Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-12 Variant Draft
ASP.NET Misconfiguration: Missing Custom Error Page
This vulnerability occurs when an ASP.NET application fails to configure custom error pages, allowing attackers to extract sensitive information from the framework's default error messages.
Definition
What is CWE-12?
This vulnerability occurs when an ASP.NET application fails to configure custom error pages, allowing attackers to extract sensitive information from the framework's default error messages.
When an ASP.NET application encounters an error, it can reveal detailed technical information in its default response, such as stack traces, server file paths, database connection details, or snippets of source code. Attackers actively probe for these errors to gather intelligence about your application's internal structure, which they can then use to plan more targeted and damaging attacks.
To prevent this, you must configure custom error pages in your web.config file. This ensures that any runtime errors display a generic, user-friendly message to visitors while logging the full technical details securely on the server. This simple configuration step acts as a critical layer of defense, stopping information leaks that could compromise your entire application.
Real-world impact
Real-world CVEs caused by CWE-12
No public CVE references are linked to this CWE in MITRE's catalog yet.
How attackers exploit it
Step-by-step attacker path
- 1
The mode attribute of the tag in the Web.config file defines whether custom or default error pages are used.
- 2
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.
- 3
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.
- 4
Another secure option is to set the mode attribute of the tag to use a custom page as follows:
Vulnerable code example
Vulnerable ASP.NET
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.
<customErrors mode="Off" /> 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-12
- System Configuration Handle exceptions appropriately in source code. ASP .NET applications should be configured to use custom error pages instead of the framework default page.
- Architecture and Design Do not attempt to process an error or attempt to mask it.
- Implementation Verify return values are correct and do not supply sensitive information about the system.
Detection signals
How to detect CWE-12
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.
CWE-12
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-12?
How serious is CWE-12?
What languages or platforms are affected by CWE-12?
How can I prevent CWE-12?
How does Plexicus detect and fix CWE-12?
Where can I learn more about CWE-12?
Frequently asked questions
What is CWE-12?
This vulnerability occurs when an ASP.NET application fails to configure custom error pages, allowing attackers to extract sensitive information from the framework's default error messages.
How serious is CWE-12?
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-12?
MITRE lists the following affected platforms: ASP.NET.
How can I prevent CWE-12?
Handle exceptions appropriately in source code. ASP .NET applications should be configured to use custom error pages instead of the framework default page. Do not attempt to process an error or attempt to mask it.
How does Plexicus detect and fix CWE-12?
Plexicus's SAST engine matches the data-flow signature for CWE-12 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-12?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/12.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-12
CWE-756 Parent
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…
CWE-7 Sibling
J2EE Misconfiguration: Missing Custom Error Page
This vulnerability occurs when a J2EE application uses the server's default error pages instead of custom ones, potentially leaking…
Resources
Further reading
- MITRE — official CWE-12 https://cwe.mitre.org/data/definitions/12.html
- Seven Pernicious Kingdoms: A Taxonomy of Software Security Errors https://samate.nist.gov/SSATTM_Content/papers/Seven%20Pernicious%20Kingdoms%20-%20Taxonomy%20of%20Sw%20Security%20Errors%20-%20Tsipenyuk%20-%20Chess%20-%20McGraw.pdf
- ASP.NET Misconfiguration: Missing Custom Error Handling https://www.owasp.org/index.php/ASP.NET_Misconfiguration:_Missing_Custom_Error_Handling
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