CVE-2026-46673

### Title Unchecked `CryptoVec` allocation and growth handling was reachable from local agent inputs in current `russh` releases and from remote SSH traffic in historical pre-`0.58.0` releases ### Summary `CryptoVec` used unchecked capacity growth, unchecked length arithmetic, and unsafe allocation/locking paths. In current `russh` releases, local SSH agent peers could still feed attacker-controlled frame lengths into buffer growth before validation. In older `russh` releases before `0.58.0`, remote SSH traffic also reached `CryptoVec` through transport and compression buffers. ### Details The underlying unsafe paths were in `CryptoVec`: - `cryptovec/src/cryptovec.rs` - unchecked capacity growth - unchecked length arithmetic in growth callers - raw allocation and reallocation paths coupled to those sizes - `cryptovec/src/platform/unix.rs` - `mlock` / `munlock` previously accepted zero-length calls and performed null-pointer validation inside the `unsafe` OS-call path There are two relevant reachability stories: 1. current local reachability in `russh` - `russh/src/keys/agent/client.rs` - `AgentClient::read_response()` read a peer-supplied `u32` length and then resized `self.buf` to that value before reading the payload - `russh/src/keys/agent/server.rs` - `Connection::run()` read a peer-supplied `u32` length and then resized `self.buf` to that value before reading the payload This is the path that still existed in current `0.60.x` releases before the fix, although by then those buffers were no longer `CryptoVec`. 2. historical remote reachability in older `russh` - before commit `712e32b` (first released in `v0.58.0`), non-secret transport and compression buffers in `russh` still used `CryptoVec` - I verified this in a detached pre-`712e32b` worktree by adding and running: - `cipher::tests::remote_packet_length_grows_transport_cryptovec_buffer` - `compression::tests::remote_compressed_payload_expands_cryptovec_output` - those tests show that remote SSH traffic could grow `CryptoVec` through: - transport packet reads - zlib decompression output Also added a constrained-memory reproduction in that historical worktree: - `compression::tests::remote_compressed_payload_can_crash_under_memory_limit` That test re-execs the test binary under `prlimit --as=134217728`, decompresses a highly compressible payload that expands to `96 MiB`, and reliably aborts in the old Unix `CryptoVec` path when `NonNull::new_unchecked()` receives a null pointer after allocation failure. The prepared patch does two things: 1. hardens `CryptoVec` itself - checked capacity growth - checked length arithmetic - immediate allocation-failure handling - zero-length `mlock` / `munlock` no-ops - explicit null-pointer validation before entering the Unix `unsafe` locking calls 2. hardens the real untrusted-input path - caps agent frame lengths at `256 * 1024` on both client and server before resizing buffers This cap matches OpenSSH’s agent framing guardrail. ### PoC The following end-to-end tests demonstrate the real untrusted-input path by feeding oversized peer-controlled agent frame lengths into the public client and server flows and asserting that they are rejected before buffer growth. Client-side agent reply path: ```rust #[test] fn oversized_agent_response_is_rejected_before_allocation() -> std::io::Result<()> { let runtime = tokio::runtime::Builder::new_current_thread() .enable_all() .build()?; runtime.block_on(async { let (mut writer, reader) = tokio::io::duplex(64); let server = tokio::spawn(async move { let mut frame = [0u8; 4]; writer.read_exact(&mut frame).await?; let len = BigEndian::read_u32(&frame) as usize; let mut body = vec![0; len]; writer.read_exact(&mut body).await?; BigEndian::write_u32(&mut frame, (MAX_AGENT_FRAME_LEN + 1) as u32); writer.write_all(&frame).await?; Ok::<(), std::io::Error>(()) }); let mut client = AgentClient::connect(reader); let err = client.request_identities().await.unwrap_err(); assert!(matches!