rustls CVE-2026-123610

Exploitation requires the optional, default-disabled brotli feature to be compiled in and enabled. A client is vulnerable when it enables brotli and connects to a malicious server; a server is vulnerable only when it enables brotli AND requests client certificate authentication, exposing it to malicious clients. Neither direction requires a trusted or valid attacker certificate, and the trigger (a five-byte Brotli header requesting up to a 1 GiB window) occurs before authentication. The primary limiting factors are: the non-default brotli configuration, the server-side requirement that client auth be requested, and that the abort only materializes when the target allocator cannot satisfy the ~1 GiB allocation (so systems with ample memory may degrade rather than crash).

This is a genuine but moderate-severity, availability-only issue, correctly reflected by the CVSS 4.0 score of 6.3 (medium). The supplied vector CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N with VA:L and VC:N/VI:N confirms a network-reachable, pre-authentication denial-of-service with no confidentiality or integrity impact. Several signals pull the real-world priority down: the vulnerable Brotli certificate decompression is disabled by default (built-in but opt-in), AT:P and AC:H indicate the abort only occurs when the target's allocator cannot satisfy the ~1 GiB request, and VulnCheck's own priority label is 'this_month' rather than act_now or emergency. No EPSS score was supplied and the vulnerability is not listed in CISA KEV, so there is no evidence of active exploitation or widespread automated attack. The references are the rustls GHSA advisory and the VulnCheck advisory - advisory write-ups, not working exploit code. Net assessment: a legitimate pre-auth DoS worth patching on a normal maintenance cadence for the subset of deployments that have actually enabled the brotli feature, not a high-urgency emergency.

An attacker operating or controlling a TLS peer sends a certificate message whose five-byte Brotli header requests a history window of up to 1 GiB. A rustls client with the brotli feature enabled (or a brotli-enabled server that requests client authentication) attempts the allocation during certificate decompression before authentication completes; if the allocator cannot satisfy it, the process aborts, producing denial of service. No trusted or valid attacker certificate is required, but no public working exploit code was identified at time of analysis.