GPT-5.5-Cyber built a zlib fuzzing lab in a day

Publisher excerpt From Trail of Bits Blog

We’re running Patch the Planet, an ongoing collaboration with OpenAI that pairs Trail of Bits engineers directly with more than 30 open-source projects. Its goal is to front-run a serious problem facing open-source maintainers: highly capable models like GPT-5.5-Cyber will soon create a firehose of bug reports, and OSS maintainers are already spread thin. Our plan is to point OpenAI’s latest models at real codebases, find the security bugs first, work with maintainers to patch them, and find ways to decrease the burden on maintainers in the long run. We’ll publish field reports like this one as the initiative progresses; follow along via the Patch the Planet tag. The expertise barrier that kept bespoke fuzzing campaigns out of reach for most attackers is gone. We watched GPT-5.5-Cyber build in a single day what would have taken weeks for a skilled security researcher: harnesses across a dozen entrypoints, sanitizer and variant builds, seeds, and multiple findings currently undergoing coordinated disclosure. This particular instance focused on zlib, a widely used data format and lossless data compression software library. We pointed GPT-5.5-Cyber at the library and drove it through Codex with the /goal command, asking it to find a specific class of bugs that are critically dangerous in compression libraries. We’ll publish the full harness and findings for inspection once the vulnerabilities are patched and a new release is cut. The lab GPT-5.5-Cyber built in a day We didn’t tell the model how to find these bugs. The obvious first move is to read the source code, but zlib has been reviewed so thoroughly that there’s little left to find that way. GPT-5.5-Cyber worked that out for itself, judged static review to be a poor use of tokens, and decided the higher value path was to build fuzz tooling to dynamically test the code. Earlier models given the same goal tend to read the code and flag whatever looks suspicious, ultimately leading to mediocre outcomes. We believe the frontier 5.5-Cyber model combined with the /goal feature is what let it execute end-to-end without hand-holding. /goal forced the objective to live across multiple turns and compactions so the model held scope, and 5.5-Cyber was smart enough to reject weak findings, expand coverage when a line of investigation died, and keep running until it had workable proof-of-concepts backed by sanitizer output. Over the next several hours, it built the campaign out one piece at a time: It used ASan and UBSan builds so memory errors became observable. It repurposed existing edge-case tests as guidance for the fuzz seed corpus. It wrote C/C++ harnesses across a dozen entrypoints, including inflate, inflateBack, uncompress2, gzFile, MiniZip, puff, blast, infback9, gzjoin, gzappend, and several contrib stream wrappers. It used compile-time variant builds (INFLATE_STRICT, BUILDFIXED, PKZIP_BUG_WORKAROUND, etc.) to reach code that the default zlib build hides. Each of these decisions is r…

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