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Backpropagate: backprop ui --auth and backprop ui --share do not enforce authentication

Critical severity GitHub Reviewed Published May 23, 2026 in mcp-tool-shop-org/backpropagate • Updated Jun 26, 2026

Package

npm @mcptoolshop/backpropagate (npm)

Affected versions

>= 1.1.0, < 1.2.0

Patched versions

1.2.0
pip backpropagate (pip)
>= 1.1.0, < 1.2.0
1.2.0

Description

Summary

In backpropagate >= 1.1.0, the optional Reflex web UI (pip install backpropagate[ui], launched via backprop ui) exposes a training control plane: dataset upload, model load, training start/stop, multi-run orchestration, GGUF export, and HuggingFace Hub push.

The CLI accepts two operator-facing flags intended as security controls:

  • --auth user:pass — documented as "require HTTP Basic authentication on every request to the UI."
  • --share — documented as "expose the UI on a public address; requires --auth."

When --auth user:pass is passed, the CLI prints Auth: enabled (user: <username>) to confirm to the operator that authentication is active, then exports BACKPROPAGATE_UI_AUTH=user:pass to the subprocess that launches the Reflex backend.

The Reflex backend (backpropagate/ui_app/**) never reads BACKPROPAGATE_UI_AUTH. No authentication middleware is registered. No request-level guard runs. No WebSocket upgrade guard runs. Any client that reaches the bound port — local or remote, depending on whether --share is used — has full UI access.

An inline comment at backpropagate/cli.py:1217-1218 in the v1.1.0 source documents the gap: "For Phase 1 the variable is exported but Reflex doesn't read it yet." This comment was internal-facing; the user-facing documentation (README, CHANGELOG, SHIP_GATE) advertised the contract as enforced.

This advisory is filed primarily because the runtime contradicted an operator-facing security claim. Code-only bugs of comparable shape (auth check missing entirely from a path) would already warrant disclosure; the additional false-promise dimension raises the severity.

Impact

An attacker who reaches the bound port can:

  • Read uploaded datasets rendered in the UI preview, including content of any JSONL/CSV/TXT file the legitimate operator has uploaded for fine-tuning.
  • Trigger arbitrary training runs against any base model the operator has installed locally or that can be downloaded from HuggingFace.
  • Trigger HuggingFace Hub pushes to repositories named via the UI input (subject to the operator's local HF token's scope — typically all repos owned by the operator).
  • Cause disk-fill DoS via the rx.upload endpoint (no size cap, no extension filter, no per-session count cap in v1.1.0 / v1.1.1).
  • Read model paths (source_model_path, dataset_path, model, uploaded_path) which are user-supplied and bypass the safe_path() helper that lives in backpropagate/ui_security.py (path validation is dead code on the Reflex surface in v1.1.0 / v1.1.1).

The combination of unauthenticated training control, HF push target spoofing, and path-input traversal makes the affected endpoint suitable for both data exfiltration (reading uploaded training data) and supply-chain attacks (pushing tampered model weights to the operator's HF account).

The local-only default (no --share) reduces exposure to a host-local attacker. The --share flag is documented as a "public URL" feature; operators who used --share --auth user:pass had no warning that the auth half was inert.

Patches

Fixed in v1.2.0 (released 2026-05-23). The patch implements real ASGI middleware via rx.App(api_transformer=basic_auth_transformer) that gates HTTP routes AND the /_event WebSocket upgrade. Four modes (no_auth_local_only / token_auto / explicit_creds / production), HMAC-signed cookie validated PRE-websocket.accept(), Host + Origin allowlists. The middleware ships alongside a 4-layer defense in depth at the cli.py / ui_app/app.py / rxconfig.py / env-strip surfaces so direct python -m reflex run invocations (bypassing the CLI guard) also enforce authentication.

Upgrade with:

  • pip: pip install --upgrade backpropagate
  • npm: npm install -g @mcptoolshop/backpropagate@latest

Full release notes: https://github.com/mcp-tool-shop-org/backpropagate/blob/main/CHANGELOG.md#120---2026-05-23

Workarounds

If users cannot upgrade immediately:

  1. Do not pass --auth or --share to backprop ui. Run the UI with no flags (backprop ui); it will bind to localhost and accept any client that can reach 127.0.0.1.
  2. For remote access, use SSH port-forwarding instead of --share: 
    # On the client:
ssh -L 7860:localhost:7860 <training-host>
# On the server:
backprop ui            # no --share
# Then open http://localhost:7860 in your local browser.
    SSH provides the authentication layer the Reflex UI did not.
  3. Audit existing deployments. If any host running backpropagate >= 1.1.0 has previously been launched with --share, treat any uploaded training data, model paths, or HF push targets visible in that UI session as potentially exposed. Re-issue HF tokens that have been in use during such sessions.

Binary distribution gap. Standalone binaries (Windows .exe / macOS .app via PyInstaller) failed to build for v1.2.0 and will land in a follow-up patch release. In the interim, users who relied on the v1.1.x binary distribution should install the patched version via pip install backpropagate==1.2.0 to receive the auth-bypass fix. The v1.2.0 PyPI package and @mcptoolshop/backpropagate@1.2.0 npm package both carry the patched code.

Credit

Discovered by the dogfood-swarm Stage A audit on 2026-05-22 (finding ID FRONTEND-A-001, classified CRITICAL). The audit also surfaced contradicting documentation in CHANGELOG / SHIP_GATE / README; those were corrected in v1.2.0 alongside the runtime fix.

References

@mcp-tool-shop mcp-tool-shop published to mcp-tool-shop-org/backpropagate May 23, 2026
Published by the National Vulnerability Database Jun 17, 2026
Published to the GitHub Advisory Database Jun 26, 2026
Reviewed Jun 26, 2026
Last updated Jun 26, 2026

Severity

Critical

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(24th percentile)

Weaknesses

Improperly Implemented Security Check for Standard

The product does not implement or incorrectly implements one or more security-relevant checks as specified by the design of a standardized algorithm, protocol, or technique. Learn more on MITRE.

Missing Authorization

The product does not perform an authorization check when an actor attempts to access a resource or perform an action. Learn more on MITRE.

Debug Messages Revealing Unnecessary Information

The product fails to adequately prevent the revealing of unnecessary and potentially sensitive system information within debugging messages. Learn more on MITRE.

CVE ID

CVE-2026-48797

GHSA ID

GHSA-f65r-h4g3-3h9h

Source code

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