Input Validation Flaw in fast-jwt by NearForm

CVE-2026-34950

What is CVE-2026-34950?

CVE-2026-34950 is a vulnerability identified in the fast-jwt library, developed by NearForm, which is designed for rapid and efficient handling of JSON Web Tokens (JWT). This library is critical for applications that require secure and fast authentication mechanisms, commonly used in web services for maintaining user sessions and permissions. The specific flaw lies within the input validation process where the publicKeyPemMatcher regex implementation does not adequately restrict leading whitespace in key strings. This oversight can lead to JWT algorithm confusion attacks, similar to a previously identified vulnerability (CVE-2023-48223), thereby undermining the integrity and security of the authentication flow. If exploited, this vulnerability could permit attackers to manipulate JWT handling mechanisms, leading to unauthorized access within systems relying on this library.

Potential impact of CVE-2026-34950

1. Authentication Bypass: The flaw allows for potentially unauthorized manipulation of JWTs, enabling attackers to impersonate legitimate users or escalate privileges, which could lead to significant security breaches.

2. Data Integrity Risks: Exploiting this vulnerability might lead to situations where sensitive user data becomes compromised or altered, affecting the data integrity of applications that depend on secure JWT management.

3. Re-exploitation of Existing Vulnerabilities: This issue reopens avenues for attacks previously mitigated by fixes for earlier vulnerabilities, meaning that systems thought to be secure could once again face serious risks from new exploitation techniques.

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