{"id":"EEF-CVE-2026-119434","summary":"Decompression bomb in ezstd:decompress_using_ddict/2 allows BEAM memory exhaustion via unbounded output buffer","details":"## Summary\n\nImproper Handling of Highly Compressed Data (Data Amplification) vulnerability in silviucpp ezstd allows an attacker who supplies a zstd frame to `ezstd:decompress_using_ddict/2` to exhaust memory and terminate the Erlang VM via a small frame that declares a very large content size.\n\n`zstd_nif_decompress_using_ddict` in `c_src/ezstd_nif.cc` reads the frame's declared content size with `ZSTD_getFrameContentSize` and passes it directly to `enif_make_new_binary`, without the 1 GiB `MAX_BUFFER_SIZE` check that `ezstd:decompress/1` applies before allocating. A valid 65,550-byte frame of RLE blocks declaring 2 GiB is rejected by `ezstd:decompress/1` but is fully allocated and decompressed by the dictionary path. The allocation happens inside a NIF, so when it exceeds the memory available to the VM the operating system kills the whole node rather than the calling process.\n\nThis issue affects ezstd: from 1.0.5 before 1.2.5.\n\n## Details\n\n**1. Declared size is attacker controlled.** A zstd frame header carries an optional Frame Content Size field. `zstd_nif_decompress_using_ddict` in `c_src/ezstd_nif.cc` reads it with `ZSTD_getFrameContentSize` and rejects only the two sentinel values `ZSTD_CONTENTSIZE_UNKNOWN` and `ZSTD_CONTENTSIZE_ERROR`.\n\n**2. Unbounded allocation.** The size is passed directly to `enif_make_new_binary`, so a frame declaring 2 GiB makes the NIF request a 2 GiB binary. The sibling `zstd_nif_decompress` rejects any declared size above `MAX_BUFFER_SIZE` (1 GiB) before allocating; the dictionary path omitted that check from the commit that introduced it.\n\n**3. Amplification.** A frame built from RLE blocks, each encoding 128 KiB of a repeated byte in four bytes, declares and genuinely decompresses to 2 GiB from 65,550 bytes of input, a ratio of about 32,760:1. `ZSTD_decompress_usingDDict` then fills the buffer. Because this happens inside a NIF, exceeding the memory available to the VM results in the operating system killing the whole BEAM process rather than an Erlang-level exception.\n\n## Proof of concept\n\n1. Build ezstd 1.2.4 and start a VM under an external memory limit, for example a container limited to 256 MiB.\n2. Create any decompression dictionary with `ezstd:create_ddict/1`; its content does not matter because the frame carries no dictionary ID.\n3. Construct a zstd frame: the magic number, a frame header descriptor selecting an 8-byte content size, a window descriptor, a little-endian content size of 2 GiB, then 16,383 RLE blocks of 128 KiB each (block header `0x02 0x00 0x10` followed by one payload byte) and a final RLE block with the last-block bit set. The frame is 65,550 bytes.\n4. Call `ezstd:decompress/1` on the frame: it returns `{error, \u003c\u003c\"frame too large or invalid\"\u003e\u003e}`.\n5. Call `ezstd:decompress_using_ddict/2` on the same frame with the dictionary: the VM is killed by the operating system (exit status 137).\n6. Repeat on 1.2.5: the call returns `{error, \u003c\u003c\"frame too large or invalid\"\u003e\u003e}` and the VM continues.\n\n## Impact\n\nAn attacker who can submit a zstd frame to a service that decompresses it with a fixed dictionary can crash the entire Erlang VM with a single request of about 65 KiB. Every application and process hosted in that VM is taken down, not only the one that called the decompressor.\n\n## Workarounds\n\nReplace calls to `ezstd:decompress_using_ddict/2` with the streaming API: create a context with `ezstd:create_decompression_context/1` using a small output buffer, attach the dictionary with `ezstd:select_ddict/2`, and decompress with `ezstd:decompress_streaming/2`. The streaming path decompresses into the caller-sized buffer and gives up with `{error, decompressor_stuck}` after 1000 chunks, so a 64 KiB buffer caps a single call at 64 MiB of output. Treat that error as a rejected input. Legitimate frames whose output exceeds 1000 times the buffer size fail the same way, so size the buffer for the largest expected payload.","aliases":["CVE-2026-119434"],"modified":"2026-11-05T20:30:03.151855300Z","published":"2026-11-05T20:15:56.311Z","database_specific":{"capec_ids":["CAPEC-130"],"cpe_ids":["cpe:2.3:a:silviucpp:ezstd:*:*:*:*:*:*:*:*"],"cwe_ids":["CWE-409"]},"references":[{"type":"WEB","url":"https://github.com/silviucpp/ezstd/releases/tag/v1.2.5"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-119434.html"},{"type":"WEB","url":"https://github.com/silviucpp/ezstd/commit/6730128b1d35eca02a2028671421471ff651f423"},{"type":"FIX","url":"https://github.com/silviucpp/ezstd/commit/e34f4a4f6dc18d990d1cece2d5fceef8e934a195"},{"type":"PACKAGE","url":"https://hex.pm/packages/ezstd"}],"affected":[{"package":{"name":"ezstd","ecosystem":"Hex","purl":"pkg:hex/ezstd"},"ranges":[{"type":"SEMVER","events":[{"introduced":"1.0.5"},{"fixed":"1.2.5"}]}],"versions":["1.0.5","1.0.6","1.0.7","1.0.8","1.0.9","1.1.0","1.2.0","1.2.1","1.2.2","1.2.3","1.2.4"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-119434.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/silviucpp/ezstd","events":[{"introduced":"6730128b1d35eca02a2028671421471ff651f423"},{"fixed":"e34f4a4f6dc18d990d1cece2d5fceef8e934a195"}]}],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-119434.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"}],"credits":[{"name":"Oz Kaiser","type":"FINDER"},{"name":"Oz Kaiser","type":"REPORTER"},{"name":"Silviu Caragea","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}