Defense Against Dark Arts

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Defense Against Dark Arts

# 1. SQLite Always Validates Its Inputs

SQLite should never crash, overflow a buffer, leak memory, or exhibit any other harmful behavior, even when presented with maliciously malformed SQL inputs or database files. SQLite should always detect erroneous inputs and raise an error, not crash or corrupt memory. Any malfunction caused by an SQL input or database file is considered a serious bug and will be promptly addressed when brought to the attention of the SQLite developers. SQLite is extensively fuzz-tested to help ensure that it is resistant to these kinds of errors.

Nevertheless, bugs happen. If you are writing an application that sends untrusted SQL inputs or database files to SQLite, there are additional steps you can take to help reduce the attack surface and prevent zero-day exploits caused by undetected bugs.

## 1.1. Untrusted SQL Inputs

Applications that accept untrusted SQL inputs should take the following precautions:

1. Set the SQLITE_DBCONFIG_DEFENSIVE flag. This prevents ordinary SQL statements from deliberately corrupting the database file. SQLite should be proof against attacks that involve both malicious SQL inputs and a maliciously corrupted database file at the same time. Nevertheless, denying a script-only attacker access to corrupt database inputs provides an extra layer of defense.

2. Reduce the limits that SQLite imposes on inputs. This can help prevent denial of service attacks and other kinds of mischief that can occur as a result of unusually large inputs. You can do this either at compile-time using -DSQLITE_MAX_... options, or at run-time using the sqlite3_limit() interface. Most applications can reduce limits dramatically without impacting functionality. The table below provides some suggestions, though exact values will vary depending on the application:

   Limit Setting | Default Value | High-security Value
   --- | --- | ---
   LIMIT_LENGTH | 1,000,000,000 | 1,000,000
   LIMIT_SQL_LENGTH | 1,000,000,000 | 100,000
   LIMIT_COLUMN | 2,000 | 100
   LIMIT_EXPR_DEPTH | 1,000 | 10
   LIMIT_COMPOUND_SELECT | 500 | 3
   LIMIT_VDBE_OP | 250,000,000 | 25,000
   LIMIT_FUNCTION_ARG | 127 | 8
   LIMIT_ATTACH | 10 | 0
   LIMIT_LIKE_PATTERN_LENGTH | 50,000 | 50
   LIMIT_VARIABLE_NUMBER | 999 | 10
   LIMIT_TRIGGER_DEPTH | 1,000 | 10

3. Consider using the sqlite3_set_authorizer() interface to limit the scope of SQL that will be processed. For example, an application that does not need to change the database schema might add an sqlite3_set_authorizer() callback that causes any CREATE or DROP statement to fail.

4. The SQL language is very powerful, and so it is always possible for malicious SQL inputs (or erroneous SQL inputs caused by an application bug) to submit SQL that runs for a very long time. To prevent this from becoming a denial-of-service attack, consider using the sqlite3_progress_handler() interface to invoke a callback periodically as each SQL statement runs, and have that callback return non-zero to abort the statement if the statement runs for too long. Alternatively, set a timer in a separate thread and invoke sqlite3_interrupt() when the timer goes off to prevent the SQL statement from running forever.

5. In extreme cases, consider compiling SQLite with the -DSQLITE_ENABLE_MEMSYS5 option and then providing SQLite with a fixed chunk of memory to use as its heap via the sqlite3_config(SQLITE_CONFIG_HEAP) interface. This will prevent malicious SQL from executing a denial-of-service attack by using an excessive amount of memory. If (say) 5 MB of memory is provided for SQLite to use, once that much has been consumed, SQLite will start returning SQLITE_NOMEM errors, rather than soaking up memory needed by other parts of the application. This also sandboxes SQLite's memory so that a write-after-free error in some other part of the application will not cause problems for SQLite, or vice versa.

## 1.2. Untrusted SQLite Database Files

Applications that accept untrusted database files should do the following:

1. Run PRAGMA integrity_check or PRAGMA quick_check on the database as the first SQL statement after opening the database files and prior to running any other SQL statements. Reject and refuse to process any database file containing errors.

2. Enable the PRAGMA cell_size_check=ON setting.

3. Do not enable memory-mapped I/O. In other words, make sure that PRAGMA mmap_size=0.

4. A maliciously crafted database might be able to inject SQL by defining new triggers or views in the schema that the application does not anticipate. There are multiple defenses:

   1. If the application does not use triggers or views, then disable the unused capabilities using:

      >     sqlite3_db_config(db,SQLITE_DBCONFIG_ENABLE_TRIGGER,0,0);
      >     sqlite3_db_config(db,SQLITE_DBCONFIG_ENABLE_VIEW,0,0);

   2. If the application does use triggers or views, then use queries to scan the sqlite_master table to verify that the triggers and views found there are expected, that there are no surplus triggers or views, existing triggers and views have not been tampered with, and that no existing ordinary tables have been replaced by malicious views.

   3. If the application uses application-defined SQL functions that have side effects, then it is recommended to set the SQLITE_DIRECTONLY flag on those SQL functions to prevent them from being used inside triggers and views. To illustrate the importance of this flag, consider an application that implements an SQL function "send_money(...)". Without the SQLITE_DIRECTONLY flag, an attacker might be able to add a trigger or view that uses that custom function, then trick a high-privilege application to run an otherwise harmless query that invokes that malicious trigger or view. The SQLITE_DIRECTONLY flag prevents the attack by requiring the "send_money()" function to be invoked directly by the application, rather than indirectly through a trigger or view.

Even if the application does not deliberately accept database files from untrusted sources, beware of attacks in which a local database file is surreptitiously altered to contain harmful content.

# 2. Summary

The precautions above are not required in order to use SQLite safely with potentially hostile inputs. However, they do provide an extra layer of defense against zero-day exploits and are encouraged for applications that pass data from untrusted sources into SQLite.