EEF-CVE-2026-119434

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EEF-CVE-2026-119434

Summary: Decompression bomb in ezstd:decompress_using_ddict/2 allows BEAM memory exhaustion via unbounded output buffer

Details: ## Summary Improper 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. `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. This issue affects ezstd: from 1.0.5 before 1.2.5. ## Details **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`. **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. **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. ## Proof of concept 1. Build ezstd 1.2.4 and start a VM under an external memory limit, for example a container limited to 256 MiB. 2. Create any decompression dictionary with `ezstd:create_ddict/1`; its content does not matter because the frame carries no dictionary ID. 3. 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. 4. Call `ezstd:decompress/1` on the frame: it returns `{error, <<"frame too large or invalid">>}`. 5. Call `ezstd:decompress_using_ddict/2` on the same frame with the dictionary: the VM is killed by the operating system (exit status 137). 6. Repeat on 1.2.5: the call returns `{error, <<"frame too large or invalid">>}` and the VM continues. ## Impact An 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. ## Workarounds Replace 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.

References: https://github.com/silviucpp/ezstd/releases/tag/v1.2.5, https://cna.erlef.org/cves/CVE-2026-119434.html, https://github.com/silviucpp/ezstd/commit/6730128b1d35eca02a2028671421471ff651f423, https://github.com/silviucpp/ezstd/commit/e34f4a4f6dc18d990d1cece2d5fceef8e934a195, https://hex.pm/packages/ezstd