CVE-2026-76840: RustDesk through 1.4.9 Heap Buffer Overflow via Unvalidated CLIPRDR FileContentsResponse Length

RustDesk's Windows clipboard redirection copies a peer-supplied length into a fixed-size caller buffer without an upper bound check. When an OLE paste consumer such as explorer.exe calls IStream::Read with a buffer of cb bytes, CliprdrStreamRead in libs/clipboard/src/windows/wfcliprdr.c requests that many bytes of a remote file through cliprdrsendrequestfilecontents and then executes CopyMemory(pv, clipboard->reqfdata, clipboard->reqfsize), where reqfsize is taken verbatim from the peer's CLIPRDR FileContentsResponse by wfcliprdrserverfilecontentsresponse (reqfsize = fileContentsResponse->cbRequested) and is never clamped to cb anywhere in the chain. The function's only length comparison, reqfsize < cb, handles the short-read case and is evaluated after the copy has already occurred. A malicious or compromised peer that answers a small file-contents read with an oversized response therefore writes attacker-chosen data past the end of the paste consumer's heap buffer when the local user pastes clipboard file contents offered by the remote side. The file is a fork of FreeRDP's client/Windows/wfcliprdr.c, where the same defect is CVE-2026-68579, fixed in FreeRDP 3.30.0.