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CVE-2026-40691 is a heap-based buffer overflow in Unbound affecting versions 1.9.0 through 1.25.1. The flaw occurs in the DNSCrypt-over-TCP reply path when Unbound encrypts a reply in place. Unlike the UDP path, the TCP path does not clamp the reply length against the destination buffer size before shifting the reply forward by 48 bytes for DNSCrypt processing. If a DNSCrypt TCP query causes Unbound to generate a reply larger than 65504 bytes, the in-place shift can write past the end of a heap buffer whose capacity is limited by msg-buffer-size. This out-of-bounds heap write can be triggered by a single malicious encrypted query and results in resolver process termination. The issue is only reachable when Unbound is built with DNSCrypt support and DNSCrypt is configured and enabled on listening interfaces.
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