The RE_BACKREF execution path read `captures[group * 2]` and
`captures[group * 2 + 1]` without verifying that the group
index fit in the allocated captures array. A pattern like
`/\1/` (no capture group, but a backreference to group 1) is
accepted by the compiler and lands in execution with `ncap = 2`
(only group 0 slots) and an instruction asking for group 1 --
a 4-byte read past the end of the allocation.
Reported by ClusterFuzz testcase
clusterfuzz-testcase-minimized-mruby_fuzzer-5474946829844480.
Add `if (group * 2 + 1 >= ncap) return FALSE;` ahead of the
captures access, mirroring the bounds guard already present in
RE_SAVE. The compiler's permissive `\<digit>` handling stays
unchanged; the runtime now treats a reference to a non-existent
group as a non-match rather than UB.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
db2845aae0