CVE-2026-101878 in Server
Summary
by MITRE • 09/29/2026
Bitwarden Server 2025.6.0 before 2026.5.0 declares the @ExternalId parameter of the User_ReadBySsoUserOrganizationIdExternalId stored procedure as NVARCHAR(50) while the column it queries stores NVARCHAR(300), silently truncating the SSO login identifier on SQL Server deployments and allowing a user whose identity-provider identifier begins with another organization member's full 50-character identifier to authenticate as that member and obtain a victim-scoped access token.
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Analysis
by VulDB Data Team • 09/29/2026
The vulnerability in Bitwarden Server versions prior to 2026.5.0 represents a critical authentication bypass rooted in improper input validation and type mismatch handling within the SQL Server backend infrastructure. Specifically, the stored procedure User_ReadBySsoUserOrganizationIdExternalId declares its @ExternalId parameter with an NVARCHAR(50) data type constraint. However, the corresponding column in the underlying database schema is defined as NVARCHAR(300). This discrepancy creates a silent truncation mechanism where any SSO login identifier exceeding fifty characters is automatically truncated to fit the narrower parameter definition before being used for query execution or identity resolution logic.
From a technical perspective, this flaw exploits the behavior of SQL Server when handling string comparisons and lookups with mismatched lengths. When an attacker provides an external identifier that begins with the exact sequence of characters comprising another user's full fifty-character identifier, the truncation process causes the system to treat these two distinct identities as identical for the purpose of lookup operations. Consequently, the authentication service fails to distinguish between the legitimate victim and the malicious actor based on their unique SSO provider identifiers, leading to a successful impersonation event where the attacker is authenticated with the privileges associated with the victim's account.
The operational impact of this vulnerability is severe, as it allows for unauthorized access to sensitive vault data belonging to other organization members. By obtaining a victim-scoped access token, an adversary can read encrypted passwords, secure notes, and identity information stored within Bitwarden without possessing valid credentials for their own account or the victim's account in the traditional sense. This undermines the fundamental security model of multi-tenant SSO integrations where unique external identifiers are expected to serve as immutable keys for user mapping. The attack requires no special privileges beyond a basic authenticated session, making it accessible to any low-level attacker who can control their own identity provider attributes or exploit configuration weaknesses in federated authentication setups.
This vulnerability aligns with CWE-20 Improper Input Validation and CWE-134 Use of Externally-Controlled Format String if the truncation leads to unexpected parsing behaviors, though primarily it falls under CWE-697 Incorrect Comparison due to implicit type conversion or length mismatch in database queries. In terms of MITRE ATT&CK framework classification, this behavior facilitates Account Manipulation and potentially Credential Access by allowing an attacker to assume the identity of another user within a trusted organization context. The lack of explicit bounds checking on input parameters before database interaction is a common pattern in legacy enterprise applications that rely heavily on stored procedures for business logic encapsulation.
Mitigation strategies must prioritize immediate patching to version 2026.5.0 or later, where the parameter declaration has been corrected to match the underlying column definition of NVARCHAR(300). Until patches are applied, administrators should enforce strict length limits at the application layer for all SSO external identifiers before they reach the database layer. Additionally, implementing input validation that rejects any identifier exceeding fifty characters with a clear error message rather than silently truncating it can prevent exploitation attempts. Organizations relying on Bitwarden Server in enterprise environments should also review their identity provider configurations to ensure that generated user IDs do not inadvertently create prefix collisions among different users within the same organization scope.