CVE-2026-18174 Vulnerability Analysis & Exploit Details

CVE-2026-18174 Vulnerability Summary

CVE-2026-18174: @fastify/forwarded resolves client addresses from the X-Forwarded-For header. In versions before 3.0.2, when the header contains two or more comma separated entries, the parser trims only space characters and does not strip horizontal tabs, even though RFC 7230 defines optional whitespace as both space and tab. As a result, an entry padded with a tab keeps the literal tab in the resolved address string. Applications that make exact string match security decisions on the resolved client IP, such as an allowlist, a blocklist, a per IP rate limit key, or audit log correlation, can be evaded because the tab corrupted string no longer matches the expected value. This does not cross the trust boundary, since a tab corrupted string is not a valid IP and cannot be mistaken for a trusted proxy. The issue is fixed in @fastify/forwarded 3.0.2.

CVE-2026-18174 References

External References

• https://cna.openjsf.org/security-advisories.html
• https://github.com/fastify/forwarded/security/advisories/GHSA-2849-m2w7-xm8f

CWE Common Weakness Enumeration

CAPEC Common Attack Pattern Enumeration and Classification

• Double Encoding CAPEC-120 The adversary utilizes a repeating of the encoding process for a set of characters (that is, character encoding a character encoding of a character) to obfuscate the payload of a particular request. This may allow the adversary to bypass filters that attempt to detect illegal characters or strings, such as those that might be used in traversal or injection attacks. Filters may be able to catch illegal encoded strings, but may not catch doubly encoded strings. For example, a dot (.), often used in path traversal attacks and therefore often blocked by filters, could be URL encoded as %2E. However, many filters recognize this encoding and would still block the request. In a double encoding, the % in the above URL encoding would be encoded again as %25, resulting in %252E which some filters might not catch, but which could still be interpreted as a dot (.) by interpreters on the target.
• Command Delimiters CAPEC-15 An attack of this type exploits a programs' vulnerabilities that allows an attacker's commands to be concatenated onto a legitimate command with the intent of targeting other resources such as the file system or database. The system that uses a filter or denylist input validation, as opposed to allowlist validation is vulnerable to an attacker who predicts delimiters (or combinations of delimiters) not present in the filter or denylist. As with other injection attacks, the attacker uses the command delimiter payload as an entry point to tunnel through the application and activate additional attacks through SQL queries, shell commands, network scanning, and so on.
• Flash Injection CAPEC-182 An attacker tricks a victim to execute malicious flash content that executes commands or makes flash calls specified by the attacker. One example of this attack is cross-site flashing, an attacker controlled parameter to a reference call loads from content specified by the attacker.
• Using Leading 'Ghost' Character Sequences to Bypass Input Filters CAPEC-3 Some APIs will strip certain leading characters from a string of parameters. An adversary can intentionally introduce leading "ghost" characters (extra characters that don't affect the validity of the request at the API layer) that enable the input to pass the filters and therefore process the adversary's input. This occurs when the targeted API will accept input data in several syntactic forms and interpret it in the equivalent semantic way, while the filter does not take into account the full spectrum of the syntactic forms acceptable to the targeted API.
• Exploiting Multiple Input Interpretation Layers CAPEC-43 An attacker supplies the target software with input data that contains sequences of special characters designed to bypass input validation logic. This exploit relies on the target making multiples passes over the input data and processing a "layer" of special characters with each pass. In this manner, the attacker can disguise input that would otherwise be rejected as invalid by concealing it with layers of special/escape characters that are stripped off by subsequent processing steps. The goal is to first discover cases where the input validation layer executes before one or more parsing layers. That is, user input may go through the following logic in an application: <parser1> --> <input validator> --> <parser2>. In such cases, the attacker will need to provide input that will pass through the input validator, but after passing through parser2, will be converted into something that the input validator was supposed to stop.
• Argument Injection CAPEC-6 An attacker changes the behavior or state of a targeted application through injecting data or command syntax through the targets use of non-validated and non-filtered arguments of exposed services or methods.
• Using Unicode Encoding to Bypass Validation Logic CAPEC-71 An attacker may provide a Unicode string to a system component that is not Unicode aware and use that to circumvent the filter or cause the classifying mechanism to fail to properly understanding the request. That may allow the attacker to slip malicious data past the content filter and/or possibly cause the application to route the request incorrectly.
• User-Controlled Filename CAPEC-73 An attack of this type involves an adversary inserting malicious characters (such as a XSS redirection) into a filename, directly or indirectly that is then used by the target software to generate HTML text or other potentially executable content. Many websites rely on user-generated content and dynamically build resources like files, filenames, and URL links directly from user supplied data. In this attack pattern, the attacker uploads code that can execute in the client browser and/or redirect the client browser to a site that the attacker owns. All XSS attack payload variants can be used to pass and exploit these vulnerabilities.
• AJAX Footprinting CAPEC-85 This attack utilizes the frequent client-server roundtrips in Ajax conversation to scan a system. While Ajax does not open up new vulnerabilities per se, it does optimize them from an attacker point of view. A common first step for an attacker is to footprint the target environment to understand what attacks will work. Since footprinting relies on enumeration, the conversational pattern of rapid, multiple requests and responses that are typical in Ajax applications enable an attacker to look for many vulnerabilities, well-known ports, network locations and so on. The knowledge gained through Ajax fingerprinting can be used to support other attacks, such as XSS.

Vulnerable Configurations

• cpe:2.3:a:fastify:fastify\/forwarded:0.1.2:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:0.1.2:*:*:*:*:node.js:*:*
• cpe:2.3:a:fastify:fastify\/forwarded:1.0.0:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:1.0.0:*:*:*:*:node.js:*:*
• cpe:2.3:a:fastify:fastify\/forwarded:2.0.0:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:2.0.0:*:*:*:*:node.js:*:*
• cpe:2.3:a:fastify:fastify\/forwarded:2.1.0:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:2.1.0:*:*:*:*:node.js:*:*
• cpe:2.3:a:fastify:fastify\/forwarded:2.2.0:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:2.2.0:*:*:*:*:node.js:*:*
• cpe:2.3:a:fastify:fastify\/forwarded:3.0.0:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:3.0.0:*:*:*:*:node.js:*:*
• cpe:2.3:a:fastify:fastify\/forwarded:3.0.1:*:*:*:*:node.js:*:*
  cpe:2.3:a:fastify:fastify\/forwarded:3.0.1:*:*:*:*:node.js:*:*

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