The vulnerability identified as CVE-2026-27932 resides in the authlib joserfc Python library, which implements JSON Object Signing and Encryption (JOSE) standards. Specifically, in versions 1.6.2 and earlier, the library fails to impose limits on the 'p2c' (PBES2 Count) parameter extracted from the protected header of JSON Web Encryption (JWE) tokens when using Password-Based Encryption (PBES2) algorithms. The 'p2c' parameter controls the iteration count for the PBKDF2 key derivation function, which is computationally expensive. An attacker can craft a malicious JWE token with an extremely high iteration count (e.g., 2^31 - 1), causing the server to perform excessive CPU-intensive operations during token decryption. Because the library does not validate or throttle this parameter, it leads to resource exhaustion and a Denial of Service (DoS) attack. This vulnerability affects all high-level JWE and JWT decryption interfaces that allow PBES2 algorithms, making it a broad risk for applications relying on joserfc for secure token handling. The attack requires no authentication or user interaction, increasing its risk profile. Although no known exploits are currently reported, the vulnerability's nature and ease of exploitation make it a significant threat. The CVSS v3.1 base score is 7.5, reflecting high severity due to network attack vector, no privileges required, no user interaction, and a high impact on availability.

The primary impact of CVE-2026-27932 is a Denial of Service (DoS) condition caused by CPU exhaustion. Organizations using the vulnerable joserfc library for decrypting JWE tokens with PBES2 algorithms may experience service outages or degraded performance when processing maliciously crafted tokens. This can disrupt authentication, authorization, or other security-critical workflows relying on token validation, potentially affecting user access and system availability. The vulnerability does not directly compromise confidentiality or integrity but can indirectly impact business operations and user trust due to service unavailability. Since the attack requires no authentication and can be launched remotely, it poses a significant risk to internet-facing services and APIs using joserfc. The broad applicability to all high-level JWE/JWT decryption interfaces that permit PBES2 algorithms increases the attack surface. Organizations with high transaction volumes or critical authentication services using this library are particularly vulnerable to large-scale or targeted DoS attacks, which could lead to financial loss, reputational damage, and operational disruption.

To mitigate CVE-2026-27932, organizations should immediately upgrade the authlib joserfc library to a version beyond 1.6.2 where this vulnerability is fixed. If an upgrade is not immediately feasible, implement strict validation and bounding of the 'p2c' parameter in the application layer before passing tokens to joserfc for decryption. Specifically, enforce a maximum iteration count threshold that balances security and performance, rejecting tokens with excessively high values. Disable or avoid using PBES2 algorithms for JWE token encryption if not strictly necessary, or restrict their use to trusted sources. Employ rate limiting and anomaly detection on endpoints that accept JWE tokens to identify and block suspicious token decryption attempts. Monitor CPU usage patterns for unusual spikes that may indicate exploitation attempts. Additionally, consider deploying Web Application Firewalls (WAFs) with custom rules to detect and block tokens with abnormal 'p2c' values. Regularly audit and update cryptographic libraries and dependencies to incorporate security patches promptly.