## 1. Summary The shared HTTP CONNECT helper parses a proxy response with `http.ReadResponse` over an unrestricted buffered reader. The production helper accepted a valid response containing a 2 MiB header in three consecutive runs. A malicious or compromised configured proxy, or an active on-path actor controlling a plaintext HTTP-proxy hop, can grow memory until the process fails. The security impact is process-wide exhaustion, not loss of access through the malicious proxy, which the proxy already controls. The victim must configure and use the proxy, so UI is Required and the rating is Medium. ## 2. Affected Assets & Attack Surface - Verified rclone revision: `a0c09f1381ae93e2a9a33c529d170186c61ad058` (`v1.74.0-240-ga0c09f138`) - Current-master check: `lib/proxy/http.go` was unchanged at master commit `961266888fe797390c535386f3b3aa46f4853602` on 2026-07-18 - Shared helper: `lib/proxy/http.go:23-81` - SFTP use: `backend/sftp/ssh_internal.go:25-45` - FTP use: `backend/ftp/ftp.go:465-479` - Proxy peer: configured malicious/compromised proxy or active on-path actor for a plaintext HTTP proxy - TLS boundary: HTTPS proxy connections authenticate the proxy before this response is parsed, so an on-path actor must also defeat TLS ## 3. Technical Root Cause Analysis `HTTPConnectDial` invokes `http.ReadResponse(br, req)` directly. This call does not inherit `http.Transport.MaxResponseHeaderBytes`. In the Go implementation used for validation, exported `textproto.Reader.ReadMIMEHeader` passes `math.MaxInt64` limits, and `textproto.NewReader` explicitly instructs callers to use `io.LimitReader` or an equivalent bound for denial-of-service resistance. Rclone supplies no bound or total CONNECT-handshake deadline. The helper additionally returns the raw connection, so a safe remediation must preserve any tunnel bytes already buffered after the CONNECT response. ## 4. Proof-of-Concept & Evidence 1. Configure the helper to use a test proxy. 2. Accept rclone's CONNECT request. 3. Return `HTTP/1.1 200 Connection Established` with an `X-Fill` header containing 2 MiB of data. 4. The actual helper parses and accepts the entire response without a fixed ceiling; this succeeded in all three reruns. The test establishes unbounded parsing behavior without intentionally exhausting the host. ## 5. Impact Assessment Large or concurrent CONNECT responses can terminate the rclone process and interrupt unrelated FTP/SFTP remotes and mounts. Runtime OOM cannot be contained by RC panic recovery. SFTP reaches this parser before SSH server authentication, so target host-key validation does not constrain a malicious proxy; HTTPS proxy authentication does constrain ordinary on-path attackers. ## 6. Remediation Guidance - Enforce a total CONNECT status/header budget before parsing. - Add a fixed total handshake deadline as well as idle deadlines. - Close the connection on an oversized or malformed response. - Return a wrapper that consumes already buffered post-response tunnel bytes before the raw connection. - Test large single/multiple headers, slow streaming, and concurrent handshakes.