CWE-603 Base Draft

Use of Client-Side Authentication

This vulnerability occurs when an application places its authentication logic solely within the client-side code, such as in a mobile app or web browser, without enforcing the same checks on the…

Definition

What is CWE-603?

This vulnerability occurs when an application places its authentication logic solely within the client-side code, such as in a mobile app or web browser, without enforcing the same checks on the server. Attackers can bypass authentication by modifying the client to skip these checks entirely.
Relying on client-side authentication creates a critical security flaw because the attacker controls the client environment. They can easily decompile an app, inspect JavaScript, or use debugging tools to analyze and remove the authentication logic. Since the server blindly trusts the client's claim of being authenticated, this allows unauthorized access to protected functions and data. To prevent this, authentication must always be verified and enforced on the server. Every request for a protected resource or action should be validated server-side using a secure session or token that the client cannot forge. Treat all client-side code as inherently untrustworthy and ensure the server is the final authority on user identity and permissions.
Real-world impact

Real-world CVEs caused by CWE-603

  • CVE-2022-33139

    SCADA system only uses client-side authentication, allowing adversaries to impersonate other users.

  • CVE-2006-0230

    Client-side check for a password allows access to a server using crafted XML requests from a modified client.

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 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

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-603

  • Architecture and Design Do not rely on client side data. Always perform server side authentication.
Detection signals

How to detect CWE-603

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-603 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

Frequently asked questions

What is CWE-603?

This vulnerability occurs when an application places its authentication logic solely within the client-side code, such as in a mobile app or web browser, without enforcing the same checks on the server. Attackers can bypass authentication by modifying the client to skip these checks entirely.

How serious is CWE-603?

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-603?

MITRE lists the following affected platforms: ICS/OT.

How can I prevent CWE-603?

Do not rely on client side data. Always perform server side authentication.

How does Plexicus detect and fix CWE-603?

Plexicus's SAST engine matches the data-flow signature for CWE-603 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-603?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/603.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-603

CWE-1390 Parent

Weak Authentication

This vulnerability occurs when a system's login or identity verification process is too easy to bypass or fool. While it attempts to check…

CWE-1391 Sibling

Use of Weak Credentials

This vulnerability occurs when a system relies on weak authentication credentials—like default passwords, hard-coded keys, or easily…

CWE-262 Sibling

Not Using Password Aging

This vulnerability occurs when a system lacks password expiration policies, allowing users to keep the same password indefinitely.

CWE-263 Sibling

Password Aging with Long Expiration

The system enforces password changes, but the time allowed between changes is excessively long, weakening security.

CWE-289 Sibling

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…

CWE-290 Sibling

Authentication Bypass by Spoofing

This weakness occurs when an application's authentication system can be tricked into accepting forged or manipulated credentials, allowing…

CWE-294 Sibling

Authentication Bypass by Capture-replay

This vulnerability occurs when an attacker can intercept and record legitimate authentication traffic, then replay it later to gain…

CWE-301 Sibling

Reflection Attack in an Authentication Protocol

A reflection attack is a flaw in mutual authentication protocols that allows an attacker to impersonate a legitimate user without knowing…

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…

Resources

Further reading

Ready when you are

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.