CWE-290 Base Incomplete

Authentication Bypass by Spoofing

This weakness occurs when an application's authentication system can be tricked into accepting forged or manipulated credentials, allowing unauthorized access without proper verification.

Definition

What is CWE-290?

This weakness occurs when an application's authentication system can be tricked into accepting forged or manipulated credentials, allowing unauthorized access without proper verification.
Authentication bypass by spoofing happens when an application fails to properly verify the source or integrity of authentication data. Instead of robustly checking credentials, the system might accept falsified information from network packets, certificates, session tokens, or identity claims. This allows attackers to impersonate legitimate users by crafting fake authentication artifacts that the flawed validation logic incorrectly approves. Common root causes include trusting easily forged data like IP addresses or HTTP headers for identity, using weak cryptographic signatures, or failing to validate the entire authentication chain. To prevent this, developers must implement complete credential verification using cryptographically secure methods, never rely on client-supplied data alone for identity, and ensure all authentication tokens are properly signed and validated on the server side.
Real-world impact

Real-world CVEs caused by CWE-290

  • CVE-2022-30319

    S-bus functionality in a home automation product performs access control using an IP allowlist, which can be bypassed by a forged IP address.

  • CVE-2009-1048

    VOIP product allows authentication bypass using 127.0.0.1 in the Host header.

How attackers exploit it

Step-by-step attacker path

  1. 1

    The following code authenticates users.

  2. 2

    The authentication mechanism implemented relies on an IP address for source validation. If an attacker is able to spoof the IP, they may be able to bypass the authentication mechanism.

  3. 3

    Both of these examples check if a request is from a trusted address before responding to the request.

  4. 4

    The code only verifies the address as stored in the request packet. An attacker can spoof this address, thus impersonating a trusted client.

  5. 5

    The following code samples use a DNS lookup in order to decide whether or not an inbound request is from a trusted host. If an attacker can poison the DNS cache, they can gain trusted status.

Vulnerable code example

Vulnerable Java

The following code authenticates users.

Vulnerable Java 
String sourceIP = request.getRemoteAddr();
  if (sourceIP != null && sourceIP.equals(APPROVED_IP)) {
  	authenticated = true;
  }
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-290

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-290

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-290 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.

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Frequently asked questions

Frequently asked questions

What is CWE-290?

This weakness occurs when an application's authentication system can be tricked into accepting forged or manipulated credentials, allowing unauthorized access without proper verification.

How serious is CWE-290?

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-290?

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

How can I prevent CWE-290?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-290?

Plexicus's SAST engine matches the data-flow signature for CWE-290 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-290?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/290.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-290

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-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…

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…

Resources

Further reading

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