Negative shared-string index causes panic in GetCellValue and GetRows

Summary

Excelize parses shared-string cell values with strconv.Atoi and checks only the upper bound before indexing the shared string slice. If an XLSX file contains a shared-string cell with <v>-1</v>, the parsed index is negative. The upper-bound check still passes (len(sharedStrings) > -1), and Excelize indexes sharedStrings[-1], causing a runtime panic.

This was reproduced on the current default branch commit 1213a8bd7c5ab360554603ac5c995ccaf6eb4314 and the latest release tag v2.10.1 (5ad5ab3af0054c55bdce09f1530085600e9f2e45). The issue is independent from the row-bound allocation report, so I am reporting it separately.

Affected package

• Package: github.com/xuri/excelize/v2
• Tested affected versions: current default branch at 1213a8bd7c5ab360554603ac5c995ccaf6eb4314, and release v2.10.1
• Fixed version: none known at the time of this report

Impact

An attacker who can provide an XLSX file to an application using Excelize can trigger a process panic when the application reads the malicious cell through common APIs such as GetCellValue or GetRows. In services that parse untrusted spreadsheets without a panic recovery boundary, this can cause denial of service.

Root cause

For shared-string cells (t="s"), xlsxC.getValueFrom() parses the cell value as a shared-string index and only checks whether the index is below len(d.SI) before indexing:

xlsxSI, _ := strconv.Atoi(strings.TrimSpace(c.V))
if len(d.SI) > xlsxSI {
    return d.SI[xlsxSI].String(), nil
}

For xlsxSI == -1, len(d.SI) > -1 is true, so the code proceeds to index d.SI[-1] and panics.

Minimal worksheet payload

<?xml version="1.0" encoding="UTF-8"?>
<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
  <sheetData>
    <row r="1"><c r="A1" t="s"><v>-1</v></c></row>
  </sheetData>
</worksheet>

The workbook also contains a normal sharedStrings.xml with one string (ok), so the failure is specifically due to accepting a negative index.

Reproduction

Calling GetCellValue("Sheet1", "A1") on the workbook panics:

== negative shared string GetCellValue ==
elapsed=0s alloc_delta=0MB
PANIC: runtime.boundsError runtime error: index out of range [-1]

Calling GetRows("Sheet1") on the same workbook also panics:

== negative shared string GetRows ==
elapsed=0s alloc_delta=0MB
PANIC: runtime.boundsError runtime error: index out of range [-1]

The same results were observed on current default branch commit 1213a8bd7c5ab360554603ac5c995ccaf6eb4314 and on release v2.10.1.

Expected behavior

Malformed shared-string indices should be rejected or treated as missing/invalid string references without panicking.

Suggested remediation

Check both lower and upper bounds before indexing the shared string table. For example:

if xlsxSI >= 0 && xlsxSI < len(d.SI) {
    return d.SI[xlsxSI].String(), nil
}

Add regression tests for GetCellValue() and GetRows() on t="s" cells whose <v> value is negative.