CVE-2026-56331: Capgo Information Disclosure Vulnerability

CVE-2026-56331 Overview

CVE-2026-56331 affects Capgo versions before 12.128.2. The vulnerability exists in the /private/accept_invitation endpoint, which returns HTTP 500 responses instead of safe 4xx errors when magic_invite_string values are invalid. Attackers can trigger the flaw using only the public key by submitting malformed magic_invite_string values. The resulting server errors leak internal processing details that assist reconnaissance. The issue is classified under [CWE-209] Generation of Error Message Containing Sensitive Information.

Critical Impact
Unauthenticated attackers can send malformed invitation strings to a public endpoint and receive server-side error details, exposing internal processing information useful for further attacks.

Affected Products

Capgo versions prior to 12.128.2
The /private/accept_invitation API endpoint
Deployments accessible over the network without authentication requirements for this endpoint

Discovery Timeline

2026-06-30 - CVE-2026-56331 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-56331

Vulnerability Analysis

The vulnerability stems from improper error handling in the invitation acceptance workflow. When the /private/accept_invitation endpoint receives a magic_invite_string that fails parsing or validation, Capgo returns HTTP 500 responses containing internal processing details. Well-designed APIs return 4xx status codes for client input problems and reserve 5xx codes for unexpected server failures. By collapsing input validation failures into unhandled exceptions, the endpoint exposes stack traces, internal identifiers, or backend framework details that should remain confidential.

The endpoint is reachable using only the public API key, so no invitation ownership or authenticated session is required to trigger the error path. This lowers the exploitation bar to any network-connected attacker with knowledge of the endpoint.

Root Cause

The root cause is missing input validation combined with verbose error propagation. Instead of validating magic_invite_string format before processing and returning a structured 4xx response, the code allows malformed input to propagate into deeper parsing or database operations that raise exceptions. Those exceptions surface to the client as HTTP 500 responses with diagnostic content.

Attack Vector

An attacker sends a POST request to /private/accept_invitation on a vulnerable Capgo instance using the public key. The request body contains a malformed magic_invite_string. The server responds with HTTP 500 and error content that reveals internal processing details. Repeated probing can map internal behavior and identify additional attack surface. Refer to the GitHub Security Advisory GHSA-34p8-fh3m-376x and the VulnCheck advisory on Capgo for technical details.

// No verified exploit code is available for CVE-2026-56331.
// See the referenced advisories for technical detail.

Detection Methods for CVE-2026-56331

Indicators of Compromise

Repeated HTTP 500 responses generated by the /private/accept_invitation endpoint in application logs
Requests to /private/accept_invitation containing malformed, truncated, or non-standard magic_invite_string values
Bursts of requests to the invitation endpoint from a single IP or public API key within a short interval

Detection Strategies

Alert on abnormal 500 response rates for the /private/accept_invitation route relative to baseline traffic
Inspect request payloads for magic_invite_string values that fail regex or length checks expected by the invitation format
Correlate error responses with source IP reputation and API key usage patterns to identify probing behavior

Monitoring Recommendations

Ingest web server, reverse proxy, and Capgo application logs into a centralized logging platform for correlation
Track error-to-success ratios per endpoint and generate alerts when the ratio for invitation endpoints exceeds normal thresholds
Retain full request and response metadata for the invitation flow to support incident investigation

How to Mitigate CVE-2026-56331

Immediate Actions Required

Upgrade Capgo to version 12.128.2 or later to receive the vendor fix
Review recent access logs for the /private/accept_invitation endpoint and investigate anomalous 500 error patterns
Restrict public exposure of the invitation endpoint where business requirements allow, such as behind a WAF or IP allowlist

Patch Information

Capgo maintainers addressed the issue in version 12.128.2. The fix changes error handling in /private/accept_invitation so invalid magic_invite_string inputs return safe 4xx responses without exposing internal processing details. Refer to the GitHub Security Advisory GHSA-34p8-fh3m-376x for release information.

Workarounds

Deploy a Web Application Firewall (WAF) rule that validates magic_invite_string format before requests reach Capgo
Configure the reverse proxy to rewrite upstream 500 responses from the invitation endpoint to generic error pages that strip diagnostic content
Rate limit requests to /private/accept_invitation per source IP and per public API key to slow enumeration attempts

bash

# Example NGINX rate limit and error masking for the invitation endpoint
limit_req_zone $binary_remote_addr zone=capgo_invite:10m rate=5r/m;

location /private/accept_invitation {
    limit_req zone=capgo_invite burst=5 nodelay;
    proxy_intercept_errors on;
    error_page 500 502 503 504 /generic_error.html;
    proxy_pass http://capgo_backend;
}