CRLF stands for Carriage Return (CR) and Line Feed (LF), two control characters used to signify the end of a line in network protocols like HTTP. When combined as \r\n, they instruct web servers and browsers to end a header line and begin a new one, forming the structure of HTTP response headers.

CRLF Injection exploits the way these characters are interpreted by web servers. When user input isn't properly sanitized, attackers can insert \r\n into query parameters or HTTP headers to manipulate the server’s response. This opens the door to HTTP response splitting, session fixation, web cache poisoning, and even cross-site scripting through indirect paths.

Although not as widely publicized as XSS or SQL Injection, CRLF Injection presents a serious risk. It undermines the integrity of web communication and, in some cases, can chain into more dangerous exploits. Left unaddressed, this vulnerability gives attackers control over how content is delivered to users—but how often are you checking for it?

Understanding CRLF Injection: How Newline Characters Compromise Web Applications

Defining CRLF Injection in Technical Terms

CRLF injection is a type of security vulnerability that occurs when an attacker manipulates newline characters—specifically Carriage Return (CR, \r) and Line Feed (LF, \n)—into server responses. By injecting these characters into HTTP headers or other structured outputs, the attacker can alter the format of a server response, potentially rendering unintended content or creating new headers entirely.

The term “CRLF” originates from legacy operating system conventions. In the context of HTTP, CRLF denotes the end of a header line and the start of either a new header or the message body. According to the HTTP/1.1 specification (RFC 7230), each header line must end with a CRLF sequence.

CRLF Characters in HTTP Response Structuring

Web servers and proxies construct HTTP responses using a well-defined structure where headers are separated by CRLF sequences. Each header line follows the pattern:

Header-Name: Header-Value\r\n

At the end of the headers section, a double CRLF (\r\n\r\n) signals that the body content follows. These sequences are not meant to be influenced by user input; however, when input validation fails, they can be inserted maliciously.

How Improper Input Handling Enables CRLF Injection

The vulnerability emerges when a web application accepts user-form input and directly incorporates it into response headers without encoding or sanitization. This unchecked input facilitates injection of raw CRLF sequences, disrupting the original structure. Attackers exploit this to:

• Inject new HTTP headers
• Spoof redirect responses (HTTP response splitting)
• Perform cache poisoning
• Execute cross-site scripting (XSS) in advanced chaining scenarios

Most modern servers prohibit direct multiline values in headers from regular input; however, misconfigurations or legacy frameworks still make these attacks feasible.

Real-World Analogy: The Tampered Mail Envelope

Imagine a postal envelope with a printed label for the recipient. An attacker finds a way to insert extra commands like "Return to sender" or “Forward to another address” by exploiting a weakness in the label’s printer. The machine stops reading after a line break—so the attacker embeds hidden instructions after a newline. Similarly, CRLF injection silently modifies server headers beyond what developers intended.

Injection Vulnerabilities: A Persistent Web Security Threat

Understanding the Broader Class of Injection Attacks

Injection-based vulnerabilities have consistently occupied a top position in web security threat models due to their capacity to expose sensitive data, disrupt application logic, or grant unauthorized access. At their core, these vulnerabilities exploit inadequate input handling, where untrusted user data is parsed and executed as part of commands or queries. The attacker manipulates this behavior by injecting malicious content.

The most recognized example is SQL Injection, where malicious SQL commands alter database queries. Others include Command Injection, targeting operating system commands, and LDAP Injection, focused on directory services. Each variation adapts to the protocol involved—reflecting how injection attacks transcend platforms and inputs.

Where CRLF Injection Fits

CRLF Injection belongs to this broader family but specifically targets the manipulation of HTTP headers through the insertion of carriage return (\r) and line feed (\n) characters. These characters, when used out of context, terminate headers and allow attackers to forge or inject new headers into the server’s response. This behavior can escalate to attacks like HTTP response splitting and cross-site scripting (XSS).

While it may not wield the same immediate impact as SQL Injection, CRLF Injection enables attackers to reshape client-server communication invisibly. It can lead to exploited cache poisoning, redirect manipulation, or cookie tampering—each a vector into deeper exploitation.

Persistent Spotlight in the OWASP Top Ten

The OWASP Top Ten 2024 classifies injection flaws under the listed category “A03:2021 – Injection.” Despite the evolving threat landscape, injection stays on this list every cycle, underscoring its severity and prevalence. CRLF Injection, though a more specific variant, is addressed under this overarching category, given its ability to subvert standard HTTP parsing behavior.

