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Authentication Bypass by Spoofing

Incomplete Base
Structure: Simple
Description

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

Extended Description

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.
Common Consequences 1
Scope: Access Control

Impact: Bypass Protection MechanismGain Privileges or Assume Identity

This weakness can allow an attacker to access resources which are not otherwise accessible without proper authentication.
Demonstrative Examples 3
The following code authenticates users.

Code Example:

Bad
Java
java
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.

ID : DX-99

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

Code Example:

Bad
C
c

Code Example:

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

ID : DX-93

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.

Code Example:

Bad
C
c

Code Example:

Bad
Java
java

Code Example:

Bad
C#
c#
IP addresses are more reliable than DNS names, but they can also be spoofed. Attackers can easily forge the source IP address of the packets they send, but response packets will return to the forged IP address. To see the response packets, the attacker has to sniff the traffic between the victim machine and the forged IP address. In order to accomplish the required sniffing, attackers typically attempt to locate themselves on the same subnet as the victim machine. Attackers may be able to circumvent this requirement by using source routing, but source routing is disabled across much of the Internet today. In summary, IP address verification can be a useful part of an authentication scheme, but it should not be the single factor required for authentication.
Observed Examples 2
CVE-2022-30319S-bus functionality in a home automation product performs access control using an IP allowlist, which can be bypassed by a forged IP address.
CVE-2009-1048VOIP product allows authentication bypass using 127.0.0.1 in the Host header.
References 1
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
Modes of Introduction
Implementation
Related Attack Patterns
Related Weaknesses
ChildOf: Weak Authentication (CWE-1390)
ChildOf: Improper Authentication (CWE-287)
Taxonomy Mapping
  • PLOVER
Notes
RelationshipThis can be resultant from insufficient verification.