Dasel: Index-out-of-range panic in dasel selector lexer on trailing backslash in quoted string

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:

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 v3.3.1 (fba653c7f248aff10f2b89fca93929b64707dfc8)

PoC

package main

import (
	"fmt"
	"runtime"
	"runtime/debug"

	"github.com/tomwright/dasel/v3/selector/lexer"
)

func main() {
	fmt.Printf("Go version: %s\n", runtime.Version())
	fmt.Printf("GOARCH: %s\n", runtime.GOARCH)
	fmt.Println()

	for _, input := range []string{`"\`, `'\`} {
		fmt.Printf("Input: %s\n", input)
		func() {
			defer func() {
				if r := recover(); r != nil {
					fmt.Printf("PANIC: %v\n", r)
					debug.PrintStack()
				}
			}()
			t := lexer.NewTokenizer(input)
			tokens, err := t.Tokenize()
			if err != nil {
				fmt.Printf("Error: %v\n", err)
			} else {
				fmt.Printf("OK: %d tokens\n", len(tokens))
			}
		}()
		fmt.Println()
	}
}

Observed output on v3.3.1 in the test environment above:

Go version: go1.26.1
GOARCH: arm64

Input: "\
PANIC: runtime error: index out of range [2] with length 2
goroutine 1 [running]:
...
github.com/tomwright/dasel/v3/selector/lexer.(*Tokenizer).parseCurRune(...)
    .../selector/lexer/tokenize.go:193 +0x1c2c
...

Input: '\
PANIC: runtime error: index out of range [2] with length 2
goroutine 1 [running]:
...
github.com/tomwright/dasel/v3/selector/lexer.(*Tokenizer).parseCurRune(...)
    .../selector/lexer/tokenize.go:193 +0x1c2c
...

Impact

An attacker who can control or influence the selector/query string passed to dasel can trigger a Go runtime panic and crash the process unless the caller explicitly recovers from panics.

The selector string is typically provided by the application developer, but there are deployment scenarios where it may be attacker-influenced:

• Web applications using dasel for dynamic data querying
• Applications that construct selectors from user input
• Shared tooling environments where selectors are passed as parameters

Suggested Fix

Add a bounds check after incrementing pos past the backslash, consistent with the existing UnexpectedEOFError handling for unterminated quoted strings:

if p.src[pos] == '\\' {
    pos++
    if pos >= p.srcLen {
        return Token{}, &UnexpectedEOFError{Pos: pos}
    }
    buf = append(buf, rune(p.src[pos]))
    pos++
    continue
}