CWE-27 Variant Draft

Path Traversal: 'dir/../../filename'

This vulnerability occurs when an application builds file paths using user input but fails to properly block sequences like 'dir/../../filename'. Attackers can exploit this by injecting multiple…

Definition

What is CWE-27?

This vulnerability occurs when an application builds file paths using user input but fails to properly block sequences like 'dir/../../filename'. Attackers can exploit this by injecting multiple '../' segments to escape the intended directory and access unauthorized files or folders elsewhere on the system.

Attackers leverage this pattern to bypass common security filters. Some defenses only strip a single '../' sequence or only check for it at the start of a path. By nesting the traversal within a subdirectory reference (like 'dir/../../'), an attacker can move up multiple directory levels, effectively sidestepping these incomplete protections. This technique highlights why simple string removal or prefix matching is insufficient for path traversal defense. Developers need canonical path resolution that normalizes the entire path before checking if it remains within the allowed directory boundary. Relying on partial sanitization creates a false sense of security that this specific pattern is designed to exploit.

Real-world impact

Real-world CVEs caused by CWE-27

CVE-2002-0298

Server allows remote attackers to cause a denial of service via certain HTTP GET requests containing a %2e%2e (encoded dot-dot), several "/../" sequences, or several "../" in a URI.

How attackers exploit it

Step-by-step attacker path

1
Identify a code path that handles untrusted input without validation.
2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
3
Deliver the payload through a normal request and observe the application's reaction.
4
Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo

// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}

Secure code example

Secure pseudo

// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}

What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.

Prevention checklist

How to prevent CWE-27

Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as "/" to avoid CWE-36. Use a list of allowable file extensions, which will help to avoid CWE-434. Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (CWE-184). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would be removed from the original string, but the remaining characters would still form the "../" string.
Implementation Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Detection signals

How to detect CWE-27

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-27 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

Get a demo Try Plexicus free

Frequently asked questions

What is CWE-27?

How serious is CWE-27?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-27?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-27?

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and…

How does Plexicus detect and fix CWE-27?

Plexicus's SAST engine matches the data-flow signature for CWE-27 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-27?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/27.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-27

CWE-23 Parent

Don't Let Security
Weigh You Down.

Stop choosing between AI velocity and security debt. Plexicus is the only platform that runs Vibe Coding Security and ASPM in parallel — one workflow, every codebase.

Get started free Book a demo

Path Traversal: 'dir/../../filename'

What is CWE-27?

Real-world CVEs caused by CWE-27

Step-by-step attacker path

Vulnerable pseudo

Secure pseudo

How to prevent CWE-27

How to detect CWE-27

Plexicus auto-detects CWE-27 and opens a fix PR in under 60 seconds.

Frequently asked questions

Weaknesses related to CWE-27

Relative Path Traversal

Path Traversal: '../filedir'

Path Traversal: '/../filedir'