CWE-795: Only Filtering Special Elements at a Specified… | Glexia
CWE-795 (Only Filtering Special Elements at a Specified Location) weakness overview with consequences, detection methods, mitigations, related CVEs and MITRE…
Glexia's Take · Automated analysis
CWE-795: Only Filtering Special Elements at a Specified Location
Only Filtering Special Elements at a Specified Location represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Executive Impact
- Integrity: Unexpected State
Developer Pattern
CWE-795 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.
Automation confidence
high confidence from CWE-795, 4.20.
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Official CWE Definition
CWE-795: Only Filtering Special Elements at a Specified Location
The product receives data from an upstream component, but only accounts for special elements at a specified location, thereby missing remaining special elements that may exist before sending it to a downstream component.
A filter might only account for instances of special elements when they occur: relative to a marker (e.g. "at the beginning/end of string; the second argument"), or at an absolute position (e.g. "byte number 10"). This may leave special elements in the data that did not match the filter position, but still may be dangerous.
Developer And Remediation Guidance
How teams prevent and detect this weakness
Causes
- The following code takes untrusted input and uses a regular expression to filter a "../" element located at the beginning of the input string. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path. Since the regular expression is only looking for an instance of "../" at the beginning of the string, it only removes the first "../" element. So an input value such as: will have the first "../" stripped, resulting in: This value is then concatenated with the /home/user/ directory: which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-22).
- The following code takes untrusted input and uses a substring function to filter a 3-character "../" element located at the 0-index position of the input string. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path. Since the if function is only looking for a substring of "../" between the 0 and 2 position, it only removes that specific "../" element. So an input value such as: will have the first "../" filtered, resulting in: This value is then concatenated with the /home/user/ directory: which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-22).
Remediation
- Use safe APIs
- Centralize the control
- Add regression tests
- Review logs and telemetry for attempted abuse
Detection
- Code review
- SAST
- DAST
- Focused regression tests
Mappings
Related CVEs, CWEs, and ATT&CK context
Related CWEs
ATT&CK Relevance
ATT&CK relevance is shown only when reviewed or responsibly inferred.