CVE-2026-71310 is a medium-severity (CVSS 5.9) Uncontrolled Resource Consumption vulnerability in github.com/rclone/rclone. A fix is available for github.com/rclone/rclone — see the affected versions and patch details below.
rclone: Unbounded HTTP CONNECT Response Headers Can Exhaust rclone Memory
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-71310.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/rclone/rcloneReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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.gowas unchanged at master commit961266888fe797390c535386f3b3aa46f4853602on 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
- Configure the helper to use a test proxy.
- Accept rclone's CONNECT request.
- Return
HTTP/1.1 200 Connection Establishedwith anX-Fillheader containing 2 MiB of data. - 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/rclone/rclone | all versions | 1.75.0go get github.com/rclone/rclone@v1.75.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/rclone/rclone, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/rclone/rclone to 1.75.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-71310 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Moderate impact denial of service flaw in rclone arises from unbounded memory growth when processing oversized HTTP CONNECT response headers via FTP or SFTP proxy connections. Exploitation requires a malicious or compromised proxy server, or an active on-path attacker, which limits the attack vector and increases…
Avoid routing rclone FTP or SFTP connections through untrusted HTTP proxies. If a proxy is required, ensure it is a trusted, internally controlled endpoint. No mitigation is needed for deployments that do not use HTTP proxies for rclone connections.Source: Red Hat security advisory for CVE-2026-71310 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-71310 in your dependencies?
Find it across Go, including transitive dependencies.