Dasel is a command-line tool and library for querying, modifying, and transforming data structures. From 3.0.0 until 3.10.1, the escape sequence handler in (*Tokenizer).parseCurRune in selector/lexer/tokenize.go increments past a trailing backslash in a quoted string such as "\ or '\ and then reads p.src[pos] without a bounds check, allowing attacker-controlled selector strings to trigger a Go index-out-of-range panic. This issue is fixed in version 3.10.1.

Affected: Go: github.com/tomwright/dasel/v3

### Summary

`dasel`'s selector lexer panics with an index-out-of-range error when tokenizing a quoted string that ends with a trailing backslash (e.g., `"\` or `'\`). A 2-byte input causes an immediate process crash via Go runtime panic.

I confirmed the issue on `v3.3.1` (`fba653c7f248aff10f2b89fca93929b64707dfc8`) and on `master` commit `0dd6132e0c58edbd9b1a5f7ffd00dfab1e6085ad`. I also verified the same code path is present in `v3.0.0` (`648f83baf070d9e00db8ff312febef857ec090a3`). No fix is available yet.

### Details

The bug is in the escape sequence handler within `(*Tokenizer).parseCurRune` in [`selector/lexer/tokenize.go#L191-L194`](https://github.com/TomWright/dasel/blob/fba653c7f248aff10f2b89fca93929b64707dfc8/selector/lexer/tokenize.go#L191-L194):

```go if p.src[pos] == '\\' { pos++ buf = append(buf, rune(p.src[pos])) // line 193: no bounds check pos++ continue } ```

When a backslash is the last character inside quotes, `pos++` increments the position past the end of the input. The subsequent `p.src[pos]` attempts to read past the end of the slice, which Go turns into a runtime panic: `runtime error: index out of range [2] with length 2`.

Notably, the same function already handles unterminated quoted strings by returning `UnexpectedEOFError`, but the escape sequence path does not perform a similar bounds check.

Minimal trigger: `"\` or `'\` (2 bytes)

Test environment:

- MacBook Air (Apple M2), macOS / Darwin `arm64` - Go `1.26.1` - dasel `v…

Affected: Ubuntu:Pro:24.04:LTS: dasel, Ubuntu:26.04:LTS: dasel

Dasel is a command-line tool and library for querying, modifying, and transforming data structures. From 3.0.0 until 3.10.1, the escape sequence handler in (*Tokenizer).parseCurRune in selector/lexer/tokenize.go increments past a trailing backslash in a quoted string such as "\ or '\ and then reads p.src[pos] without a bounds check, allowing attacker-controlled selector strings to trigger a Go index-out-of-range panic. This issue is fixed in version 3.10.1.