CVE-2021-39216: Wasmtime Use After Free Passing `externref`s Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-v4cp-h94r-m7xf",
  "modified" : "2024-11-19T18:00:37Z",
  "published" : "2021-09-20T19:54:05Z",
  "aliases" : [ "CVE-2021-39216" ],
  "summary" : "Use after free passing `externref`s to Wasm in Wasmtime",
  "details" : "### Impact\n\nThere was a use-after-free bug when passing `externref`s from the host to guest Wasm content.\n\nTo trigger the bug, you have to explicitly pass multiple `externref`s from the host to a Wasm instance at the same time, either by\n\n* passing multiple `externref`s as arguments from host code to a Wasm function,\n* or returning multiple `externref`s to Wasm from a multi-value return function defined in the host.\n \nIf you do not have host code that matches one of these shapes, then you are not impacted.\n\nIf Wasmtime's [`VMExternRefActivationsTable`](https://github.com/bytecodealliance/wasmtime/blob/37c094faf53f1b356aab3c79d451395e4f7edb34/crates/runtime/src/externref.rs#L493) became filled to capacity after passing the first `externref` in, then passing in the second `externref` could trigger a garbage collection. However the first `externref` is not rooted until we pass control to Wasm, and therefore could be reclaimed by the collector if nothing else was holding a reference to it or otherwise keeping it alive. Then, when control was passed to Wasm after the garbage collection, Wasm could use the first `externref`, which at this point has already been freed.\n\nWe have reason to believe that the effective impact of this bug is relatively small because usage of `externref` is currently quite rare.\n\n### Patches\n\nThe bug has been fixed, and users should upgrade to Wasmtime 0.30.0.\n\nAdditionally, we have updated [our primary `externref` fuzz target](https://github.com/bytecodealliance/wasmtime/blob/37c094faf53f1b356aab3c79d451395e4f7edb34/fuzz/fuzz_targets/table_ops.rs) such that it better exercises these code paths and we can have greater confidence in their correctness going forward.\n\n### Workarounds\n\nIf you cannot upgrade Wasmtime yet, you can avoid the bug by disabling reference types support in Wasmtime by passing `false` to [`wasmtime::Config::wasm_reference_types`](https://docs.rs/wasmtime/0.29.0/wasmtime/struct.Config.html#method.wasm_reference_types).\n\n### References\n\n* [The reference types Wasm proposal, which introduces `externref`](https://github.com/WebAssembly/reference-types/)\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n\n* Reach out to us on [the Bytecode Alliance Zulip chat](https://bytecodealliance.zulipchat.com/#narrow/stream/217126-wasmtime)\n* Open an issue in [the `bytecodealliance/wasmtime` repository](https://github.com/bytecodealliance/wasmtime/)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "wasmtime"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.30.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "wasmtime"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.30.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-v4cp-h94r-m7xf"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-39216"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/bytecodealliance/wasmtime/commit/101998733b74624cbd348a2366d05760b40181f3"
  }, {
    "type" : "WEB",
    "url" : "https://crates.io/crates/wasmtime"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/bytecodealliance/wasmtime"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/bytecodealliance/wasmtime-py/compare/0.29.0...0.30.0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/wasmtime/PYSEC-2021-320.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/WAVBRYDDUIY2ZR3K3FO4BVYJKIMJ5TP7"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/Z2Z33FTXFQ6EOINVEQIP4DFBG53G5XIY"
  }, {
    "type" : "WEB",
    "url" : "https://rustsec.org/advisories/RUSTSEC-2021-0110.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-416" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-09-17T20:05:21Z",
    "nvd_published_at" : "2021-09-17T20:15:00Z"
  }
}