Organizations adhering to secure coding standards and automated input sanitization frameworks still face CRLF risks when legacy systems, custom header logic, or overlooked input vectors come into play. This places CRLF Injection firmly on the radar of penetration testers, security engineers, and threat analysts alike.

Dissecting CRLF Injection: Mechanics Behind the Exploit

Breaking Down the Attack Flow

CRLF injection exploits revolve around manipulating how a web server constructs headers or log content. The goal: inject malicious input containing special characters—carriage return (\r, ASCII 13) and line feed (\n, ASCII 10)—which are interpreted by the server to split content into unintended segments.

Here's what happens step by step:

• Step 1: An attacker finds an unsanitized input field. This could be a form parameter, a query string, or part of a URL path.
• Step 2: The malicious payload is submitted. For example, the attacker crafts input that injects %0D%0A (URL-encoded \r\n).
• Step 3: The server processes the input and includes it directly in response headers or logs, without sanitizing or encoding it.
• Step 4: The CRLF characters force the server to interpret the injected content as new headers or log entries.

Example of an Unvalidated Input Scenario

Consider this PHP snippet that sets a location header based on user input:

<?php
  $redirect = $_GET['url'];
  header("Location: " . $redirect);
?>

A URL like https://example.com/redirect.php?url=%0D%0ASet-Cookie:%20malicious=1 results in the server sending:

HTTP/1.1 302 Found
Location: 
Set-Cookie: malicious=1

The injected CRLF sequence ends the original header line and introduces a new one—Set-Cookie in this case, manipulated by the attacker. Browsers may process this just like a legitimate cookie header.

When HTTP Response Splitting Happens

CRLF injection serves as a gateway to HTTP Response Splitting. Once the attacker controls line breaks in response headers, they gain the ability to:

• Insert Arbitrary Headers: Manipulate headers such as Set-Cookie, Content-Type, or Cache-Control.
• Trigger Redirection Attacks: Redirect users to attacker-controlled domains via falsified Location headers.
• Launch Header-Based XSS: Inject JavaScript through malformed Content-Type or Refresh headers and bypass usual client-side mitigations.

This exploitation vector becomes particularly dangerous when combined with cross-site scripting. By inserting scripts into headers processed by browsers, attackers sidestep content filters and same-origin policies, opening a path to session hijacking or data theft.

How Attackers Exploit CRLF Injection: Use Cases and Real Scenarios

Exploiting HTTP Response Splitting to Inject Fake Headers

CRLF injection commonly enables HTTP response splitting—a tactic that allows attackers to interfere with how browsers interpret server responses. When an application reflects unsanitized input directly into HTTP headers, inserting %0D%0A (URL-encoded carriage return and line feed) breaks the response structure.

For example, imagine a URL like https://targetsite.com/welcome?name=John. If the server generates an HTTP header like Content-Disposition: inline; filename="John" based on this input, an attacker could modify the request:

https://targetsite.com/welcome?name=John%0D%0ASet-Cookie:%20sessionid=steal

The injected Set-Cookie header forces the browser to store a malicious cookie. Any following pages on the site may then use this attacker-crafted cookie, creating a basis for session fixation or hijacking.

Tampering with Server Logs: Log Poisoning Attacks

Another practical scenario involves log poisoning. Many web servers log HTTP requests along with headers or parameters. When input is not sanitized, an attacker can inject entries that alter the formatting of server logs, insert false information, or truncate log content.

For instance, a crafted request like:

GET /index.html?user=%0D%0A[Error]:%20Unauthorized%20access HTTP/1.1

can lead to log entries such as:

[2024-04-18] Request from 192.168.0.5: [Error]: Unauthorized access

Security analysts reviewing logs may misunderstand the injected line as a legitimate system error. With enough injections, attackers can mask their tracks or trigger false alarms to divert attention.

Compromising Cache Integrity with Cache Poisoning

Many web caching systems make content decisions based on HTTP headers. When an attacker manipulates headers via CRLF injection, cache poisoning becomes possible. By injecting a Location or even a Content-Type header, attackers can poison a shared cache with malicious responses.

Take a request like this:

https://trustedsite.com/page?lang=en%0D%0ALocation:%20https://evil.com

If the application reflects this input into response headers and the response is cached, subsequent visitors to the original site could be redirected to the attacker's domain—without any DNS-level tampering.

