CVE-2026-107213: Excelize: Nil-pointer dereference in GetSlicers when a worksheet has extLst present but no drawing element

### Summary GetSlicers guards only `if ws.ExtLst == nil { return }` (slicer.go:816-818) and then immediately dereferences `ws.Drawing.RID` with no nil check on ws.Drawing itself. `Drawing` and `ExtLst` are two independently optional child elements of <worksheet> in xl/worksheets/sheetN.xml, populated separately by encoding/xml depending on whether each tag is present. I executed a PoC: took a plain NewFile() workbook (no drawing, no slicer), saved it, then injected a bare `<extLst></extLst>` immediately before `</worksheet>` in xl/worksheets/sheet1.xml (no other change, no drawing element present) and called GetSlicers. Result: `runtime error: invalid memory address or nil pointer dereference`, confirmed at the ws.Drawing.RID line via stack trace. ### Reachability / who can trigger this Unauthenticated: any application that opens an untrusted .xlsx and calls the public `File.GetSlicers(sheet)` API. The trigger is a single empty XML tag with no valid slicer or drawing content required at all, making it an even smaller/simpler payload than candidate 1. ### Proof of Concept / Reproduction **Method:** 1) Reused the existing clone at C:\Users\vatsa\AppData\Local\Temp\claude\D--\59901fa2-b549-40e1-a433-30c8a3d01718\scratchpad\CVE-Hunt-10k\repos\qax-os__excelize (origin=https://github.com/qax-os/excelize.git, branch master up to date, commit e81f05008b33232151a58f06474fad5f70c4d060 dated 2026-09-27, `git describe`=v2.11.0-42-ge81f050, i.e. 42 commits past the v2.11.0 tag). 2) Manually re-read the cited source myself (not trusting the claim): slicer.go:805-820 (GetSlicers) and xmlWorksheet.go:21-72 (xlsxWorksheet struct + xlsxDrawing struct) -- confirmed verbatim, matches claim exactly. 3) Confirmed local toolchain: go version go1.27.0 windows/amd64 (/c/Program Files/Go/bin/go). 4) Built the whole package with `go build ./...` from the repo root -- succeeded with zero errors, proving the current checkout compiles cleanly. 5) Ran the pre-existing PoC test file already present in this checkout (slicer_extlst_poc_test.go, TestGetSlicersNilDrawingPanic) via `go test -run TestGetSlicersNilDrawingPanic -v .`: it builds a benign excelize.NewFile() workbook, saves it, reopens it, loads the raw zip entry xl/worksheets/sheet1.xml, string-replaces `</worksheet>` with `<extLst></extLst></worksheet>` (adds nothing else, no <drawing> anywhere), rebuilds the zip byte-for-byte otherwise unchanged (rezipReplacing helper, plain archive/zip re-encode), reopens the crafted file with excelize.OpenReader, and calls the public GetSlicers("Sheet1") API inside a recover(). 6) For a fully independent, non-test-harness reproduction, I additionally wrote and built my own separate standalone Go module+program from scratch (excelize-nilptr-repro/{go.mod,main.go} in my scratchpad dir) that imports the real, unmodified excelize package as an external library dependency via a `replace github.com/xuri/excelize/v2 => ../CVE-Hunt-10k/repos/qax-os__excelize` directive (no copy-pasted/reimplemented function -- the actual library code, used through its real public API from a separate consumer program, simulating "any application that opens an untrusted .xlsx"). This program performs the identical craft-and-open sequence but calls GetSlicers with NO recover() at all. Built with `go build -o repro.exe .` (succeeded) and ran `./repro.exe` directly, observing an unhandled process crash. **Evidence:** SOURCE VERIFICATION (current HEAD, read directly): slicer.go:816-820 is exactly as claimed -- `if ws.ExtLst == nil { return slicers, err }` followed immediately by `target := f.getSheetRelationshipsTargetByID(sheet, ws.Drawing.RID)` with no nil check on ws.Drawing. xmlWorksheet.go:54 `Drawing *xlsxDrawing `xml:"drawing"`` and xmlWorksheet.go:64 `ExtLst *xlsxExtLst `xml:"extLst"`` are both independently-optional pointer fields on xlsxWorksheet, populated only when their respective XML tag is present in the worksheet part -- confirming Drawing and ExtLst are unrelated/independent. getSheetRelationshipsTargetByID(sheet, rID string) string (sheet.go:721) takes a plain string, so `ws.Drawing.RID` is evaluated (dereferencing the nil pointer) at the call site itself. RUN 1 -- existing repo test, `go test -run TestGetSlicersNilDrawingPanic -v .`: === RUN TestGetSlicersNilDrawingPanic slicer_extlst_poc_test.go:38: original sheet1.xml: ...<sheetData>...</sheetData></worksheet> (no drawing, no extLst) slicer_extlst_poc_test.go:48: tampered sheet1.xml: ...</sheetData><extLst></extLst></worksheet> slicer_extlst_poc_test.go:59: CONFIRMED: GetSlicers panicked on nil ws.Drawing dereference: runtime error: invalid memory address or nil pointer dereference --- PASS: TestGetSlicersNilDrawingPanic (0.01s) PASS ok github.com/xuri/excelize/v2 0.100s RUN 2 -- my own independent standalone consumer program (real unmodified library via go.mod replace, NO recover installed), `./repro.exe`: [*] Saved benign workbook: 6052 bytes [*] Original sheet1.xml: ...<sheetData>...</sheetData></worksheet> [*] Tampered sheet1.xml (attacker payload -- bare empty <extLst>, still no <drawing>): ...<extLst></extLst></worksheet> [*] Crafted malicious .xlsx: 6059 bytes [*] Crafted workbook accepted by OpenReader. Calling the public GetSlicers("Sheet1") API now, no recover() installed... panic: runtime error: invalid memory address or nil pointer dereference [signal 0xc0000005 code=0x0 addr=0x20 pc=0x7ff7fc8456b8] goroutine 1 [running]: github.com/xuri/excelize/v2.(*File).GetSlicers(0x27fa8874908, {0x7ff7fc8a5e8a, 0x6}) C:/.../qax-os__excelize/slicer.go:820 +0x98 main.main() C:/.../excelize-nilptr-repro/main.go:122 +0x67f EXIT CODE: 2 The stack trace pinpoints slicer.go:820 exactly (the ws.Drawing.RID dereference), from an ordinary external caller with no recover(), on a workbook whose only "malicious" content is one empty `<extLst></extLst>` tag and zero drawing/slicer content -- confirming the claim end-to-end: nil-pointer dereference, unrecovered panic, DoS, matching CWE-476.