The flaw in rustls before version 0.23.46 allows a malicious peer to trigger a denial‑of‑service by sending a certificate whose Brotli header specifies an excessively large history window. The decoder accepts the header, attempts to allocate the requested space, and if the allocator cannot satisfy the request the client or server aborts, interrupting normal operation. The weakness is categorized as CWE‑770, excessive allocation of memory resources, which directly compromises availability.

The vulnerability affects the rustls project’s rustls library in all releases earlier than 0.23.46. Systems that integrate rustls and have Brotli certificate decompression enabled are at risk.

The CVSS score of 6.3 places this issue in the medium severity range, but the absence of an EPSS score suggests that it is not a commonly seen exploit yet. Since the attack requires an inbound connection to a rustls‑based server or client, the likely vector is remote over a network protocol such as TLS. The vulnerability is not listed in CISA KEV, yet the potential for widespread impact exists if the library is used widely without mitigations.