CVE-2026-56331 in Capgo
Summary
by MITRE • 07/01/2026
Capgo before 12.128.2 contains improper error handling in the /private/accept_invitation endpoint that returns HTTP 500 instead of safe 4xx errors when magic_invite_string is invalid. Attackers can trigger this vulnerability using only the public key by submitting malformed magic_invite_string values to cause server errors and leak internal processing details.
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Analysis
by VulDB Data Team • 07/01/2026
The vulnerability in Capgo versions prior to 12.128.2 represents a critical improper error handling flaw that exposes the system to potential information disclosure and service disruption risks. This weakness manifests specifically within the /private/accept_invitation endpoint where the application fails to properly validate input parameters before processing requests. When an attacker submits a malformed magic_invite_string value, the system responds with an HTTP 500 internal server error instead of implementing proper input validation that would generate a more appropriate 4xx client error response. This improper error handling creates a significant security risk as it reveals internal system processing details and potentially exposes sensitive information about the application's internal state and error handling mechanisms.
The technical implementation of this vulnerability stems from inadequate input sanitization and validation procedures within the invitation acceptance workflow. The endpoint accepts public key information as a parameter but fails to properly validate the magic_invite_string format before attempting to process it, creating an opportunity for attackers to probe the system with malformed data. This type of error handling misconfiguration falls under CWE-209, which specifically addresses improper error handling that may expose internal system details to unauthorized parties. The vulnerability is particularly concerning because it requires minimal attacker input - only the public key is needed to trigger the problematic code path, making exploitation accessible and straightforward.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable more sophisticated attack vectors. When the server returns HTTP 500 errors instead of proper 4xx responses, it may inadvertently leak stack traces, internal file paths, or other system-specific details that could aid attackers in developing further exploits. This misbehavior can also contribute to service availability issues as repeated malformed requests may cause the application to crash or enter unstable states. The vulnerability creates a potential denial of service condition where legitimate users might be affected by cascading server errors, while simultaneously providing attackers with valuable reconnaissance information. According to ATT&CK framework tactic T1211, this represents an information gathering technique that can be used to understand system behavior and identify additional weaknesses in the application's defenses.
Mitigation strategies should focus on implementing proper input validation and error handling mechanisms throughout the application stack. The /private/accept_invitation endpoint must validate the magic_invite_string parameter against expected formats before attempting any processing, returning appropriate 4xx errors for malformed inputs rather than allowing them to propagate to higher-level error handlers. Organizations should implement comprehensive logging of all error conditions while ensuring that error messages do not contain sensitive system information. Additionally, the application should be configured to return generic error responses to clients while maintaining detailed internal logs for debugging purposes. Regular security testing including penetration testing and vulnerability scanning should be conducted to identify similar improper error handling patterns throughout the application codebase. The fix should also include implementing proper rate limiting and input sanitization mechanisms to prevent abuse of this vulnerability and ensure that all external inputs are properly validated before processing.