JSON: reject deeply nested input instead of overflowing the stack - #33
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Every generated simpleRestJson and jsonRpc2 server calls smithy::json::Decode() directly on the untrusted request body. A body of deeply nested arrays/objects (~20k levels) overflowed the stack in nlohmann's recursive-descent parser — and again in FromBackend — and crashed the process: an unauthenticated remote DoS. CBOR already guards this via DecodeValue's depth counter; JSON did not. Add an O(n), iterative pre-scan that rejects input past a fixed nesting depth (512, far beyond any legitimate document) before the recursive parser runs. Brackets inside strings don't nest, so the scan tracks string/escape state. Regression test drives 100k-deep arrays and objects through Decode and asserts a clean error, plus a bracket-heavy-but-flat document that must still parse. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_019ytv3VMrURFYP2mdWhk3un
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What
Fixes an unauthenticated remote DoS. Every generated simpleRestJson and jsonRpc2 server calls
smithy::json::Decode()directly on the untrusted request body (examples/*/generated/src/server.cc). A body of deeply nested arrays or objects (~20k levels) overflows the stack in nlohmann's recursive-descent parser — and again in our recursiveFromBackend— and crashes the process with SIGSEGV. I confirmed the crash locally: depth 10k decodes fine, depth 20k+ segfaults.CBOR already guards against exactly this with
DecodeValue's depth counter (kMaxDepth = 64); JSON had no equivalent.The fix adds an O(n), iterative pre-scan (
ExceedsMaxDepth) that rejects input past a fixed nesting depth (512 — far beyond any legitimate document, well under the overflow threshold) before the recursive parser runs. Brackets inside strings don't nest, so the scan tracks string/escape state.Testing
RejectsDeeplyNestedInputInsteadOfStackOverflow: drives 100k-deep[and{bombs throughDecodeand asserts a clean error (not a crash), plus a bracket-heavy-but-flat document that must still parse (guarding against the string-state logic over-counting).Found while auditing for other latent crashes in the vein of the SIGPIPE fix from the previous PR. See the PR discussion for the broader hostile-input test surface I'd suggest next.
Checklist
🤖 Generated with Claude Code
https://claude.ai/code/session_019ytv3VMrURFYP2mdWhk3un
Generated by Claude Code