CVE-2026-102820: pageant: Out-of-bounds read / oversized allocation in `pageant` MemoryMap::read via a malicious Pageant agent (Windows)

MemoryMap::read in the pageant crate (part of the russh workspace, used by russh’s SSH-agent client on Windows via AgentClient::connect_pageant) copies a peer-controlled number of bytes out of an 8192-byte shared-memory view with no bounds check — unlike the sibling MemoryMap::write, which correctly rejects oversize access with Error::Overflow. The byte count comes straight from a u32 length prefix that the responding “Pageant” process writes into the shared mapping. A malicious local process that answers as the Pageant agent can therefore cause:

• an out-of-bounds read past the 8 KiB view (access violation → process crash; or disclosure of adjacent process memory if the following page is committed)
• an allocation of up to ~4 GiB from a single u32 (vec![0; n]).

This was reproduced end-to-end against the real, unmodified pageant crate (not a model) on x86_64-pc-windows-gnu under Wine; see “Proof of concept”.