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

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.

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.

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.

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.