Jwcrypto CVE-2026-39373

The CVSS score of 5.3 with attack vector network, low complexity, no privileges required, and low availability impact reflects a moderate denial-of-service severity. The EPSS score and exploitation probability are not explicitly provided but the unauthenticated, network-accessible nature (AV:N/PR:N) and simple attack mechanism (AC:L) suggest practical exploitability. No KEV status is indicated, meaning this is not confirmed as actively exploited by CISA at the time of analysis. The real-world risk is particularly acute for memory-constrained environments (edge devices, containerized services, resource-limited deployments) where a single malicious JWE token can trigger complete service failure. However, the impact is limited to availability (A:L) with no confidentiality or integrity compromise, and the attack requires an application actively processing untrusted JWE tokens with compression enabled. This is a solid priority for services handling JWT/JWE from untrusted sources, especially in resource-limited contexts, but lower urgency than critical RCE or authentication bypass issues.

An attacker crafts a JWE token with ZIP compression containing a highly-compressible payload (e.g., repetitive data) that is approximately 250KB or less in compressed form but decompresses to ~100MB. The attacker sends this token to a vulnerable JWCrypto application over the network without requiring authentication. When the application processes the JWE token, the library decompresses the payload without enforcing output size limits, causing rapid memory consumption that exhausts available RAM and crashes the service or degrades performance on memory-constrained systems. No public exploit code is explicitly referenced, but the attack is trivial to construct given the compression mechanics.