rclone: Unbounded HTTP CONNECT Response Headers Can Exhaust rclone Memory

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.