Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-756 Base Incomplete
Missing Custom Error Page
This vulnerability occurs when an application fails to display its own user-friendly error pages, instead falling back to default system messages that can leak sensitive technical details.
Definition
What is CWE-756?
This vulnerability occurs when an application fails to display its own user-friendly error pages, instead falling back to default system messages that can leak sensitive technical details.
When an application crashes or encounters an unexpected condition, the web server or framework often generates a default error response. These generic pages can be a goldmine for attackers, revealing stack traces, database schemas, server versions, internal file paths, or snippets of code. This exposed information significantly lowers the effort required for further exploitation, as it provides clear clues about the underlying technology and potential weaknesses.
To prevent this, developers should implement a centralized error handler that intercepts all uncaught exceptions and HTTP error statuses (like 404 or 500). This handler must then respond with a consistent, branded page that informs the user a problem occurred—without any technical details—while logging the full diagnostic information securely on the server side for internal debugging. This simple practice improves both security and user experience.
Real-world impact
Real-world CVEs caused by CWE-756
No public CVE references are linked to this CWE in MITRE's catalog yet.
How attackers exploit it
Step-by-step attacker path
- 1
In the snippet below, an unchecked runtime exception thrown from within the try block may cause the container to display its default error page (which may contain a full stack trace, among other things).
- 2
The mode attribute of the tag in the Web.config file defines whether custom or default error pages are used.
- 3
In the following insecure ASP.NET application setting, custom error message mode is turned off. An ASP.NET error message with detailed stack trace and platform versions will be returned.
- 4
A more secure setting is to set the custom error message mode for remote users only. No defaultRedirect error page is specified. The local user on the web server will see a detailed stack trace. For remote users, an ASP.NET error message with the server customError configuration setting and the platform version will be returned.
- 5
Another secure option is to set the mode attribute of the tag to use a custom page as follows:
Vulnerable code example
Vulnerable Java
In the snippet below, an unchecked runtime exception thrown from within the try block may cause the container to display its default error page (which may contain a full stack trace, among other things).
Public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
try {
...
} catch (ApplicationSpecificException ase) {
logger.error("Caught: " + ase.toString());
}
} Secure code example
Secure ASP.NET
A more secure setting is to set the custom error message mode for remote users only. No defaultRedirect error page is specified. The local user on the web server will see a detailed stack trace. For remote users, an ASP.NET error message with the server customError configuration setting and the platform version will be returned.
<customErrors mode="RemoteOnly" /> 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-756
- 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-756
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
Plexicus auto-detects CWE-756 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.
Frequently asked questions What is CWE-756?
How serious is CWE-756?
What languages or platforms are affected by CWE-756?
How can I prevent CWE-756?
How does Plexicus detect and fix CWE-756?
Where can I learn more about CWE-756?
Frequently asked questions
What is CWE-756?
This vulnerability occurs when an application fails to display its own user-friendly error pages, instead falling back to default system messages that can leak sensitive technical details.
How serious is CWE-756?
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-756?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-756?
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-756?
Plexicus's SAST engine matches the data-flow signature for CWE-756 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-756?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/756.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-756
CWE-755 Parent
Improper Handling of Exceptional Conditions
This vulnerability occurs when software fails to properly manage unexpected situations or errors, leaving it in an unstable or insecure…
CWE-209 Sibling
Generation of Error Message Containing Sensitive Information
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CWE-248 Sibling
Uncaught Exception
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CWE-274 Sibling
Improper Handling of Insufficient Privileges
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CWE-280 Sibling
Improper Handling of Insufficient Permissions or Privileges
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CWE-333 Sibling
Improper Handling of Insufficient Entropy in TRNG
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CWE-390 Sibling
Detection of Error Condition Without Action
This weakness occurs when software successfully identifies an error condition but then fails to take any meaningful action to address it.…
CWE-392 Sibling
Missing Report of Error Condition
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CWE-395 Sibling
Use of NullPointerException Catch to Detect NULL Pointer Dereference
Using a try-catch block for NullPointerException as a substitute for proper null checks is an anti-pattern. This approach masks the root…
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