fwupd and xz metadata – Technical Blog of Richard Hughes

A few people (and multi-billion dollar companies!) have asked for my response to the xz backdoor. The fwupd metadata that millions of people download every day is a 9.5MB XML file — which thankfully is very compressible. This used to be compressed as gzip by the LVFS, making it a 1.6MB download for end-users, but in 2021 we switched to xz compression instead.

What actually happens behind the scenes is that the libxmlb library loads the optionally compressed metadata into a mmap-able binary blob, and then it gets used by fwupd to look for new updates for specific hardware. In libxmlb 0.3.3 we added support for xz as a compression format. Then fwupd 1.8.7 was released with xz support, preferring the xz format to the “legacy” gz format — as the metadata became a 1.1MB download, saving significant amounts of data from the CDN.

Then this week we learned that xz wasn’t the kind of thing we want to depend on. Out of an abundance of caution (and to be clear — my understanding is there is no fwupd or LVFS security problem of any kind) I’ve switched the LVFS to also generate zstd metadata, make libxmlb no longer hard depend on lzma and switched fwupd to prefer the zstd metadata over the xz metadata if the installed version of libjcat supports it. The zstd metadata is also ~3% smaller than xz (and faster to decompress), but the real benefit is that I now trust it a lot more than xz.

I’ll be doing new libxmlb and fwupd releases with the needed changes next week.