CWE-560 Variant Draft

Use of umask() with chmod-style Argument

This vulnerability occurs when a program incorrectly uses the `umask()` system call with an argument formatted for `chmod()`, leading to unintended and overly permissive file permissions.

Definition

What is CWE-560?

This vulnerability occurs when a program incorrectly uses the `umask()` system call with an argument formatted for `chmod()`, leading to unintended and overly permissive file permissions.
The `umask()` function sets a process's file mode creation mask, which restricts permissions on newly created files. However, developers sometimes mistakenly pass it an octal value (like 0644 or 0777) intended for `chmod()`, which sets permissions directly. Since `umask()` interprets its argument as a mask that *blocks* permissions, this inversion results in files being created with far more access than intended, potentially exposing sensitive data. For example, passing `umask(0)` (a common `chmod`-style intent to grant all permissions) actually creates a mask of zero, meaning no permissions are blocked. This causes new files to be created with full read, write, and execute access for all users. The core issue is a semantic confusion between a mask that subtracts permissions (`umask`) and a mode that adds them (`chmod`), a mistake that directly weakens file system security.
Real-world impact

Real-world CVEs caused by CWE-560

No public CVE references are linked to this CWE in MITRE's catalog yet.

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 pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// 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

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-560

  • Implementation Use umask() with the correct argument.
  • Testing If you suspect misuse of umask(), you can use grep to spot call instances of umask().
Detection signals

How to detect CWE-560

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.

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Frequently asked questions

Frequently asked questions

What is CWE-560?

This vulnerability occurs when a program incorrectly uses the `umask()` system call with an argument formatted for `chmod()`, leading to unintended and overly permissive file permissions.

How serious is CWE-560?

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-560?

MITRE lists the following affected platforms: C.

How can I prevent CWE-560?

Use umask() with the correct argument. If you suspect misuse of umask(), you can use grep to spot call instances of umask().

How does Plexicus detect and fix CWE-560?

Plexicus's SAST engine matches the data-flow signature for CWE-560 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-560?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/560.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-560

CWE-687 Parent

Function Call With Incorrectly Specified Argument Value

This vulnerability occurs when a function is called with an argument that holds an incorrect or unexpected value, leading to unintended…

Resources

Further reading

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