulting namespace string becomes ambiguous to the underlying communication layer. The driver interprets the input literally as part of the identifier structure rather than treating the entire string as a single logical unit for the database component. Consequently, any special characters present in the unescaped database name are interpreted as structural delimiters within the MongoDB wire protocol message. This misinterpretation causes the client to direct write operations toward an unintended target namespace that differs from what was explicitly specified by the application developer or user input.
The operational impact of this vulnerability is severe and directly relates to data integrity and confidentiality risks associated with unauthorized access and modification. An attacker who can inject a database name containing reserved separator characters into a BulkWrite call could redirect writes to arbitrary databases and collections within the same MongoDB instance. This capability effectively bypasses intended access controls if those controls are scoped per-database or per-collection, as the write operation lands in a location where the application may not have expected it to occur. In scenarios involving multi-tenant architectures or applications with strict data isolation requirements between different logical units, this flaw could lead to cross-tenant data leakage or corruption. The attacker does not need to exploit a separate injection vulnerability such as SQLi; rather, they leverage the driver's own parsing logic against itself to achieve namespace confusion and unauthorized write access.
This issue aligns with CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component, specifically where special elements are not properly escaped before being passed to another component that interprets them structurally. It also maps closely to the ATT&CK technique T1059 Command and Scripting Interpreter if viewed through the lens of manipulating system commands via input injection, though more accurately it falls under data manipulation techniques like T1486 Data Encrypted for Impact or T1565 Data Manipulation depending on the attacker's ultimate goal. The core failure is a lack of strict validation against expected character sets and proper encoding of special characters that have semantic meaning in the target protocol format.
Mitigation strategies must focus on rigorous input validation at the application layer before passing data to the driver, as well as potential updates to the driver itself if available. Developers should implement allow-listing for database names, ensuring that only alphanumeric characters and safe hyphens or underscores are permitted, thereby excluding any reserved separator characters entirely from user-supplied inputs. If dynamic naming is required based on untrusted input, strict regex validation must be applied to reject any string containing dots or other namespace delimiters. Additionally, organizations should review their use of the BulkWrite API in contexts where database names originate from external sources such as HTTP parameters, configuration files, or message queues. Upgrading to a patched version of the MongoDB Go Driver that implements proper escaping mechanisms for reserved characters is essential if allow-listing is not feasible due to application requirements. Security teams should also audit existing codebases for similar patterns in other driver methods where namespace construction occurs without adequate sanitization checks.