COMMON WEAKNESS ENUMERATIONIncomplete

CWE-1236

Improper Neutralization of Formula Elements in a CSV File

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.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

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.

Terminology

Alternate terms and usages

CSV Injection

Formula Injection

Excel Macro Injection

Root cause

How and when does the problem arise?

Lifecycle phase

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.
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.

Risk

What does successful exploitation lead to?

Read Application DataExecute Unauthorized Code or Commands

Affected security scopes: Confidentiality

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.

Defence

How is it prevented and fixed?

Implementation

Implementation-phase defence

Detail
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).

Effectiveness: Moderate

Unfortunately, there is no perfect solution, since different spreadsheet products act differently.

Implementation

Implementation-phase defence

Detail
If a field starts with a formula character, prepend it with a ' (single apostrophe), which prevents Excel from executing the formula.

Effectiveness: Moderate

It is not clear how effective this mitigation is with other spreadsheet software.

Architecture and Design

Architecture and design approach

Detail
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.

Effectiveness: Limited

This mitigation has limited effectiveness because it often depends on end users opening spreadsheet software safely.

Verification

How is it detected?

Automated Static Analysis

High
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.)
CWE-1236: Common Weakness Enumeration Entry CWE-1236 · DayBreach Sözlük · DayBreach