Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-403 Base Draft
Exposure of File Descriptor to Unintended Control Sphere ('File Descriptor Leak')
This vulnerability occurs when a parent process launches a child process without first closing sensitive file descriptors. The child process inherits these open handles, potentially gaining…
Definition
What is CWE-403?
This vulnerability occurs when a parent process launches a child process without first closing sensitive file descriptors. The child process inherits these open handles, potentially gaining unauthorized access to files, sockets, or other resources it shouldn't be able to interact with.
When a system creates a child process through forking or execution, that new process automatically receives copies of all file descriptors currently open in its parent. This inherited access persists even if the child process runs with lower system privileges than its parent. The core risk emerges because the child can perform read or write operations through these inherited descriptors, bypassing normal permission checks that would block direct access to the underlying files or network connections.
To prevent this, developers must proactively close sensitive file descriptors before spawning child processes, especially when dropping privileges or executing less-trusted code. This is a common oversight in privilege-separation architectures and can lead to information leaks, data corruption, or escalation of privilege, as the child process operates with unintended access rights handed down from its parent.
Real-world impact
Real-world CVEs caused by CWE-403
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Server leaks a privileged file descriptor, allowing the server to be hijacked.
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File descriptor leak allows read of restricted files.
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Access to restricted resource using modified file descriptor for stderr.
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Open file descriptor used as alternate channel in complex race condition.
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Program does not fully drop privileges after creating a file descriptor, which allows access to the descriptor via a separate vulnerability.
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User bypasses restrictions by obtaining a file descriptor then calling setuid program, which does not close the descriptor.
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Terminal manager does not properly close file descriptors, allowing attackers to access terminals of other users.
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Module opens a file for reading twice, allowing attackers to read files.
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 pseudo
MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} 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-403
- 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-403
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-403
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-403?
How serious is CWE-403?
What languages or platforms are affected by CWE-403?
How can I prevent CWE-403?
How does Plexicus detect and fix CWE-403?
Where can I learn more about CWE-403?
Frequently asked questions
What is CWE-403?
This vulnerability occurs when a parent process launches a child process without first closing sensitive file descriptors. The child process inherits these open handles, potentially gaining unauthorized access to files, sockets, or other resources it shouldn't be able to interact with.
How serious is CWE-403?
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-403?
MITRE lists the following affected platforms: C, Unix.
How can I prevent CWE-403?
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-403?
Plexicus's SAST engine matches the data-flow signature for CWE-403 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-403?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/403.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-403
CWE-402 Parent
Transmission of Private Resources into a New Sphere ('Resource Leak')
This vulnerability occurs when an application unintentionally exposes internal resources, like files, memory, or database connections, to…
CWE-619 Sibling
Dangling Database Cursor ('Cursor Injection')
A dangling database cursor occurs when a database cursor is not properly closed, potentially allowing other users to access it while it…
Resources
Further reading
- MITRE — official CWE-403 https://cwe.mitre.org/data/definitions/403.html
- File descriptors and setuid applications https://blogs.oracle.com/paulr/entry/file_descriptors_and_setuid_applications
- Introduction to Secure Coding Guide https://developer.apple.com/library/archive/documentation/Security/Conceptual/SecureCodingGuide/Articles/AccessControl.html
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