CWE-1236: Improper Neutralization of Formula Elements in a CSV File
| Abstraction | Structure | Status |
|---|---|---|
| None | Simple | Incomplete |
Description
The product saves user-provided information into a Comma-Separated Value (CSV) file, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as a command when the file is opened by a spreadsheet product.
Alternate Terms
- CSV Injection:
- Formula Injection:
- Excel Macro Injection:
Related Weaknesses
| Nature | ID | View ID | Name |
|---|---|---|---|
| ChildOf | CWE-74 | 1000 | Improper Neutralization of Special Elements in Output Used by a Downstream Component (‘Injection’) |
| ChildOf | CWE-74 | 1003 | Improper Neutralization of Special Elements in Output Used by a Downstream Component (‘Injection’) |
Background Details
User-provided data is often saved to traditional databases. This data can be exported to a CSV file, which allows users to read the data using spreadsheet software such as Excel, Numbers, or Calc. This software interprets entries beginning with '=' as formulas, which are then executed by the spreadsheet software. The software's formula language often allows methods to access hyperlinks or the local command line, and frequently allows enough characters to invoke an entire script.
Modes of Introduction
| Phase | Note |
|---|---|
| Implementation | The weakness is in the implementation of a software’s CSV export feature, in particular how it formats formula entries as the output gets flattened into a text file. |
Applicable Platforms
Languages
Class: Not Language-Specific
Technologies
Class: None
Common Consequences
| Scope | Impact | Note |
|---|---|---|
| Confidentiality | Read Application Data, Execute Unauthorized Code or Commands | Attackers can populate data fields which, when saved to a CSV file, may attempt information exfiltration or other malicious activity when automatically executed by the spreadsheet software. Note that current versions of Excel warn users of untrusted content. |
Detection Methods
Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect “sources” (origins of input) with “sinks” (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
Effectiveness: High
Potential Mitigations
Implementation
When generating CSV output, ensure that formula-sensitive metacharacters are effectively escaped or removed from all data before storage in the resultant CSV. Risky characters include ‘=’ (equal), ‘+’ (plus), ‘-’ (minus), and ‘@’ (at).
Implementation
If a field starts with a formula character, prepend it with a ’ (single apostrophe), which prevents Excel from executing the formula.
Architecture and Design
Certain implementations of spreadsheet software might disallow formulas from executing if the file is untrusted, or if the file is not authored by the current user.
Observed Examples
- CVE-2019-12134: Low privileged user can trigger CSV injection through a contact form field value
- CVE-2019-4521: Cloud management product allows arbitrary command execution via CSV injection
- CVE-2019-17661: CSV injection in content management system via formula code in a first or last name