Public bug reported:
fonts-katex is installed by default on Kubuntu 26.04: it's listed
explicitly in the installer's package set in /var/log/apt/history.log,
not pulled in as a dependency of anything else. With it present,
fontconfig segfaults in FcCharSetHasChar and Plasma can't start.
Affected: two machines, both stock Kubuntu 26.04 installs from April
2026, fully updated on 2026-09-22. This report was filed from the second
one before applying the workaround.
SYMPTOMS
plasmashell SIGSEGV crash loop at login, kioworker crashing repeatedly, and the
plasma desktop never loads. /tmp (tmpfs) filled completely during the crash
loop. Also saw "The screen locker is broken and unlocking is not possible
anymore."
$ coredumpctl list kscreenlocker_great plasmashell 2>/dev/null | tail
TIME PID UID GID SIG COREFILE EXE
SIZE
Fri 2026-05-22 09:01:48 EDT 2486 1000 1000 SIGSEGV missing
/usr/bin/plasmashell -
Fri 2026-05-22 09:02:09 EDT 34328 1000 1000 SIGSEGV missing
/usr/bin/plasmashell -
Wed 2026-09-23 16:00:15 EDT 402103 1000 1000 SIGSEGV present
/usr/bin/plasmashell 13.6M
Wed 2026-09-23 16:00:20 EDT 404899 1000 1000 SIGSEGV present
/usr/bin/plasmashell 34M
Wed 2026-09-23 16:00:23 EDT 405894 1000 1000 SIGSEGV present
/usr/bin/plasmashell 13.2M
BACKTRACE (plasmashell)
#4 libfontconfig.so.1 (+0xc4fc)
#5 FcCharSetHasChar
#6 QFontEngineMultiFontConfig::shouldLoadFontEngineForCharacter(...)
#7 QFontEngineMulti::stringToCMap(...)
#14 QQuickLabel::componentComplete()
#20 Klipper::Klipper(QObject*)
FONT MATCHING ON THE AFFECTED SYSTEM
$ fc-match sans
KaTeX_AMS-Regular.woff: "Noto Sans" "<unknown style>"
$ fc-match -v sans | grep -E 'file|fontversion'
file: "/usr/share/fonts/truetype/katex/KaTeX_AMS-Regular.woff"(s)
fontversion: 2147483647(i)(s)
VERSIONS
fonts-katex / libjs-katex 0.16.10+~cs6.1.0-5ubuntu1
libfontconfig1 / fontconfig 2.17.1-3ubuntu1 (unchanged since initial install in
April)
Plasma 6.6.6, Qt 6.10.2, kernel 7.0.0-34
The September updates installed before the breakage included no font,
freetype, harfbuzz, or Qt packages. Possibly related: libexpat1
2.7.4-1ubuntu0.1 (fontconfig parses its config via expat), ghostscript /
libgs 10.06.0~dfsg-3ubuntu1.1, and glib 2.88.0-1ubuntu0.1.
WORKAROUND (confirmed on two systems)
sudo apt purge fonts-katex # also removes libjs-katex; nothing else depends
on them
sudo rm -rf /var/cache/fontconfig/*
rm -rf ~/.cache/fontconfig
sudo fc-cache -f
$ fc-match sans
NotoSans-Regular.ttf: "Noto Sans" "Regular"
Clearing only the user cache (~/.cache/fontconfig) fixed it for a single
login before the crash returned. The system cache has to be rebuilt too.
RELATED
Same crash, reported by someone who got fonts-katex as a dependency of
lemonade-server: https://github.com/lemonade-sdk/lemonade/issues/3655
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: fonts-katex 0.16.10+~cs6.1.0-5ubuntu1
ProcVersionSignature: Ubuntu 7.0.0-31.31-generic 7.0.14
Uname: Linux 7.0.0-31-generic x86_64
NonfreeKernelModules: zfs
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Wed Sep 23 16:01:55 2026
Dependencies:
InstallationDate: Installed on 2026-05-22 (125 days ago)
InstallationMedia: Kubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423)
PackageArchitecture: all
RebootRequiredPkgs: Error: path contained symlinks.
SourcePackage: node-katex
UpgradeStatus: No upgrade log present (probably fresh install)
** Affects: node-katex (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute wayland-session
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https://bugs.launchpad.net/bugs/2168311
Title:
Default-installed fonts-katex crashes fontconfig (FcCharSetHasChar);
Plasma won't start
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