Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-289 Base Incomplete
Authentication Bypass by Alternate Name
This vulnerability occurs when a system checks access based on a resource or user name, but fails to account for all the different names or aliases that could refer to the same entity, allowing…
Definition
What is CWE-289?
This vulnerability occurs when a system checks access based on a resource or user name, but fails to account for all the different names or aliases that could refer to the same entity, allowing attackers to bypass authentication.
This flaw often appears in systems that rely on simple string matching for access control. For example, a file server might block access to 'admin.txt' but overlook 'ADMIN.TXT', 'Admin.Txt', or a symlink pointing to the same file. Similarly, a user authentication system might verify 'administrator' but not check for 'admin', 'root', or other common aliases for privileged accounts. Attackers exploit this by simply using an alternate name the system doesn't recognize as restricted.
To prevent this, developers must implement canonicalization, where all names are resolved to a single, standard form before the security check. This means normalizing file paths, converting usernames to a consistent case, and resolving symbolic links. Access control logic should never rely on raw, unvalidated input strings. Instead, it should compare the canonical identity of the resource or user against a policy that is enforced consistently for all access attempts.
Real-world impact
Real-world CVEs caused by CWE-289
-
Protection mechanism that restricts URL access can be bypassed using URL encoding.
-
Bypass of authentication for files using "\" (backslash) or "%5C" (encoded backslash).
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.
// 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-289
- Architecture and Design Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
- 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.
- 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-289
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
Plexicus auto-detects CWE-289 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.
Frequently asked questions What is CWE-289?
How serious is CWE-289?
What languages or platforms are affected by CWE-289?
How can I prevent CWE-289?
How does Plexicus detect and fix CWE-289?
Where can I learn more about CWE-289?
Frequently asked questions
What is CWE-289?
This vulnerability occurs when a system checks access based on a resource or user name, but fails to account for all the different names or aliases that could refer to the same entity, allowing attackers to bypass authentication.
How serious is CWE-289?
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-289?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-289?
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names. 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…
How does Plexicus detect and fix CWE-289?
Plexicus's SAST engine matches the data-flow signature for CWE-289 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-289?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/289.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-289
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Weak Authentication
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CWE-1391 Sibling
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CWE-262 Sibling
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Password Aging with Long Expiration
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CWE-290 Sibling
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CWE-294 Sibling
Authentication Bypass by Capture-replay
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CWE-301 Sibling
Reflection Attack in an Authentication Protocol
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CWE-302 Sibling
Authentication Bypass by Assumed-Immutable Data
This vulnerability occurs when an authentication system incorrectly treats certain data as unchangeable, when in fact an attacker can…
CWE-303 Sibling
Incorrect Implementation of Authentication Algorithm
This weakness occurs when a developer implements a standard authentication algorithm, but makes critical mistakes in the code that cause…
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