The vulnerability CVE-2026-27932 affects the authlib project's joserfc Python library, which provides implementations for JOSE standards including JSON Web Encryption (JWE). Specifically, the issue lies in the handling of the p2c (PBES2 Count) parameter within the protected header of JWE tokens when using PBES2 algorithms for password-based encryption. The p2c parameter defines the iteration count for the PBKDF2 key derivation function, which is computationally expensive. In versions 1.6.2 and earlier, joserfc does not impose any upper limit or validation on this iteration count. Consequently, an attacker can specify an extremely large value (e.g., 2^31 - 1), forcing the library to perform an excessive number of PBKDF2 iterations during token decryption. This leads to significant CPU resource consumption, effectively causing a Denial of Service (DoS) by exhausting server processing capacity. The vulnerability exists at the JSON Web Algorithms (JWA) layer and affects all high-level JWE and JWT decryption interfaces that permit PBES2 algorithms according to the application's policy. Exploitation requires no authentication or user interaction, making it accessible to remote attackers. The vulnerability does not impact confidentiality or integrity but severely affects availability. No patches or fixes are linked yet, and no known exploits have been reported in the wild as of the published date. The CVSS v3.1 score is 7.5, reflecting high severity due to network attack vector, low attack complexity, no privileges required, and no user interaction needed.

This vulnerability can lead to Denial of Service (DoS) conditions on servers or services that use the vulnerable joserfc library for JWE token decryption with PBES2 algorithms. By sending a single maliciously crafted token with an extremely high iteration count, an attacker can cause the server to consume excessive CPU resources, potentially degrading performance or causing service outages. This can disrupt authentication and authorization workflows relying on JWT/JWE tokens, impacting availability of critical applications and services. Organizations using joserfc in their security infrastructure, especially those processing untrusted tokens from external sources, are at risk. The attack requires no authentication and can be launched remotely, increasing the threat surface. While confidentiality and integrity are not directly compromised, the availability impact can affect business continuity, user access, and downstream systems dependent on these tokens. The lack of current known exploits provides a window for mitigation, but the vulnerability's ease of exploitation and potential for widespread disruption warrant urgent attention.

To mitigate this vulnerability, organizations should immediately upgrade joserfc to a version that enforces limits on the p2c iteration count parameter once such a patch is released by the vendor. Until a patch is available, implement strict input validation and filtering at the application or API gateway level to reject JWE tokens with suspiciously high iteration counts in the PBES2 header. Limit or disable the use of PBES2 algorithms for JWE decryption if feasible, especially in scenarios where tokens originate from untrusted sources. Employ rate limiting and anomaly detection to identify and block repeated attempts to exploit this resource exhaustion. Monitor CPU usage patterns on services handling JWE tokens to detect unusual spikes indicative of exploitation attempts. Consider isolating token processing in dedicated environments with resource quotas to contain potential DoS impact. Engage with the vendor or open-source community for updates and patches, and review cryptographic policy configurations to minimize exposure to this class of vulnerabilities.