(err, Error::AgentProtocolError)); server.await.expect("server task")?; Ok::<(), std::io::Error>(()) })?; Ok(()) } ``` Server-side agent request path: ```rust #[test] fn oversized_agent_request_is_rejected_before_allocation() -> std::io::Result<()> { let runtime = tokio::runtime::Builder::new_current_thread() .enable_all() .build()?; runtime.block_on(async { let (server, mut client) = tokio::io::duplex(64); let connection = Connection { lock: Lock(std::sync::Arc::new(std::sync::RwLock::new(crate::CryptoVec::new()))), keys: KeyStore(std::sync::Arc::new(std::sync::RwLock::new( std::collections::HashMap::new(), ))), agent: Some(()), s: server, buf: Vec::new(), }; let server = tokio::spawn(async move { connection.run().await }); let mut frame = [0u8; 4]; BigEndian::write_u32(&mut frame, (MAX_AGENT_FRAME_LEN + 1) as u32); client.write_all(&frame).await?; drop(client); let err = server.await.expect("server task").unwrap_err(); assert!(matches!(err, Error::AgentProtocolError)); Ok::<(), std::io::Error>(()) })?; Ok(()) } ``` These tests pass on the fixed branch and fail on unfixed `v0.60.2`, where oversized agent frame lengths are not rejected at the framing boundary. For historical `russh < 0.58.0`, I also verified remote reachability into `CryptoVec` in a detached pre-`712e32b` worktree (`91d431d`, package version `0.57.1`). Transport packet read path: ```rust #[test] fn remote_packet_length_grows_transport_cryptovec_buffer() -> std::io::Result<()> { let runtime = tokio::runtime::Builder::new_current_thread() .enable_all() .build()?; runtime.block_on(async { let packet_len = MAXIMUM_PACKET_LEN; let (mut writer, mut reader) = tokio::io::duplex(packet_len + 4); let writer_task = tokio::spawn(async move { let mut packet = vec![0u8; packet_len + 4]; packet[..4].copy_from_slice(&(packet_len as u32).to_be_bytes()); writer.write_all(&packet).await?; Ok::<(), std::io::Error>(()) }); let mut buffer = SSHBuffer::new(); let mut cipher = clear::Key; let n = read(&mut reader, &mut buffer, &mut cipher).await.unwrap(); assert_eq!(n, packet_len + 4); assert_eq!(buffer.buffer.len(), packet_len + 4); assert_eq!(&buffer.buffer[..4], &(packet_len as u32).to_be_bytes()); writer_task.await.expect("writer task")?; Ok::<(), std::io::Error>(()) })?; Ok(()) } ``` Compression growth path: ```rust #[test] fn remote_compressed_payload_expands_cryptovec_output() { let payload = vec![b'A'; 64 * 1024]; let compression = Compression::new(&ZLIB); let mut compressor = Compress::None; let mut decompressor = Decompress::None; compression.init_compress(&mut compressor); compression.init_decompress(&mut decompressor); let mut compressed = CryptoVec::new(); let encoded = compressor .compress(&payload, &mut compressed) .expect("compress") .to_vec(); let mut output = CryptoVec::new(); let decoded = decompressor .decompress(&encoded, &mut output) .expect("decompress"); assert_eq!(decoded.len(), payload.len()); assert_eq!(decoded, payload.as_slice()); assert!(encoded.len() < output.len()); } ``` Constrained-memory crash reproduction for the historical remote compression path: ```rust #[test] fn remote_compressed_payload_can_crash_under_memory_limit() { const CHILD_ENV: &str = "RUSSH_REMOTE_COMPRESS_CRASH_CHILD"; if std::env::var_os(CHILD_ENV).is_some() { let payload = vec![b'A'; 96 * 1024 * 1024]; let compression = Compression::new(&ZLIB); let mut compressor = Compress::None; let mut decompressor = Decompress::None; compression.init_compress(&mut compressor); compression.init_decompress(&mut decompressor); let mut compressed = CryptoVec::new(); let encoded = compressor .compress(&payload, &mut compressed) .expect("compress") .to_vec(); let mut output = CryptoVec::new(); let decoded = decompressor .decompress(&encoded, &mut output) .expect("decompress"); assert_eq!(decoded.len(), payload.len()); return; } let exe = std::env::current_exe().expect("current exe"); let status = Command::new("prlimit") .args([ "--as=134217728", "--", exe.to_str().expect("utf8 exe path"), "--exact", "compression::tests::remote_compressed_payload_can_crash_under_memory_limit", "--nocapture", ]) .env(CHILD_ENV, "1") .status() .expect("spawn child"); assert!