feat: new crlfFilter appender to prevent log forging

docs/appenders.md

@@ -23,6 +23,7 @@ The following appenders are included with log4js. Some require extra dependencie
 
 - [categoryFilter](categoryFilter.md)
 - [console](console.md)
+- [crlfFilter](crlfFilter.md)
 - [dateFile](dateFile.md)
 - [file](file.md)
 - [fileSync](fileSync.md)

docs/crlfFilter.md

@@ -0,0 +1,36 @@
+# CRLF Filter
+
+This is not strictly an appender - it wraps around another appender and to prevent log forging by replacing starting the line with a `>` prefix after line-breaks.
+
+## Configuration
+
+- `type` - `"crlfFilter"`
+- `appender` - `string | Array<String>` - the name of the appender to filter. (exists for backward compatibility)
+- `appenders` - `string | Array<String>` - the names of the appenders to filter.
+
+## Example
+
+```javascript
+log4js.configure({
+  appenders: {
+    everything: { type: "file", filename: "all-the-logs.log" },
+    crlfFilter: {
+      type: "crlfFilter",
+      appenders: ["everything"],
+    },
+  },
+  categories: {
+    default: { appenders: ["crlfFilter"], level: "debug" },
+  },
+});
+
+const logger = log4js.getLogger();
+logger.info(
+  "Some debug messages\n[2023-01-17T11:58:38.150] [INFO] default - Log forging"
+);
+```
+
+```
+[2023-01-17T11:58:38.150] [INFO] default - Some debug messages
+> [2023-01-17T11:58:38.150] [INFO] default - Log forging
+```

lib/appenders/crlfFilter.js

@@ -0,0 +1,47 @@
+const debug = require('debug')('log4js:crlfFilter');
+
+function crlfFilter(appenders) {
+  return (logEvent) => {
+    const filteredData = logEvent.data.map((i) => {
+      if (typeof i.replace === 'function') return i.replace(/\n/g, '\n> '); // add a > prefix
+      return i;
+    });
+    logEvent.data = filteredData;
+    appenders.forEach((appender) => {
+      appender(logEvent);
+    });
+  };
+}
+
+function configure(config, layouts, findAppender) {
+  const addToSet = function (input, set = new Set()) {
+    const addAppender = function (appenderName) {
+      debug(`actual appender is ${appenderName}`);
+      const appender = findAppender(appenderName);
+      if (!appender) {
+        debug(`actual appender "${config.appender}" not found`);
+        throw new Error(`crlfFilter appender "${config.appender}" not defined`);
+      } else {
+        set.add(appender);
+      }
+    };
+
+    if (input && !Array.isArray(input)) input = [input];
+    if (input) input.forEach((appenderName) => addAppender(appenderName));
+
+    return set;
+  };
+
+  const appenders = new Set();
+  addToSet(config.appender, appenders);
+  addToSet(config.appenders, appenders);
+
+  if (appenders.size === 0) {
+    debug(`no appender found in config ${config}`);
+    throw new Error('crlfFilter appender must have an "appender" defined');
+  }
+
+  return crlfFilter(appenders);
+}
+
+module.exports.configure = configure;