CWE-201: Insertion of Sensitive Information Into Sent Data

Definition

What is CWE-201?

This vulnerability occurs when an application sends data to an external party, but accidentally includes sensitive information—like passwords, keys, or personal data—that the recipient should not be able to access.

This flaw often happens because of overly broad data selection or logging mechanisms that package more information than necessary for a given transaction. For example, an API response might return an entire user object, including internal fields like hashed passwords or security questions, instead of just the needed profile data. Developers should carefully review all data payloads sent to clients, third-party services, or logs, ensuring they follow the principle of least privilege and expose only what is explicitly required. Catching these leaks manually is challenging, as sensitive data can be buried deep in nested objects or only appear under specific conditions. While SAST tools can flag patterns of data exposure, Plexicus uses AI to analyze data flows in context and suggest precise code fixes—such as implementing stricter output filters or using data sanitization libraries—saving hours of manual review and reducing the risk of accidental disclosure across your entire application stack.

Real-world impact

Real-world CVEs caused by CWE-201

CVE-2022-0708

Collaboration platform does not clear team emails in a response, allowing leak of email addresses

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 SQL

The following is an actual MySQL error statement:

Vulnerable SQL

Warning: mysql_pconnect(): Access denied for user: 'root@localhost' (Using password: N1nj4) in /usr/local/www/wi-data/includes/database.inc on line 4

Secure code example

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-201

Requirements Specify which data in the software should be regarded as sensitive. Consider which types of users should have access to which types of data.
Implementation Ensure that any possibly sensitive data specified in the requirements is verified with designers to ensure that it is either a calculated risk or mitigated elsewhere. Any information that is not necessary to the functionality should be removed in order to lower both the overhead and the possibility of security sensitive data being sent.
System Configuration Setup default error messages so that unexpected errors do not disclose sensitive information.
Architecture and Design Compartmentalize the system to have "safe" areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area. Ensure that appropriate compartmentalization is built into the system design, and the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.

Detection signals

How to detect CWE-201

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

CWE-201

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Frequently asked questions

What is CWE-201?

This vulnerability occurs when an application sends data to an external party, but accidentally includes sensitive information—like passwords, keys, or personal data—that the recipient should not be able to access.

How serious is CWE-201?

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-201?

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

How can I prevent CWE-201?

Specify which data in the software should be regarded as sensitive. Consider which types of users should have access to which types of data. Ensure that any possibly sensitive data specified in the requirements is verified with designers to ensure that it is either a calculated risk or mitigated elsewhere. Any information that is not necessary to the functionality should be removed in order to lower both the overhead and the possibility of security sensitive data being sent.

How does Plexicus detect and fix CWE-201?

Plexicus's SAST engine matches the data-flow signature for CWE-201 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-201?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/201.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-201

CWE-200 Parent

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Insertion of Sensitive Information Into Sent Data

What is CWE-201?

Real-world CVEs caused by CWE-201

Step-by-step attacker path

Vulnerable SQL

Secure pseudo

How to prevent CWE-201

How to detect CWE-201

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

Frequently asked questions

Weaknesses related to CWE-201

Exposure of Sensitive Information to an Unauthorized Actor

Exposure of Sensitive System Information Due to Uncleared Debug Information

Device Unlock Credential Sharing

Debug Messages Revealing Unnecessary Information

Driving Intermediate Cryptographic State/Results to Hardware Module Outputs

Observable Discrepancy

Generation of Error Message Containing Sensitive Information

Exposure of Sensitive Information Due to Incompatible Policies

Insertion of Sensitive Information Into Debugging Code

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`