CWE-420 Base Draft

Unprotected Alternate Channel

This vulnerability occurs when an application secures its main communication path but leaves a backup or alternative channel with weaker or no protection.

Definition

What is CWE-420?

This vulnerability occurs when an application secures its main communication path but leaves a backup or alternative channel with weaker or no protection.

Think of an application that uses strong encryption for its primary web login but sends password reset codes via unencrypted SMS. An attacker can ignore the fortified main door and easily intercept data through this neglected side channel. This creates a false sense of security, as the overall system is only as strong as its weakest point. Developers often focus all their security efforts on the obvious primary path, forgetting that attackers actively seek out these alternate, less-guarded routes. To prevent this, you must perform a thorough threat model that maps all data flows, not just the primary ones. Apply consistent security controls—like encryption, authentication, and integrity checks—across every channel that handles sensitive data or critical commands. Regularly test both primary and alternate paths during security assessments to ensure your defenses are uniform and comprehensive.

Real-world impact

Real-world CVEs caused by CWE-420

CVE-2020-8004

When the internal flash is protected by blocking access on the Data Bus (DBUS), it can still be indirectly accessed through the Instruction Bus (IBUS).
CVE-2002-0567

DB server assumes that local clients have performed authentication, allowing attacker to directly connect to a process to load libraries and execute commands; a socket interface also exists (another alternate channel), so attack can be remote.
CVE-2002-1578

Product does not restrict access to underlying database, so attacker can bypass restrictions by directly querying the database.
CVE-2003-1035

User can avoid lockouts by using an API instead of the GUI to conduct brute force password guessing.
CVE-2002-1863

FTP service can not be disabled even when other access controls would require it.
CVE-2002-0066

Windows named pipe created without authentication/access control, allowing configuration modification.
CVE-2004-1461

Router management interface spawns a separate TCP connection after authentication, allowing hijacking by attacker coming from the same IP address.

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 Verilog

assign addr_auth = (address == 32'hF00) ? 1: 0;

Secure code example

Secure Verilog

The bugged line of code is repeated in the Bad example above. The weakness arises from the fact that the SECURE_ME register can be modified by writing to the shadow register COPY_OF_SECURE_ME. The address of COPY_OF_SECURE_ME should also be included in the check. That buggy line of code should instead be replaced as shown in the Good Code Snippet below.

Secure Verilog

assign addr_auth = (address == 32'hF00 || address == 32'h800F00) ? 1: 0;

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-420

Architecture and Design Identify all alternate channels and use the same protection mechanisms that are used for the primary channels.

Detection signals

How to detect CWE-420

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.

CWE-420

Don't catalog this weakness. Prove it's reachable.

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Frequently asked questions

What is CWE-420?

This vulnerability occurs when an application secures its main communication path but leaves a backup or alternative channel with weaker or no protection.

How serious is CWE-420?

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-420?

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

How can I prevent CWE-420?

Identify all alternate channels and use the same protection mechanisms that are used for the primary channels.

How does Plexicus detect and fix CWE-420?

Plexicus's SAST engine matches the data-flow signature for CWE-420 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-420?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/420.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-420

CWE-923 Parent

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Unprotected Alternate Channel

What is CWE-420?

Real-world CVEs caused by CWE-420

Step-by-step attacker path

Vulnerable Verilog

Secure Verilog

How to prevent CWE-420

How to detect CWE-420

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-420

Improper Restriction of Communication Channel to Intended Endpoints

Sensitive Cookie with Improper SameSite Attribute

Reliance on IP Address for Authentication

Improper Validation of Certificate with Host Mismatch

Channel Accessible by Non-Endpoint

Unprotected Primary Channel

Improper Verification of Source of a Communication Channel

Incorrectly Specified Destination in a Communication Channel

Permissive Cross-domain Security Policy with Untrusted Domains

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`