CVE-2026-49755: Req Req Vulnerable To Unbounded Archive/compression Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-655f-mp8p-96gv",
  "modified" : "2026-07-29T15:23:16Z",
  "published" : "2026-07-29T15:23:16Z",
  "aliases" : [ "CVE-2026-49755" ],
  "summary" : "Req vulnerable to unbounded archive/compression extraction triggered by response content-type",
  "details" : "### Summary\n\nReq's default response pipeline auto-decodes archive and compressed bodies based on the server-supplied `content-type` (or URL extension) and materialises the full decompressed contents in memory with no size cap. An attacker who controls (or can redirect a victim into) an HTTP endpoint reached by `Req.get!/1` can return a tiny \"decompression bomb\" that expands to many gigabytes on the client and exhausts the BEAM's memory.\n\n### Details\n\n**1. Archive auto-decoding.** `Req.Steps.decode_body/1` in `lib/req/steps.ex` dispatches on the response `content-type` (or URL extension) and calls Erlang's archive libraries with `:memory`, returning a `[{name, bytes}]` list of every entry fully decompressed in RAM: `application/zip` → `:zip.extract(body, [:memory])`, `application/x-tar` → `:erl_tar.extract({:binary, body}, [:memory])`, `application/gzip` / `.tgz` → `:erl_tar.extract({:binary, body}, [:memory, :compressed])`. No byte cap is enforced before decoding and no per-entry size limit is passed to `:zip` / `:erl_tar`.\n\n**2. content-encoding chaining.** `Req.Steps.decompress_body/1` walks the `content-encoding` header and chains `:zlib` / `:brotli` / `:ezstd` decoders, so a response advertising `content-encoding: gzip, gzip, gzip, …` inflates through multiple layers without bound.\n\n**3. Default-on, attacker-chosen decoder.** Both steps are part of Req's default pipeline. The caller does not need to opt in, and the attacker chooses which decoder fires by setting `content-type` and `content-encoding` on their own server (or on any host reached via Req's automatic redirect following).\n\n### PoC\n\n1. Run an HTTP server that responds 200 with `content-type: application/zip` and a body that is a zip archive whose single entry is ~400 MB of zero bytes (compressed wire payload: a few hundred KB).\n2. From the victim process, call `Req.get!(url)` against that server (no special options, no opt-in to archive decoding).\n3. `decode_body/1` dispatches on `content-type`, invokes `:zip.extract(body, [:memory])`, and the response body becomes `[{~c\"bomb.bin\", <<400 MB of zero bytes>>}]`. A sub-MB request produces hundreds of MB resident memory; layering gzip on the `content-encoding` path or increasing entry size scales arbitrarily.\n\n### Impact\n\nMemory-exhaustion denial of service against any Elixir application that uses Req with its default step pipeline to fetch URLs influenced by an untrusted party, including webhook senders, link previews, OAuth/OIDC discovery clients, package mirrors, image proxies, and any `Req.get!/1` call that may follow redirects to attacker-controlled hosts. No authentication is required; a single response can crash the BEAM and take down unrelated workloads on the same VM.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "req"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.1.0"
      }, {
        "fixed" : "0.6.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/wojtekmach/req/security/advisories/GHSA-655f-mp8p-96gv"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-49755"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/wojtekmach/req/commit/84977e5b1a83f26e749d55ad06e3625464af4e8d"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-49755.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/wojtekmach/req"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-49755"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-409" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-29T15:23:16Z",
    "nvd_published_at" : "2026-06-08T16:16:43Z"
  }
}