The flaw exists because PyMongo decodes percent‑encoded characters in the host portion of a connection string before it splits the host list on its delimiters. When an application inserts a hostname that an unauthenticated party can influence into the connection string, the driver may add additional, attacker‑controlled servers to its database client. As the client initializes authentication and sends database operations, those rogue servers can observe limited request data and return crafted responses, allowing an attacker to alter query results or glean sensitive information. This enables unauthorized data manipulation and potential data leakage.

MongoDB’s official Python driver, PyMongo, is impacted when it processes host values containing percent‑encoded delimiters prior to host splitting. All driver releases issued before the 4.18.2 update are vulnerable; the 4.18.2 release onward includes the vendor patch that prevents the premature decoding of host strings.

The CVSS score of 8.3 marks this vulnerability as high severity. The EPSS score of 0.00257 (< 1%) indicates a very low probability of exploitation, and it is not listed in the CISA KEV catalog. The flaw can be exploited remotely if an attacker can supply a hostname into application configuration or user‑generated data. In such scenarios the attack vector is likely HTTP or configuration‑based injection, with the attacker able to redirect the database client to a malicious server.