hMailServer: failed database writes reported as success, losing mail and bans

Misinterpretation of SQL statement text in the database layer of Progressive Robot hMailServer 6.0.0-B1 through 6.3.7 allows a remote unauthenticated attacker to have a failed database write reported to the server as successful. The layer answers a failed statement as a success when it finds the marker [IGNORE-ERRORS], which is meant only for the server's own schema statements (its upgrade scripts and the database log's tables), and statements carry values from outside - an envelope address, the user name of a failed logon, a message's headers - in their text on MySQL/MariaDB and PostgreSQL, and in the batches of the database log on every engine. Up to 6.2.18 the marker was honoured anywhere in a statement, a value included, on MySQL/MariaDB and PostgreSQL. From 6.2.19 it was honoured only inside a comment, but the layer read a backslash inside a string literal as an escape, which SQL Server, SQL Server Compact and PostgreSQL with standard_conforming_strings on (its default) do not, so that on those engines a value holding a backslash could make the layer take part of the statement's literals for statement text and find the marker there: on PostgreSQL from 6.3.3 for every statement carrying an outside value that holds a backslash, with no setting changed for a message delivered into a local mailbox (through an internationalised (SMTPUTF8) envelope sender) and for the IP range an auto-ban writes (named after the user name of the failed logon), and on SQL Server and SQL Server Compact for the database log, when the server logs to the database (logging is off, and the log device is files, by default). A statement run outside a transaction that then failed - a constraint, a deadlock, a lock or statement timeout, a full disk, and up to 6.2.18 a lost connection - was answered as a success, and its caller went on as if the row had been written: a message delivered into a local mailbox could be counted as delivered with no row to show it and no failure notice to its sender, and a message accepted with 250 could be left with no queue row and never delivered (up to 6.2.18, and on PostgreSQL from 6.3.3 when every recipient's address carries the marker too, which needs an address that takes any local part); an auto-ban whose IP range was not written could be logged and counted as applied, with no error reported, while the address stayed unbanned; and message metadata (written only when message indexing is on), entries of the database log and rows of features an administrator turns on could be lost without an error. On MySQL/MariaDB up to 6.2.18 the caller was handed whatever identifier the client library still held for the connection - 0, or that of an insert its last successful statement made, in whatever table - under which a queued message's recipients were then written, so that another message still in the queue could be delivered to them as well. This flaw gives a value no way to change what a statement does, and no privilege is gained.