[LOG4J2-439] Create a LogEventPatternConverter to escape newlines and HTML special characters

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.0-beta9
• Fix Version/s: 2.0-rc2
• Component/s: Layouts
• Assignee: Ralph Goers
• Reporter: Bruce Brouwer
• Created: 28/Oct/13 21:38
• Updated: 08/Mar/16 16:45
• Resolved: 01/May/14 06:07

Description

To prevent log forging and HTML based attacks from viewing logs in a browser, we could add a LogEventPatternConverter that escapes newlines and HTML special characters. ESAPI has a method to do this, but it doesn't have any of the nice API features that Log4j 2 has.

I was able to create a LogEventPatternConverter to do this. Note, this is only a proof of concept. I didn't try to exhaustively list all the special characters that might need to be replaced. I also didn't provide any configuration so we could choose to not escape HTML, for example.

With this configuration:

<PatternLayout pattern="%d %-5p [%t] %c %encode{%m}%n"/>

And logging this message:

LOG.warn("hi\n & <h1> there");

Would result in this being logged:

2013-10-28 16:31:21,606 WARN  [main] example.Test hi\n & <h1> there 

instead of this (which shows the potential for log forging):

2013-10-28 16:31:21,606 WARN  [main] example.Test hi
& <h1> there 

This is roughly the code I used:

@Plugin(name = "escape", category = "Converter")
@ConverterKeys({ "escape" })
public final class EscapingReplacementConverter extends LogEventPatternConverter {

    private final List<PatternFormatter> formatters;

    private EscapingReplacementConverter(final List<PatternFormatter> formatters) {
        super("escape", "escape");
        this.formatters = formatters;
    }

    public static EscapingReplacementConverter newInstance(final Configuration config,
                                                           final String[] options) {
        if (options.length != 1) {
            LOGGER.error("Incorrect number of options on escape. Expected 1, received "
                    + options.length);
            return null;
        }
        if (options[0] == null) {
            LOGGER.error("No pattern supplied on escape");
            return null;
        }
        final PatternParser parser = PatternLayout.createPatternParser(config);
        final List<PatternFormatter> formatters = parser.parse(options[0]);
        return new EscapingReplacementConverter(formatters);
    }

    @Override
    public void format(final LogEvent event, final StringBuilder toAppendTo) {
        final StringBuilder buf = new StringBuilder();
        for (final PatternFormatter formatter : formatters) {
            formatter.format(event, buf);
        }
        toAppendTo.append(buf.toString()
                             .replaceAll("\\r", "\\\\r")
                             .replaceAll("\\n", "\\\\n")
                             .replaceAll("&", "&amp;")
                             .replaceAll("<", "&lt;")
                             .replaceAll(">", "&gt;"));
    }
}

If this sounds good, I would like to hear feedback and ideas on how to make this better. I will then contribute this to the project. Do you think this could this get in 2.0?

Issue Links

• is duplicated by: LOG4J2-1203 Allow filtering of line breaks in layout pattern