v2.15.1

librdkafka v2.15.1 is a maintenance release:

• Update bundled OpenSSL and libcurl dependencies, and refresh the Windows
  build toolchain (msys2, vcpkg) (#5579).
• Fix int_latency metric calculation, that was reporting bogus values when
  message.timeout.ms is set to 0 (infinite) (#5335).
• IPv6 addresses are wrapped in square brackets following RFC 3986, allowing to connect to those compressed IPv6 addresses (#5544).
• IPv6 addresses are correctly passed to OpenSSL as IPs not as hostnames with brackets and the certificate is validated against its iPAddress entries instead of against dNSName entries (#5544).

Upgrade considerations

• If you're parsing the nodename as received by connect_cb, stats_cb, ssl_cert_verify_cb or throttle_cb, make sure you're correctly parsing it following RFC 3986. Metadata API calls are unaffected: they return the host and port as separate fields.
• Make sure the brokers' certificates carry the IP in an iPAddress entry, not a dNSName entry. Previously the certificate was validated against dNSName (falling back to the subject CN) whenever the address could not be parsed as an IP literal: with OpenSSL < 3.0 or BoringSSL for any IP address, and on all OpenSSL versions for an IPv6 address configured in brackets in bootstrap.servers ([::1]:9092), which never verified. A scoped address (fe80::1%eth0) was likewise dNSName-matched on every version and now has the zone stripped before matching. IPv6 addresses ending in :: could not be connected to at all, as the nodename became host:::port and failed name resolution. In the remaining cases, a bare IPv4 or IPv6 literal on OpenSSL >= 3.0, the iPAddress entry was already used, and behaviour is unchanged.

Security considerations

Bundled dependencies were further upgraded as follows:
OpenSSL 3.5.6 → 3.5.7 for source/autoconf builds, and 3.6.2 → 3.6.3 for
vcpkg-based packages; libcurl 8.20.0 → 8.21.0, now used by both
source/autoconf builds and vcpkg (previously pinned to 8.19.0 in vcpkg,
so vcpkg-based packages also pick up the fixes below that source/autoconf
builds already got from 8.20.0).

• OpenSSL upgrade (3.5.6 → 3.5.7 for source/autoconf, 3.6.2 → 3.6.3 for
  vcpkg) addresses:

  • Both branches: CVE-2026-34180, CVE-2026-34181, CVE-2026-34182,
    CVE-2026-34183, CVE-2026-42764, CVE-2026-42766, CVE-2026-42767,
    CVE-2026-42768, CVE-2026-42769, CVE-2026-42770, CVE-2026-45445,
    CVE-2026-45446, CVE-2026-45447, CVE-2026-7383, CVE-2026-9076.
  • Only the vcpkg 3.6.2 → 3.6.3 branch (3.5.6/3.5.7 were not affected):
    CVE-2026-35188, CVE-2026-42765.

• libcurl upgrade (8.20.0 → 8.21.0) addresses: CVE-2026-8286,
  CVE-2026-8458, CVE-2026-8924, CVE-2026-8925, CVE-2026-8926,
  CVE-2026-8927, CVE-2026-8932, CVE-2026-9079, CVE-2026-9080,
  CVE-2026-9545, CVE-2026-9546, CVE-2026-9547, CVE-2026-10536,
  CVE-2026-11352, CVE-2026-11564, CVE-2026-11586, CVE-2026-11856,
  CVE-2026-12064.
  Since libcurl is now at the same version (8.21.0) for both
  source/autoconf and vcpkg builds, this also closes the gap noted in
  the previous release, where vcpkg-pinned 8.19.0 still contained
  CVE-2026-4873, CVE-2026-5545, CVE-2026-5773, CVE-2026-6253,
  CVE-2026-6276, CVE-2026-6429, CVE-2026-7168.

• zlib vcpkg port revision bump (1.3.2#0 → 1.3.2#1): no upstream version
  change and no associated CVE; packaging-only update.

Fixes

Producer fixes

• Issues: #5555.
  Fix int_latency metric calculation. It was derived from the message
  timeout timestamp (now + message.timeout.ms - rkm_ts_timeout), which
  yields a large negative value when message.timeout.ms is 0 (infinite)
  and rkm_ts_timeout is INT64_MAX. It's now computed directly as
  now - rkm_ts_enq, the actual time the message spent in the queue,
  regardless of the timeout setting.
  Happening since 0.11.0 (#5335).

Checksums

Release asset checksums:

• v2.15.1.zip SHA256 1ca308c10c4234962695fbf9c7f1ab0649ab43c7ad03d384f1645db7b74c92ce
• v2.15.1.tar.gz SHA256 23c8575c7d1ced07246cb9cf200c11325b72201fd4134a02414ca869fbdd8ed3