GHSA-xhf4-832v-7xcr BIT-rclone-2026-71310 CVE-2026-71310 GO-2026-6199
rclone: Unbounded HTTP CONNECT Response Headers Can Exhaust rclone Memory
Medium
Go
- Severity
- 5.9 / 10
- Medium · CVSS 3.1
- Exploit chance (EPSS)
- 0.61%
- Chance of exploitation in the next 30 days. Higher than 49.25% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-400 Uncontrolled Resource Consumption
- CWE-770 Allocation of Resources Without Limits or Throttling
- Fix
1.75.0- Upgrade github.com/rclone/rclone to this version or later
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 versions
Go
| Package | Vulnerable | Fixed in |
|---|---|---|
| github.com/rclone/rclone | < 1.75.0 |
1.75.0 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
github.com/rclone/rclone Go → 1.75.0
- go
go get github.com/rclone/rclone@v1.75.0 && go mod tidy
Severity breakdown
CVSS 3.1 5.9
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Weakness
CWE-400: Uncontrolled Resource Consumption
The product does not properly control the allocation and maintenance of a limited resource.
How to prevent it
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Implementation: Ensure that all failures in resource allocation place the system into a safe posture.
CWE-770: Allocation of Resources Without Limits or Throttling
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
How to prevent it
- Requirements: Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
- Architecture and Design: Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place, and it will help the administrator to identify who is committing the abuse. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, typically by using increasing time delays uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution can be difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply requires more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Architecture and Design, Implementation: If the program must fail, ensure that it fails gracefully (fails closed). There may be a temptation to simply let the program fail poorly in cases such as low memory conditions, but an attacker may be able to assert control before the software has fully exited. Alternately, an uncontrolled failure could cause cascading problems with other downstream components; for example, the program could send a signal to a downstream process so the process immediately knows that a problem has occurred and has a better chance of recovery. Ensure that all failures in resource allocation place the system into a safe posture.
- Operation, Architecture and Design: Use quotas or other resource-limiting settings provided by the operating system or environment. For example, when managing system resources in POSIX, setrlimit() can be used to set limits for certain types of resources, and getrlimit() can determine how many resources are available. However, these functions are not available on all operating systems. When the current levels get close to the maximum that is defined for the application (see CWE-770), then limit the allocation of further resources to privileged users; alternately, begin releasing resources for less-privileged users. While this mitigation may protect the system from attack, it will not necessarily stop attackers from adversely impacting other users. Ensure that the application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-38xv-hf3p-h7mq rclone: source object names can escape the configured root on upload | Medium | Also affects github.com/rclone/rclone |
| GHSA-2p48-j3qc-rx9f rclone: S3 multipart declared-length memory exhaustion | High | Also affects github.com/rclone/rclone |
| GHSA-p569-5gjg-9cmj rclone: RC per-server auth-proxy bypass | Critical | Also affects github.com/rclone/rclone |
| GHSA-xwwr-4h3p-r22c rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass | Critical | Also affects github.com/rclone/rclone |
| GHSA-c476-6w5q-jw77 rclone: FTP cross-session auth-proxy backend confusion | High | Also affects github.com/rclone/rclone |
| GHSA-f8g7-2xjc-7mfh rclone: Directory metadata (chmod/chown/chtimes) applied through a planted symlink in rclone local --links escapes the destination | High | Also affects github.com/rclone/rclone |
| GHSA-p6m2-r3w9-mpxw rclone local: crafted Range request against a translated symlink panics (DoS) | Medium | Also affects github.com/rclone/rclone |
| GHSA-66hp-wgxq-6f5q rclone archive/zip: Zip Slip via unsanitized zip entry names lets a malicious archive escape its own namespace | Medium | Also affects github.com/rclone/rclone |
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