Format String in Cassandra

Apache Cassandra is written in Java, so classic C‑style format string bugs are rare, but they can still appear when developer‑controlled strings are passed to Java’s formatting APIs such as String.format, java.util.Formatter, or logging frameworks that ultimately use those APIs. In Cassandra, a typical vulnerable path is the handling of error messages or audit logs where user‑supplied values (e.g., a column name from a CQL statement) are inserted into a format string without validation.

For example, a custom user‑defined function (UDF) or a Java‑based storage handler might log an unexpected condition like this:

String msg = String.format("Unexpected value for column %s: %s", columnName, value);
logger.error(msg);

If columnName comes directly from a client‑provided CQL query and contains format specifiers such as %n (newline) or %s (string), the formatter will treat them as directives, potentially reading from the stack or causing a denial‑of‑service. An attacker could inject %n%n%n to generate excessive output or leak memory contents through the formatter’s internal buffer.

Although no public CVE directly ties a format string flaw to Apache Cassandra, the same pattern has been exploited in other Java projects (e.g., CVE-2014-0050 in Apache Struts2). In Cassandra, the risk is heightened when:

• Custom UDFs are deployed and accept raw user input.
• Audit logging or error reporting uses String.format with unsanitized fields.
• Third‑party plugins that interface with Cassandra’s native transport pass user data to format methods.

Understanding these code paths helps defenders focus on input validation and safe formatting practices.

middleBrick’s black‑box scanner includes an Input Validation check that actively probes for format string leakage. When scanning a Cassandra endpoint (e.g., the native binary protocol port 9042 or a REST gateway), the engine sends a series of crafted payloads containing format specifiers (%s, %n, %08x, etc.) in parameters that are reflected in error messages or log output.

If the reflector returns a response where the payload appears unchanged or where the response length changes anomalously (indicating the formatter consumed stack data), middleBrick flags the finding as a potential format string vulnerability. The scanner does not need source code; it observes the behavior from the outside.

Example of a probing request sent by middleBrick to a Cassandra REST endpoint that returns query errors:

GET /api/v1/query?cql=SELECT%20*%20FROM%20users%20WHERE%20name%20%3D%20'%25s%25s%25s' HTTP/1.1
Host: example.com

If the server replies with an error message that still contains the literal %s%s%s (or shows a stack trace indicating a java.util.MissingFormatArgumentException), the scanner records:

• Severity: Medium
• Category: Input Validation → Format String
• Evidence: Reflected format specifier in error response
• Remediation Guidance: Avoid passing user input directly to String.format or similar APIs.

Because the test is unauthenticated and runs in 5–15 seconds, it fits middleBrick’s scanning model without requiring agents or credentials.

The fix is to ensure that any user‑supplied data is never used as the format pattern itself. Instead, treat the data as an argument to the formatter, or switch to a logging framework that uses placeholder syntax (e.g., SLF4J’s {}) which does not interpret the input as a format string.

Vulnerable code (illustrative, not from Cassandra core):

// Vulnerable: userInput used as format string
public void logColumn(String userInput, Object value) {
    String msg = String.format("Column %s has value %s", userInput, value);
    logger.error(msg);
}

Remediated version:

// Safe: userInput is passed as an argument, not as the format pattern
public void logColumn(String userInput, Object value) {
    String msg = String.format("Column %s has value %s", userInput, value);
    logger.error(msg);
}

If the intent was to log the column name directly, use a constant format string:

// Safe: constant format string, userInput as argument
public void logColumn(String userInput, Object value) {
    String msg = String.format("Column %s has value %s", userInput, value);
    logger.error(msg);
}

Alternatively, with SLF4J:

logger.error("Column {} has value {}", userInput, value);

For audit logs that must avoid any formatting interpretation, consider escaping percent signs before passing the string to String.format:

String safeInput = userInput.replace("%", "%%");
String msg = String.format("Value: %s", safeInput);

These changes eliminate the format string vector while preserving the intended logging or error‑reporting functionality. middleBrick will rescan the endpoint and, upon confirming that reflected format specifiers are no longer processed, update the security score accordingly.