A corrupt or hostile input produces an error, never anything worse. The decoders read untrusted data:

• ZstdDecompressor.Decompress returns OperationStatus.InvalidData for a frame libzstd refuses, and for a block over RFC 8878's size limit, which libzstd only partly enforces.
• Lz4Block.TryDecompress returns false, and Lz4Block.Decompress throws InvalidDataException, for a literal or a match past either end, an offset of zero or before the start, and a block that does not end with literals.

Neither reads nor writes outside the spans it is given, and neither allocates what the input asks for: a decompressor's memory is bounded by the format's block size and by the dictionary the caller passes, and its output by the destination the caller passes. The tests fuzz both decoders with each buffer placed against an inaccessible memory page, so that a single byte read or written out of bounds faults.

• A frame's declared content size is a claim. ZstdDecompressor.TryGetFrameContentSize reads it from the header, unchecked: allocate from it only up to a limit you choose.
• Integrity without a checksum. A Zstandard frame's checksum is verified when the frame has one, but a corruption that leaves a frame without one valid decodes to wrong content, and the LZ4 block format has no checksum at all. Neither format authenticates anything: protection against tampering needs a MAC or a signature around the compressed data.
• Confidentiality. Compressing a secret together with data an attacker controls can reveal the secret through the compressed size (CRIME, BREACH). Nothing here runs in constant time.
• Bounded compression costs. A compressor's memory and time grow with its level and its input; a service that lets an untrusted party choose either has to bound them.

Report a vulnerability through the repository's Security tab, with "Report a vulnerability". That opens a private advisory visible only to you and the maintainers. A public issue is the wrong place, and cannot be taken back.

Include the frame or block that reproduces it, and the call that reads or writes it. A crash, a hang or an out-of-bounds access on a corrupt input is a bug within the scope of the guarantee above, and is treated as one.