CVE-2026-108586 in agent
Summary
by MITRE • 10/10/2026
1MCP Agent (@1mcp/agent) 0.20.0 through 0.39.0 contains an incorrect authorization vulnerability that allows authenticated clients to bypass OAuth tag-scope enforcement using negated advanced tag-filter expressions. Attackers holding a single-tag token can send a filter like not <granted-tag> to list and invoke tools on backend MCP servers outside their granted scopes.
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Analysis
by VulDB Data Team • 10/10/2026
The 1MCP Agent, specifically versions ranging from 0.20.0 through 0.39.0, suffers from a critical authorization bypass vulnerability rooted in the flawed implementation of OAuth tag-scope enforcement mechanisms. This flaw allows authenticated clients to circumvent intended access controls by exploiting how negated advanced tag-filter expressions are processed within the application logic. In standard secure architectures, an agent operating under specific OAuth scopes should be strictly limited to invoking tools or accessing resources associated with those granted tags. However, in this vulnerable implementation, the parsing engine fails to correctly interpret logical NOT operators when combined with existing scope restrictions, creating a significant gap between the declared permissions and the actual enforced boundaries.
The technical core of this vulnerability lies in the incorrect handling of negated advanced tag-filter expressions. When an attacker possesses a token limited to a single specific tag, they can construct a malicious request containing a filter expression that explicitly excludes their granted tag using a NOT operator. Due to the logical error in the authorization middleware or service layer, the system interprets this negation not as a restriction but potentially as a wildcard or an exclusion of irrelevant data rather than a block on access itself. Consequently, the agent proceeds to list and invoke tools on backend Model Context Protocol servers that fall outside the scope originally granted by the OAuth provider. This effectively nullifies the principle of least privilege, allowing the client to interact with resources it was explicitly denied permission to touch.
The operational impact of this vulnerability is severe for any organization relying on 1MCP Agent for secure integration with external services or internal tools. An attacker who has obtained a low-privilege token can escalate their effective permissions by leveraging these negated filters to access sensitive backend systems, exfiltrate data, or execute arbitrary commands through exposed tool endpoints. This bypass undermines the integrity of identity and access management strategies that depend on granular tag-based scoping. It enables unauthorized lateral movement within an ecosystem where trust is established via OAuth tokens, potentially leading to broader system compromise if those backend servers hold higher privileges or contain sensitive business logic.
This vulnerability aligns with CWE-269, which describes Improper Authorization, specifically illustrating a case where access control checks are bypassed through logical manipulation of input parameters rather than direct authentication failure. From an offensive security perspective, this technique mirrors tactics found in the MITRE ATT&CK framework under privilege escalation and defense evasion categories, as it involves manipulating system logic to gain unauthorized access without triggering typical alerting mechanisms associated with brute-force or credential stuffing attacks. The exploitation relies on a misunderstanding of logical operators within the authorization context rather than breaking cryptographic protections.
To mitigate this risk, immediate action is required for all organizations running 1MCP Agent versions between 0.20.0 and 0.39.0. The primary remediation step is to upgrade to a patched version where the tag-scope enforcement logic has been corrected to properly evaluate negated expressions against granted scopes. If upgrading is not immediately feasible, administrators should implement strict input validation at the API gateway level to reject requests containing complex logical filters that attempt to negate existing scope constraints. Additionally, reviewing and tightening OAuth token policies to limit the granularity of available tags can reduce the attack surface. Continuous monitoring for anomalous tool invocation patterns originating from single-tag tokens is also recommended to detect potential exploitation attempts in real-time.