Описание
Oj: Stack Buffer Overflow in Oj::Doc#each_child via Deeply Nested Input
Summary
Oj::Doc#each_child, when invoked recursively over a deeply nested JSON
document, overflows a fixed-size stack buffer and aborts the process. This is a
denial of service reachable from untrusted JSON.
Details
Two-step chain in ext/oj/fast.c:
-
doc_each_child(~line 1501) incrementsdoc->wherepast thewhere_path[MAX_STACK = 100]array with no bounds check, and never restores it (doc->where--is missing). Callingeach_childrecursively from inside the yield block therefore drivesdoc->wherebeyond the array. -
On the next entry (~line 1478) the function copies the path into a stack-local buffer:
Leaf save_path[MAX_STACK]; // 800-byte stack buffer size_t wlen = doc->where - doc->where_path; if (0 < wlen) { memcpy(save_path, doc->where_path, sizeof(Leaf) * (wlen + 1)); }When the previous recursive call left
doc->wherepastwhere_path[100],wlenexceedsMAX_STACKand thememcpyoverflowssave_pathon the C stack.
The Oj::Doc parser imposes no JSON nesting-depth limit (it relies on a
C-stack pressure check), so deeply nested attacker input reaches this path.
Proof of Concept
Recursion depth <= 99 iterates normally; depth >= 101 aborts. lldb backtrace
on the affected build (ruby 3.3.8 / arm64-darwin24):
Impact
Reliable denial of service: any endpoint that calls
Oj::Doc.open(untrusted) { |d| d.each_child ... } recursively can be crashed
with a small deeply-nested payload. On builds with a stack protector (the
default, -fstack-protector-strong) the canary aborts the process before the
saved return address is used. The Step-1 heap OOB writes into struct _doc
fields do occur, but are masked in practice because the Step-2 stack overflow
crashes first; turning them into anything beyond a crash has not been
demonstrated.
Patches
Fixed in 3.17.3: doc_each_child now bounds-checks before incrementing
doc->where (raising Oj::DepthError) and restores doc->where after the
loop, matching the existing each_leaf pattern. Verified on the fixed build:
depth >= 101 raises a clean Oj::DepthError instead of aborting.
Credit
Reported by Zac Wang (@7a6163).
Пакеты
oj
< 3.17.3
3.17.3
Связанные уязвимости
Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.3, Oj::Doc#each_child, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process, leading to DoS. In a two-step chain in ext/oj/fast.c, doc_each_child increments doc->where past the where_path[MAX_STACK = 100] array with no bounds check and never restores it (the doc->where-- is missing), so calling each_child recursively from inside the yield block drives doc->where beyond the array. On the next entry the function copies the path into the 800-byte stack-local buffer save_path[MAX_STACK] using wlen = doc->where - doc->where_path, so when the previous recursive call left doc->where past where_path[100] the wlen exceeds MAX_STACK and the memcpy overflows save_path on the C stack; because the Oj::Doc parser imposes no JSON nesting-depth limit (relying on a C-stack pressure check), deeply nested attacker input reac...
Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.3, Oj::Doc#each_child, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process, leading to DoS. In a two-step chain in ext/oj/fast.c, doc_each_child increments doc->where past the where_path[MAX_STACK = 100] array with no bounds check and never restores it (the doc->where-- is missing), so calling each_child recursively from inside the yield block drives doc->where beyond the array. On the next entry the function copies the path into the 800-byte stack-local buffer save_path[MAX_STACK] using wlen = doc->where - doc->where_path, so when the previous recursive call left doc->where past where_path[100] the wlen exceeds MAX_STACK and the memcpy overflows save_path on the C stack; because the Oj::Doc parser imposes no JSON nesting-depth limit (relying on a C-stack pressure check), deeply nested attacker input reac...
Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.3, Oj::Doc#each_child, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process, leading to DoS. In a two-step chain in ext/oj/fast.c, doc_each_child increments doc->where past the where_path[MAX_STACK = 100] array with no bounds check and never restores it (the doc->where-- is missing), so calling each_child recursively from inside the yield block drives doc->where beyond the array. On the next entry the function copies the path into the 800-byte stack-local buffer save_path[MAX_STACK] using wlen = doc->where - doc->where_path, so when the previous recursive call left doc->where past where_path[100] the wlen exceeds MAX_STACK and the memcpy overflows save_path on the C stack; because the Oj::Doc parser imposes no JSON nesting-depth limit (relying on a C-stack pressure check), deeply nested attacker input reaches
Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as ...