CWE-1236: Improper Neutralization of Formula Elements in a CSV File

Definición

What is CWE-1236?

This vulnerability occurs when an application writes user-supplied data into a CSV file without properly sanitizing special characters. Spreadsheet programs like Excel or Google Sheets interpret entries starting with '=' as executable formulas, which can lead to malicious command execution when the file is opened.

Many applications export data to CSV files for easy viewing in spreadsheet software. However, these programs treat cells beginning with characters like =, +, -, or @ as formulas. If an attacker can inject data like `=HYPERLINK("http://malicious-site.com", "Click")` or a formula that calls external scripts, simply opening the CSV file can trigger data theft, remote code execution, or other harmful actions on the user's machine. Developers often overlook this risk because CSV files are seen as simple data dumps, not executable content. To prevent this, you must sanitize or escape formula-initiating characters in all user-controlled fields before writing them to a CSV. This ensures that data is displayed as static text, not as an active command, when the file is imported into a spreadsheet application.

Impacto en el mundo real

Real-world CVEs caused by CWE-1236

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

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

1
Identifica una ruta de código que maneje entrada no confiable sin validación.
2
Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.
3
Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.
4
Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.

Ejemplo de código vulnerable

Vulnerable Other

Hyperlinks or other commands can be executed when a cell begins with the formula identifier, '='

Vulnerable Other

=HYPERLINK(link_location, [friendly_name])

Payload del atacante

Hyperlinks or other commands can be executed when a cell begins with the formula identifier, '='

Payload del atacante Other

=HYPERLINK(link_location, [friendly_name])

Ejemplo de código seguro

Secure Other

Stripping the leading equals sign, or simply not executing formulas from untrusted sources, impedes malicious activity.

Seguro Other

HYPERLINK(link_location, [friendly_name])

What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.

Lista de prevención

How to prevent CWE-1236

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.

Señales de detección

How to detect CWE-1236

SAST High

Ejecuta análisis estático (SAST) sobre el código buscando el patrón inseguro en el flujo de datos.

DAST Moderate

Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.

Runtime Moderate

Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.

Code review Moderate

Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.

CWE-1236

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Preguntas frecuentes

Frequently asked questions

¿Qué es CWE-1236?

This vulnerability occurs when an application writes user-supplied data into a CSV file without properly sanitizing special characters. Spreadsheet programs like Excel or Google Sheets interpret entries starting with '=' as executable formulas, which can lead to malicious command execution when the file is opened.

¿Qué gravedad tiene CWE-1236?

MITRE no ha publicado una calificación de probabilidad de explotación para esta debilidad. Trátala como de impacto medio hasta que tu modelo de amenazas demuestre lo contrario.

¿Qué lenguajes o plataformas se ven afectados por CWE-1236?

MITRE lists the following affected platforms: Not OS-Specific, Not Architecture-Specific, Other.

¿Cómo puedo prevenir CWE-1236?

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

¿Cómo detecta y corrige Plexicus CWE-1236?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-1236 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.

¿Dónde puedo aprender más sobre CWE-1236?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/1236.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-1236

CWE-74 Padre

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Improper Neutralization of Formula Elements in a CSV File

What is CWE-1236?

Real-world CVEs caused by CWE-1236

Ruta del atacante paso a paso

Vulnerable Other

Secure Other

How to prevent CWE-1236

How to detect CWE-1236

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-1236

Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')

Failure to Sanitize Special Elements into a Different Plane (Special Element Injection)

Improper Neutralization of Special Elements used in a Command ('Command Injection')

Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')

Improper Neutralization of Special Elements used in an SQL Command ('SQL Injection')

XML Injection (aka Blind XPath Injection)

Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`