CVE-2026-31863: Authentication Bypass via Brute Force in Anytype Heart gRPC API

Anytype operates using a decoupled architecture where the user interface interacts with a local middleware library known as Anytype Heart (anytype-heart). This middleware exposes a local gRPC client API that processes data synchronization, local storage management, and cryptographic operations. To prevent unauthorized local applications from interacting with this API, Anytype implements a challenge-based authentication mechanism.

This authentication mechanism relies on a 4-digit code generated by the Anytype desktop application or CLI. Legitimate clients must supply this code to the middleware to establish an authorized session. The system relies entirely on the secrecy of this PIN to gate access to the underlying gRPC service.

The vulnerability, classified as CWE-307 (Improper Restriction of Excessive Authentication Attempts), exists because the middleware does not implement rate limiting, backoff, or account lockout policies for the authentication endpoint. An attacker can submit continuous, rapid requests to the gRPC API without triggering any defensive mechanisms or session invalidation.

As a result, the intended security boundary between local processes is nullified. An unprivileged local attacker can programmatically iterate through all possible PIN combinations until the correct sequence is identified, successfully bypassing the authentication requirement.

The root cause of CVE-2026-31863 is an architectural failure to restrict stateful authentication attempts over the local gRPC interface. When an authentication challenge is initiated, the system expects a 4-digit numeric code, resulting in an inherently small keyspace of exactly 10,000 possible combinations (0000 through 9999).

In secure implementations of small-keyspace authentication (such as ATM PINs or mobile device lock screens), the system must compensate for the low entropy by strictly limiting the number of submission attempts. Anytype Heart failed to implement this secondary control. The gRPC endpoint processes authentication requests asynchronously and efficiently, allowing for high-throughput request processing.

Without a mechanism to track failed attempts per session or globally lock the authentication state, the endpoint acts as a fast oracle for PIN validation. The attacker incurs no time penalty for incorrect guesses. The system continues to evaluate requests at the maximum speed the local loopback interface and the gRPC server can handle.

The diagram above illustrates the uninhibited cyclic flow of the attack. The vulnerable state machine returns immediately to a receptive state after a failed guess, enabling the brute-force condition.

Exploitation of CVE-2026-31863 requires local access to the target machine where the Anytype desktop application or middleware is actively running. The attacker does not require elevated privileges (root or SYSTEM) but must be able to execute arbitrary code or scripts within the local user context.

The attacker first performs local reconnaissance to identify the specific TCP port bound by the Anytype Heart gRPC server. Because the port may vary or be dynamically assigned, the attacker typically queries local process network bindings or inspects the Anytype configuration files located in the user's application data directories.

> [!NOTE] > The CVSS Attack Complexity is rated High (AC:H) because exploitation depends on the middleware being in a specific operational state where it is actively accepting authentication challenges, or the attacker must possess the ability to artificially trigger the challenge initiation phase.

Once the port is identified and the challenge is active, the attacker uses a standard gRPC client to connect to the authentication endpoint. The exploit script loops from 0000 to 9999, submitting each value as the challenge response. Given the lack of network latency on a loopback interface, the entire keyspace can be exhausted in a matter of seconds. Upon receiving a success response from the server, the script halts and utilizes the established session to execute subsequent API calls.

The successful exploitation of this vulnerability grants the attacker unauthorized access to the Anytype Heart gRPC client API. This API functions as the primary data controller for the Anytype application, meaning the attacker gains the same level of access to the application data as the legitimate user.

The confidentiality impact is classified as Low according to the CVSS vector, but within the context of the application, it allows the attacker to query, read, and export the user's local notes, credentials, and synchronized spaces. The attacker circumvents the intended privacy boundaries established by the application's local access controls.

The integrity impact is similarly classified as Low. The attacker can issue gRPC commands to modify local data, delete entries, or potentially alter synchronization states. Because Anytype operates as a local-first application, these modifications will be processed by the middleware and subsequently synchronized to other devices if the user's account is connected to the network.

While the vulnerability requires local access, it is particularly relevant in shared computing environments or multi-user systems. It also serves as a critical secondary capability for malware that has achieved low-privileged execution on a device and seeks to extract sensitive information stored within the Anytype ecosystem.

To remediate CVE-2026-31863, Anytype has released patched versions across its affected application ecosystem. Administrators and users must upgrade Anytype Heart to version 0.48.4, Anytype CLI to version 0.1.11, and the Anytype Desktop client (anytype-ts) to version 0.54.5. These updates introduce necessary cryptographic and state-management controls to the authentication flow.

The underlying fix involves implementing a strict rate-limiting mechanism and an account lockout policy for the gRPC authentication endpoint. By tracking consecutive failed attempts within a specified timeframe, the middleware will now reject further guesses after a defined threshold is reached, permanently neutralizing the brute-force attack vector.

In environments where immediate patching is not feasible, there are no direct configuration workarounds to disable this specific gRPC behavior without breaking application functionality. Users are advised to rely on operating system-level process isolation and restrict execution permissions to untrusted local binaries.

Developers implementing challenge-response systems with low-entropy secrets (such as PINs) must always pair the authentication mechanism with robust rate limiting. Furthermore, utilizing exponential backoff for failed attempts and logging repeated authentication failures are critical defense-in-depth strategies against localized brute-force techniques.