{"id":"EEF-CVE-2026-97200","summary":"Unbounded response body buffering and decompression in hackney","details":"## Summary\n\nAllocation of Resources Without Limits or Throttling vulnerability in `hackney_conn` in benoitc hackney allows a malicious HTTP server to exhaust client memory via an oversized or highly compressed response body. `hackney:request/5` and `hackney:body/1` return the whole response body, and nothing bounds how much the connection process buffers. Any server the client requests can trigger it.\n\n`hackney_conn:read_full_body/2` and the HTTP/2 and HTTP/3 accumulators append every chunk until the body ends. With `{auto_decompress, true}` the buffered body is then inflated in one step with `zlib:gunzip/1` or `zlib:inflate/2`, which return the whole output at once.\n\nThis issue affects hackney: from 2.0.0-beta.1 before 4.9.0.\n\n## Details\n\n**1. Unbounded accumulation.** `hackney:request/5` with a body, and `hackney:body/1` after `start_response/1`, read the response in `src/hackney_conn.erl` through `read_full_body/2`, which appends each chunk to a binary accumulator until the body ends. The HTTP/2 and HTTP/3 synchronous paths accumulate stream data the same way. No option caps the size.\n\n**2. One-step decompression.** With `{auto_decompress, true}` hackney sends `Accept-Encoding: gzip, deflate`. Once the body is complete, `maybe_decompress_body/2` dispatches on `Content-Encoding` to `decompress_gzip/1` (`zlib:gunzip/1`) or `decompress_deflate/1` (`zlib:inflate/2`). Both return the whole output at once, so the inflated size is bounded only by the compression ratio. The report measured a 1 MB gzip body inflating to 1 GB on hackney 4.8.4.\n\n## Proof of concept\n\n1. Build a gzip stream whose inflated size is 1 GB from a reused 64 KB chunk of zero bytes (deflate with window bits 31, best compression). The stream is about 1 MB on the wire.\n2. Start a TCP listener on loopback that accepts one connection, reads the request, and answers `HTTP/1.1 200 OK` with `Content-Encoding: gzip`, a `Content-Length` matching the stream, `Connection: close`, and the stream as body.\n3. From a node running a vulnerable hackney, call `hackney:request(get, Url, [], \u003c\u003c\u003e\u003e, [{auto_decompress, true}, with_body])` against that listener.\n4. Observe the returned body of 1,048,576,000 bytes and the VM memory growing by about 1.8 GB for a 1,019,197 byte response. A larger stream, or an endless body without `auto_decompress`, drives the node out of memory.\n\n## Impact\n\nA server the client requests can make the client allocate memory proportional to the body it sends, or to its decompressed size, until the node runs out of memory.\n\n## Workarounds\n\nLeave `auto_decompress` disabled (the default). Without it `hackney_conn:maybe_decompress_body/2` never inflates the body, which closes the amplification half. The uncapped raw body read has no workaround.","aliases":["CVE-2026-97200"],"modified":"2026-10-26T14:00:04.842482189Z","published":"2026-10-26T13:49:40.392Z","database_specific":{"capec_ids":["CAPEC-130"],"cpe_ids":["cpe:2.3:a:benoitc:hackney:*:*:*:*:*:*:*:*"],"cwe_ids":["CWE-409","CWE-770"]},"references":[{"type":"ADVISORY","url":"https://github.com/benoitc/hackney/releases/tag/4.9.0"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-97200.html"},{"type":"WEB","url":"https://github.com/benoitc/hackney/pull/980"},{"type":"WEB","url":"https://github.com/benoitc/hackney/commit/dc41b79339ca0c41e16d3f2dd17172bc2de855d6"},{"type":"FIX","url":"https://github.com/benoitc/hackney/commit/493e8bf83a391548e5101792276ab3fc428fa76c"},{"type":"PACKAGE","url":"https://hex.pm/packages/hackney"}],"affected":[{"package":{"name":"hackney","ecosystem":"Hex","purl":"pkg:hex/hackney"},"ranges":[{"type":"SEMVER","events":[{"introduced":"2.0.0-beta.1"},{"fixed":"4.9.0"}]}],"versions":["2.0.0","2.0.0-beta.1","2.0.1","3.0.0","3.0.1","3.0.2","3.0.3","3.1.0","3.1.1","3.1.2","3.2.0","3.2.1","4.0.0","4.0.1","4.0.2","4.0.3","4.1.0","4.2.0","4.2.1","4.2.2","4.2.3","4.3.0","4.4.0","4.4.1","4.4.2","4.4.3","4.4.5","4.5.0","4.5.1","4.5.2","4.6.0","4.6.1","4.7.0","4.7.1","4.7.2","4.7.3","4.7.4","4.8.0","4.8.1","4.8.2","4.8.3","4.8.4","4.8.5","4.8.6","4.8.7"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-97200.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/benoitc/hackney","events":[{"introduced":"dc41b79339ca0c41e16d3f2dd17172bc2de855d6"},{"fixed":"493e8bf83a391548e5101792276ab3fc428fa76c"}]}],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-97200.json"}}],"schema_version":"1.9.0","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"}],"credits":[{"name":"Martin Karrer","type":"FINDER"},{"name":"Quinn Wilton","type":"FINDER"},{"name":"Quinn Wilton","type":"REPORTER"},{"name":"Martin Karrer","type":"REMEDIATION_DEVELOPER"},{"name":"Benoit Chesneau","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}