Simulated Attack: CRLF Injection Exploit in Action

Consider a test endpoint that echoes a user name in the response headers:

curl -i "https://vulnerable.example.com/greet?name=Alice"

returns:

HTTP/1.1 200 OK Content-Type: text/html X-User: Alice

Now, inject a CRLF sequence with an arbitrary header:

curl -i "https://vulnerable.example.com/greet?name=Bob%0D%0AX-Admin: true"

The modified response becomes:

HTTP/1.1 200 OK Content-Type: text/html X-User: Bob X-Admin: true

In systems that make authorization decisions based on header data, this could have operational impact. Although a simple simulation, it demonstrates how CRLF sequences break the integrity of server responses.

Where CRLF Injection Slips In: Key Entry Points

CRLF injection flaws don’t appear out of thin air—they originate from predictable, often overlooked sources. Patterns in application design and user input handling expose clear pathways for these attacks. Understanding these entry points lets security professionals pinpoint high-risk surfaces and harden applications accordingly.

Unvalidated User Input: The Core Entry Vector

Any time user input feeds directly into a response without sanitization, the door opens for injected carriage return and line feed characters.

• URLs and query parameters: If a URL parameter like ?redirect= accepts user-defined targets without filtering, an attacker can slip in encoded CR (%0D) or LF (%0A) sequences. This allows header injection or manipulation of downstream responses.
• Form fields: Fields such as comment boxes, username inputs, or feedback messages—especially when surfaced in HTTP headers or response content—can carry malicious CRLF payloads if the application doesn’t sanitize values before reflection.

HTTP Headers: A Breeding Ground for Header Injection

Web servers build response headers with help from incoming values. When these values come from users and aren’t strictly controlled, CRLF injection can shift the structure of the entire response.

• User-Agent: Sent with every HTTP request, the User-Agent string may be logged, echoed in diagnostics, or appear in response headers. If an application directly uses it without escaping or filtering, it can serve as a point of injection.
• Referer: Much like the User-Agent, the Referer header is often incorporated into analytics, debug output, or headers for security enforcement. Injecting CRLF sequences here lets an attacker tamper with headers issued to clients or downstream proxies.
• Custom input passed to redirect or content headers: Applications that dynamically build Location: headers (typically in response to redirects) with user input become exposed when special characters like carriage returns and line feeds go unchecked. Similar implications arise when custom HTTP headers take values derived from front-end forms or JavaScript APIs.

Ever followed a link and seen your browser redirect unexpectedly or present a fake login page? Chances are, a vulnerable redirect mechanism was manipulated using CRLF injection. This technique thrives where developers trust input too much and validate too little.

Unpacking CRLF Injection by Linking It to Related Vulnerabilities

CRLF injection rarely operates in isolation. Attackers often combine it with other vulnerabilities to amplify its impact. When a response header or log file gets manipulated, the entry point might be a CRLF sequence, but supporting vulnerabilities often carry the payload. Understanding these interconnections reveals the wider attack surface and clarifies why mitigating CRLF requires a broader view of application security.

Cross-Site Scripting (XSS)

While XSS primarily deals with injecting malicious scripts into the client-side context, CRLF can assist by providing a foothold in the HTTP response structure. In certain server environments, injecting a CRLF sequence allows an attacker to smuggle in headers such as Content-Type: text/html, followed by an injected script-containing body. When the browser encounters this manipulated sequence, it interprets the body as HTML, enabling script execution.

• CRLF alters the response boundary, creating space for XSS payloads.
• The injected payload can bypass client-side XSS defenses that rely only on HTML-based sanitization.

Here, CRLF doesn't exploit the DOM directly but reshapes how the server delivers the response, enabling malicious browser-side behaviors.

Header Injection

HTTP headers use CR (carriage return) and LF (line feed) to separate individual headers and mark the end of the header section. By inserting %0d%0a, an attacker can fake a new header block. If the application’s input isn't sanitized, this results in unsolicited headers being interpreted as legitimate.

• Fake Set-Cookie headers can overwrite or add unauthorized cookies.
• Injected headers like Location can lead to redirects to malicious domains.

Header injection introduces broader control over response behavior, which compounds the risks associated with CRLF injection.

Log Poisoning

Many systems automatically log user input—URLs, headers, or parameters—directly into server logs without structure validation. When CRLF characters are inserted into these inputs, the attacker can manipulate the format and content of the logs themselves.

• Injection of false log entries makes incident analysis unreliable.
• Malicious strings in logs can be designed for reverse injection into loggers or dashboards.

Log poisoning changes logs from static records into attack vectors, often used in later phases of a compromise or to confuse defenders.

URL Encoding and Filter Evasion

Security filters that block direct CRLF sequences can be sidestepped using percent-encoding. Since the ASCII characters for carriage return and line feed are 0x0d and 0x0a respectively, encoding them as %0d%0a allows the sequence to pass through unsanitized.