( !status.success(), "expected child to fail under constrained address space" ); } ``` On that historical worktree, the constrained-memory child aborts in the old Unix `CryptoVec` path with: ```text unsafe precondition(s) violated: NonNull::new_unchecked requires that the pointer is non-null thread caused non-unwinding panic. aborting. ``` To run the reproduced checks: ```bash cargo test -p russh oversized_agent_response_is_rejected_before_allocation -- --nocapture cargo test -p russh oversized_agent_request_is_rejected_before_allocation -- --nocapture cargo test -p russh-cryptovec ``` Historical pre-`0.58.0` checks were run from the detached `91d431d` worktree with: ```bash cargo test --offline -p russh remote_packet_length_grows_transport_cryptovec_buffer -- --nocapture cargo test --offline -p russh remote_compressed_payload_expands_cryptovec_output -- --nocapture cargo test --offline -p russh remote_compressed_payload_can_crash_under_memory_limit -- --nocapture ``` ### Impact This is a memory-safety hardening issue with demonstrated untrusted-input reachability. What is demonstrated: - current local agent peers could previously reach allocation growth directly from attacker-controlled frame lengths - historical remote SSH traffic could previously reach `CryptoVec` through transport and compression buffers in `russh < 0.58.0` - under constrained memory, the historical remote compression path can be turned into a process abort in the old Unix `CryptoVec` code - the fixed code now rejects oversized agent frames early and hardens the underlying allocation paths What is not demonstrated: - practical code execution - a demonstrated integrity or confidentiality break

`CryptoVec` used unchecked capacity growth, unchecked length arithmetic, and unsafe allocation and locking paths. In affected `russh` releases, attacker-controlled input could reach these code paths through buffer resizing operations. Two affected reachability paths were identified: * **Current `russh` releases (`0.60.x` before the fix)** Local SSH agent peers could provide attacker-controlled frame lengths that were used to resize internal buffers before validation in: * `AgentClient::read_response` * `agent::server::Connection::run` * **Historical `russh` releases before `0.58.0`** `CryptoVec` was also used for non-secret transport and compression buffers, allowing remote SSH traffic to trigger `CryptoVec` growth through: * transport packet reads * zlib decompression output These remote paths were removed in `0.58.0` when `CryptoVec` stopped being used for those buffers. Under constrained memory conditions, historical `russh` versions prior to `0.58.0` can abort the process when remote compressed payload expansion causes allocation failure in `CryptoVec`. This was reproduced through the compression path and resulted in process termination in the Unix allocation/locking implementation after null pointer allocation failure. For current affected releases, oversized local SSH agent frame lengths could trigger untrusted-input-driven buffer growth prior to validation. No practical remote code execution, integrity or confidentiality impact has been demonstrated. Fixed by validating CryptoVec growth operations and rejecting oversized SSH agent frame lengths before buffer allocation.

Both the SSH agent server and client accepted peer-controlled frame lengths without enforcing a maximum frame size. This could cause large memory allocations while parsing a maliciously crafted agent frame. A malicious peer could advertise an oversized frame length, causing the client or server to attempt a large memory allocation before validating the frame, potentially leading to memory exhaustion or process termination. This is fixed by enforcing a maximum agent frame size of 256 KiB and rejecting oversized frames before buffer allocation.