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. 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 Verilog

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.

Plexicus auto-fix

Plexicus auto-detects CWE-420 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-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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CWE-1275 Sibling

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CWE-291 Sibling

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CWE-297 Sibling

Improper Validation of Certificate with Host Mismatch

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CWE-300 Sibling

Channel Accessible by Non-Endpoint

This vulnerability occurs when a system fails to properly verify who is on the other end of a communication link or to secure the channel…

CWE-419 Sibling

Unprotected Primary Channel

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CWE-940 Sibling

Improper Verification of Source of a Communication Channel

This vulnerability occurs when an application accepts incoming communication requests without properly checking where they originate from,…

CWE-941 Sibling

Incorrectly Specified Destination in a Communication Channel

This vulnerability occurs when an application establishes an outgoing communication channel but fails to correctly define or enforce the…

CWE-942 Sibling

Permissive Cross-domain Security Policy with Untrusted Domains

This vulnerability occurs when a web application's cross-domain security policy, like a Content Security Policy (CSP), explicitly allows…

Resources

Further reading

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