• Encoded CRLF becomes functional when decoded by the server’s request parser.
• Web proxies or intermediate services may fail to sanitize the encoded form.

The URL-encoded variant of CRLF often becomes the first tool in bypassing simple input filters or flawed WAF (Web Application Firewall) configurations.

Detection: How to Find CRLF Injection Flaws

Manual Testing Using Crafted Payloads

Start by pinpointing input fields that interact with HTTP headers, log files, or response bodies. Then inject payloads that simulate CRLF sequences. For example, supplying %0d%0a—the URL-encoded form of a carriage return and line feed—lets you determine how the application handles raw line breaks.

Use controlled inputs like the following:

• Test 1: ?name=User%0d%0aSet-Cookie:evil=true — to check for HTTP response header injection.
• Test 2: ?redirect=%0d%0aLocation:https://attacker.com — to exploit open redirection with injected headers.
• Test 3: ?q=%0d%0aLog:injected-text — targeting log poisoning scenarios.

Observe the raw HTTP response. If the response includes additional headers, reflects the injected values, or returns malformed structure, the endpoint leaks control over header construction.

Automated Tools and Scanners

Several leading security tools integrate CRLF detection modules. Burp Suite Professional, for instance, includes active and passive scanning detections. When using Active Scanner, configure payload insertion points specifically on parameters suspected of influencing header output.

Other tools like OWASP ZAP, Nikto, and Nmap support CRLF injection detection as part of their web application assessment modules. For in-depth analysis, combine scanner outputs with manual review to reduce false positives.

Several GitHub-hosted scripts—such as crlf-injector or HeaderInjectionScanner—offer lightweight alternatives suited for CI pipelines or quick endpoint verification.

Analyzing Logs for Anomalies

CRLF injection often leaves fingerprints in application and server logs. Search for malformed entries that contain unintended line breaks or added headers. If a simple input results in split log entries, repeated user agents, or unplanned response codes, the application likely parses the string containing CRLF characters without sanitization.

Indicators include:

• Entries with embedded HTTP headers.
• Multiple log lines originating from a single request.
• Logs showing garbled output or broken formatting after parameterized input.

Review both access and error logs, especially after fuzzing or manual testing. Unexpected behaviors in logs map directly to injection vectors and help isolate the vulnerable endpoint.

Stop CRLF Injection at the Source: Proven Mitigation and Protection Strategies

Input Validation: Preempt Malicious Payloads

Unsanitized user input creates a shortcut for attackers. Validating inputs ensures they conform to expected formats, eliminating opportunities to introduce newline characters like \r (carriage return) and \n (line feed). The most effective approach uses allow-list filtering—also called whitelisting—where only known safe values are accepted.

• Whitelist example: Accepting only predefined HTTP methods such as "GET" or "POST" for a parameter intended to set the request method.
• Blacklist example: Trying to remove “\r” or “\n” characters manually. This often fails, as attackers can bypass with encoded or obfuscated variations.

Relying on blacklists introduces false negatives. Instead of attempting to neutralize all conceivable bad input, define exactly what good input looks like—anything else gets rejected.

URL Encoding: Neutralizing Special Characters

Sensitive input destined for headers or redirect locations must pass through a URL-encoding process. Encoding replaces special characters with percent-encoded representations (%0d for carriage return, %0a for line feed), neutralizing their ability to modify headers or inject responses.

For instance, if building a redirect URL using user-supplied query parameters, encode all values using built-in libraries before concatenating them with the target path. Never embed user data directly into HTTP headers or location strings without transformation.

Secure Coding Practices: Build with Defense in Mind

CRLF vulnerabilities frequently appear when developers accept user input and dynamically generate HTTP headers. This can produce unintended side effects. For example, the code snippet below exposes a common flaw:

response.addHeader("Location", userInput);

If userInput contains CRLF characters, the response header can be split into multiple lines, enabling header injection or even HTTP response splitting. Instead, use libraries or frameworks that safely manage values internally and perform automatic sanitization.

• In Java, use HttpServletResponse.encodeRedirectURL() before assigning user-generated content to headers or URLs.
• Frameworks like Express.js automatically handle many encoding responsibilities—avoid overriding default behavior with custom string concatenation.

Code defensively: assume any untrusted input is malicious until proven safe through strict validation and encoding.

Content Security Policies: Reduce Post-Injection Impact

While CSPs (Content Security Policies) do not prevent CRLF injection directly, they serve as a containment layer. If an attacker attempts to chain CRLF injection with XSS by injecting malicious scripts into a forged response, a well-defined CSP restricts what that script can do.

