PDF /GoToR action argv injection enables single-click RCE via --gtk-module dlopen

Summary

Atril Document Viewer is the default document reader of the MATE desktop environment for Linux. A single-click remote code execution vulnerability in versions prior to 1.26.3 and 1.28.4 allows an attacker to achieve arbitrary code execution as the user by tricking them into clicking a link inside a malicious PDF document. The PDF can be packaged as a polyglot file that is simultaneously a valid PDF and a valid ELF shared library, making the attack a single-file, single-click, configuration-independent RCE on stock atril installations. The root cause is `shell/ev-application.c:ev_spawn`, which builds a command line from attacker-controlled PDF link-destination fields without applying `g_shell_quote`. The cmdline is then handed to `g_app_info_create_from_commandline`, which shell-parses it back into argv — splitting any embedded `--gtk-module=PATH` into a separate argv element. GTK then `dlopen()`s the path during init, running any `__attribute__((constructor))` it finds. Versions 1.26.3 and 1.28.4 contain a patch for the issue. This is the same defect class as CVE-2023-51698 (CBT `--checkpoint-action` injection in `comics-document.c`, fixed in 1.6.2) but in a different code path (`shell/ev-application.c`) that the original patch did not touch.