Description

A flaw was found in jwcrypto. A remote attacker can send a specially crafted JSON Web Encryption (JWE) token containing numerous period delimiters. This malformed token can force the JWE.deserialize() function to allocate excessive memory, leading to a MemoryError. This issue results in a denial of service (DoS) for services that process untrusted JWE values.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.9",
  "pubDate": "2026-08-28T00:18:20.337Z",
  "pubdate": "2026-08-28T00:18:20.337Z",
  "executiveSummary": "A memory exhaustion vulnerability exists within the jwcrypto library, specifically impacting the JWE.deserialize() function. This flaw allows a remote, unauthenticated attacker to trigger a Denial of Service (DoS) condition by submitting a maliciously crafted JSON Web Encryption (JWE) token.\nThe vulnerability is characterized by improper handling of excessive period delimiters within the token structure, which forces the application to perform disproportionate memory allocation during the deserialization process. This leads to a MemoryError, causing the service to crash or become unresponsive.\nThe risk implication is high for services that process untrusted JWE input, as the exploitation is straightforward and requires no specific authentication or elevated privileges. Because the vulnerability triggers during the initial parsing phase of the token, the attack vector is effectively exposed over any network interface where the library is utilized to decode input. Successful exploitation results in complete service unavailability for legitimate users of the affected application, necessitating immediate remediation or the implementation of strict input validation mechanisms.",
  "technicalDetails": "The vulnerability resides within the JWE deserialization logic of the jwcrypto library. The root cause is an algorithmic complexity issue triggered during the parsing of compact serialized JWE strings. In the JSON Web Encryption specification, compact serialization typically involves five components separated by period ('.') characters. The implementation of JWE.deserialize() fails to adequately sanitize or limit the number of these delimiters before processing the input string.\nWhen a remote attacker supplies a specially crafted JWE token containing a massive number of period delimiters, the parser attempts to split or iterate over the components of the string. The internal handling of these excessive delimiters leads to an uncontrolled growth in memory consumption. Specifically, the library's internal structures or buffers created during the splitting or tokenization of the input string consume system memory at a rate proportional to the length and character distribution of the malicious input. This triggers an unhandled or unrecoverable MemoryError within the Python environment.\nThe attack flow follows a deterministic pattern: 1) The attacker crafts a JWE payload containing an abnormally high density of period delimiters, exceeding the expected format of a legitimate JWE. 2) The attacker submits this payload to an endpoint that utilizes jwcrypto to deserialize the input, such as an authentication service, a JWT/JWE validator, or a token-based API gateway. 3) The JWE.deserialize() function begins parsing the token, failing to enforce a 'maximum length' or 'maximum component count' constraint before initiating memory-heavy operations. 4) The application runtime exhausts the allocated heap or memory limit while attempting to handle the malformed input structure. 5) The application process terminates or enters a hanging state, resulting in a Denial of Service.\nThis vulnerability is particularly dangerous because it occurs before any cryptographic validation, signature verification, or decryption steps. Consequently, there are no authentication or privilege requirements to trigger the exploit; the network-exposed component that accepts the token is sufficient to facilitate the attack. The lack of input length validation and the failure to handle malformed string sequences as exceptions within the deserialization routine remain the primary technical gaps allowing for this heap-based resource exhaustion."
}