• Restrict the script-src directive to trusted domains only.
• Avoid allowing inline scripts by setting 'unsafe-inline' to none.
• Disallow mixed content by enforcing upgrade-insecure-requests.

With tight policies in place, successful injection attempts often result in script failures or incomplete attack execution. Treat CSP as a second line of defense, reinforcing other efforts around validation, encoding, and secure design.

Aligning CRLF Injection with the OWASP Top Ten

CRLF injection sits squarely within multiple categories of the OWASP Top Ten 2021, confirming its relevance in modern web security practice. The vulnerability doesn’t exist in isolation. Instead, it reflects broader patterns of architectural and operational weakness in applications and services.

A03:2021 – Injection

OWASP classifies injection flaws—like CRLF injection—under A03:2021 – Injection, which ranks as one of the most prevalent and impactful risks. In CRLF attacks, unsanitized user input directly manipulates HTTP headers or responses by injecting carriage return and line feed characters. This mirrors the definition OWASP uses for this category: untrusted data sent to an interpreter as part of a command or query. The server processes injected CRLF sequences as actual header delimiters, leading to response splitting, cache poisoning, and other attack outcomes.

When developers fail to validate, sanitize, or encode input before processing or rendering, injection vectors like CRLF become trivial to exploit. Applications that accept dynamic input without strict input handling are especially vulnerable.

A05:2021 – Security Misconfiguration

CRLF injection maps secondarily to A05:2021 – Security Misconfiguration. Web servers and application frameworks that respond with unstructured headers or improper status codes often lack hardened configurations. If response splitting is possible at all, that’s usually a sign of missing security headers, permissive proxies, or unintentionally exposed debug modes.

In this context, CRLF injection succeeds not because of a single coding error, but due to broader misconfiguration across layers—application, server, and network. Mismanaged error responses and overly permissive header policies extend the attack surface dramatically.

A09:2021 – Security Logging and Monitoring Failures

Successful exploitation of CRLF flaws often goes unnoticed, especially in environments lacking proper observability. This brings the attack within the scope of A09:2021 – Security Logging and Monitoring Failures. When attackers inject malicious headers or script content and those manipulations fail to generate logs or alerts, detection becomes nearly impossible.

Without centralized log analysis and header inspection, attempts to exploit CRLF issues silently degrade the integrity of the response stream. Moreover, a compromised log trail can cause incident response teams to miss the root cause entirely, or misattribute malicious activity to benign sources.

Embedding OWASP Categories into the Development Pipeline

A development workflow that aligns with OWASP Top Ten categories—not as checkboxes, but as deep design and testing principles—builds resistance to risks like CRLF injection. Use robust input validation libraries. Harden web server configurations. Ensure log visibility across headers, cookies, and status codes.

• Integrate category-specific threat models into sprint planning.
• Extend CI/CD pipelines with in-depth static and dynamic analyzers targeting header structures.
• Conduct security code and configuration reviews across all response-generating components.

CRLF injection reflects a breakdown in layered defense. OWASP Top Ten offers the taxonomy to trace those breakdowns and re-engineer the system for resilience.

Moving Beyond CRLF Injection: Lessons for Developers

CRLF injection operates quietly but impacts application behavior dramatically. A single unvalidated input can allow attackers to craft malicious HTTP responses, poison caches, or evade security mechanisms. The precision of the attack combined with its subtlety makes it a persistent threat in web infrastructure.

Developers control the first line of defense. The security posture of an application starts at the keyboard—at the intersection of syntax and intent. Decisions made when handling user input, generating headers, logging data, or constructing redirects dictate exposure levels.

What Developers Can Do Immediately

• Write secure code: Adopt a whitelist approach when validating inputs. Treat every input as untrusted, especially those that originate from external requests, HTTP headers, or query parameters.
• Test more aggressively: Go beyond unit tests. Integrate security-focused test cases in CI/CD pipelines using tools that simulate malicious payloads and check for response splitting or header injection.
• Embed security into your knowledge base: Learn beyond the known. Attend secure coding workshops, subscribe to vulnerability databases, and explore RFC specifications that define how headers and line breaks behave in HTTP protocols.

Take action today. Review common injection surfaces in your stack—look at how headers are sent, how cookies are set, how logs are written. Rethink validation logic around newline characters. Misplaced trust in input boundaries leads to compromised systems.

After reading this, which parts of your application pose hidden risks? Where does user input control headers or response structure? Don't postpone the audit. Attackers won't.