CVE-2026-79575 in yfexam-exam
Summary
by MITRE • 09/08/2026
The JWT signing secret in yfexam-exam v2.0 is derived from the username and the current month instead of a random server-side key, making the secret key easily obtainable via a bruteforce attack.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified in yfexam-exam version 2.0 represents a critical failure in cryptographic implementation, specifically falling under CWE-798: Use of Hard-coded Credentials and CWE-330: Use of Insufficiently Random Values. The core technical flaw lies in the method used to derive the JSON Web Token signing secret. Instead of generating a high-entropy, cryptographically secure random key stored securely on the server side, the application constructs this secret by concatenating the authenticated user's username with the current month number. This deterministic approach means that for any given user and time period, the signing key is predictable if an attacker can identify or guess these two components. Since usernames are often public information or easily enumerated through standard registration processes, and months follow a fixed calendar cycle, the entropy of this secret is drastically reduced compared to a random string.
This architectural weakness allows for efficient brute-force attacks against the JWT signature verification process. An attacker who has obtained a valid token from any user can attempt to reverse-engineer the signing key by iterating through possible usernames and month combinations. Given that there are only twelve months in a year, the search space is significantly smaller than what would be required for a truly random secret of equivalent length. Once an attacker successfully identifies the correct username-month combination used as the seed, they can generate valid JWTs with arbitrary claims, effectively bypassing authentication mechanisms entirely. This undermines the integrity and authenticity guarantees that JSON Web Tokens are designed to provide, allowing unauthorized access to protected resources or privilege escalation depending on how token claims are processed by the application logic.
The operational impact of this vulnerability is severe, as it compromises the entire trust model of the application's session management system. Attackers can forge tokens for high-privilege accounts if they know the associated usernames, leading to unauthorized data access, modification of user records, or complete compromise of the backend systems that rely on these tokens for authorization decisions. This type of flaw is particularly dangerous because it may not trigger traditional security monitoring alerts related to injection attacks or buffer overflows; instead, it exploits a logical error in key management. The vulnerability aligns with MITRE ATT&CK technique T1078: Valid Accounts, as attackers leverage legitimate credentials (or guessed usernames) combined with the weak cryptographic implementation to maintain persistent access without detection by standard intrusion prevention systems that focus on network-level anomalies rather than application-layer logic flaws.
To mitigate this risk, immediate remediation is required to replace the deterministic key derivation method with a robust, server-side random generation process. The signing secret must be generated using a cryptographically secure pseudo-random number generator such as /dev/urandom or equivalent OS-specific APIs, ensuring sufficient entropy and unpredictability. This strong secret should then be stored securely in environment variables or a dedicated secrets management service, never embedded within the application code or derived from user-supplied input like usernames. Additionally, implementing token expiration policies with short lifespans can limit the window of opportunity for brute-force attacks to succeed. Regular security audits and static analysis tools configured to detect weak cryptographic patterns should be employed during development to prevent similar implementation errors in future releases.