--- title: Big Fish: Exploiting a 37-Year-Old Kernel Bug in OpenBSD description: A security research team details a chain of vulnerabilities in OpenBSD's kernel, tracing a 37-year-old bug in the SysV shared memory attachment counter (a... canonical: https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Big Fish: Exploiting a 37-Year-Old Kernel Bug in OpenBSD | daily.dev og:description: A security research team details a chain of vulnerabilities in OpenBSD's kernel, tracing a 37-year-old bug in the SysV shared memory attachment counter (a... og:url: https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt og:image: https://api.daily.dev/og/posts/u6Hc8g4lt.png og:image:alt: Big Fish: Exploiting a 37-Year-Old Kernel Bug in OpenBSD og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Big Fish: Exploiting a 37-Year-Old Kernel Bug in OpenBSD **[Hacker News](https://daily.dev/sources/hn)** · 6 min read · 0 upvotes · 0 comments ## Summary A security research team details a chain of vulnerabilities in OpenBSD's kernel, tracing a 37-year-old bug in the SysV shared memory attachment counter (a signed 16-bit short) back to the original BSD source. By forking 259 processes to overflow this counter and racing a use-after-free, combined with an out-of-bounds write in IP multicast options handling, attackers can bypass SMAP/SMEP and KARL kernel address randomization to achieve local root without any information disclosure. The writeup walks through heap grooming via sysctl metrics, winning the race with IPC_RMID, and overwriting predictable kernel pointers (lo0/enc0 ioctl handlers) to gain code execution. The bugs were patched via OpenBSD errata 026 and 018. The researchers also propose several mitigations: safe-unlinking for linked lists, randomizing the self-referential PTE mapping and kernel allocator base addresses, and compiler-level defenses like array bounds sanitizer and Alloc Tokens. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How was the OpenBSD shm_nattch shared memory attachment counter vulnerability exploited to get root? An attacker maps 127 SysV shared memory segments, unmaps them, then forks 259 processes to wrap the signed 16-bit shm_nattch counter to -32,643, triggering a use-after-free race condition won via IPC_RMID. Combined with an out-of-bounds write in IP_MULTICAST_TTL handling, this overwrites a predictable kernel pointer (lo0/enc0 ioctl handlers) to flip a page table entry from user to supervisor mode, bypassing SMAP/SMEP and KARL for local root. _Teams hardening BSD kernels can follow writeups like this on daily.dev to stay ahead of exploit techniques._ ### Which OpenBSD patches fix the shm_nattch integer overflow and shmat out-of-bounds write vulnerabilities? OpenBSD 7.9 errata 026 addresses the SysV shared memory attachment counter integer overflow (shm_nattch), while errata 018 fixes a separate out-of-bounds write bug in shmat used in combination with it for privilege escalation. The shm_nattch counter bug traces back roughly 37 years to the original BSD source code. _Admins tracking OpenBSD security patches can follow this kind of errata coverage on daily.dev._ ### Why doesn't OpenBSD's KARL kernel address randomization prevent all kernel exploitation attacks? KARL only relinks the kernel binary to randomize function and data addresses, but OpenBSD's boot-time and best-fit memory allocators remain deterministic and lack base address randomization. This means early boot allocations, including structures with function pointers like lo0 and enc0 ioctl handlers, land at predictable locations across boots on the same hardware, making them exploitable targets even with KARL enabled. _Developers evaluating kernel hardening trade-offs can track mitigation research like this via daily.dev._ ## Community take How the wider developer community reacted, aggregated from 2 discussions and 45 comments across hackernews, lobsters (as of 2026-10-16). **TL;DR:** Discussion centers less on the exploit itself and more on a meta-argument about whether the writeup is AI-generated and whether OpenBSD's reputation for being 'heavily audited' and secure is overstated given its small team and marketing-driven slogans. **Sentiment:** 10% positive · 35% mixed · 55% skeptical **The case for** - The writeup's mitigation discussion is seen as solid even by critics of the rest of the piece. - Some view OpenBSD's minimalism and reduced default attack surface as a genuinely good security design choice. **The pushback** - Several think the write-up reads as AI-generated or jargon-heavy bullet points rather than a clear human explanation. - Commenters push back hard on calling OpenBSD 'heavily audited', noting it has only a handful of active developers. - The 'only two remote holes in default install' claim is called misleading since almost nothing is enabled by default, so many services' vulnerabilities don't count. - Some argue C's inherent unsafety, not auditing quality, is the real root cause of this bug class persisting for decades. **By community** - hackernews (heated): A long back-and-forth argues over whether the post is AI-written and whether OpenBSD's security reputation is deserved, with sharp disagreement throughout. **Hottest debate:** Whether OpenBSD's 'only two remote holes in default install' reputation is a meaningful security claim or a misleading marketing slogan given how little runs by default. **Open questions** - Could fuzzing or static analysis realistically have caught this bug class, or did it require a human devising the exploit primitive? - Are agentic/AI tools being used to improve the frequency and quality of OpenBSD's code audits? **Highlights** > >in one of the most heavily audited codebases The idea of "OpenBSD is super duper secure" is mostly a fiction. Their prime claim to fame, "only two remote holes in the default install in a squillion years", is pretty meaningless because the default install doesn't really do anything, so as soon as you install packages to make the system useful you're beyond their security claims. Classified workstations in National Security contexts? They run Windows. Red network routers? They're Cisco, Juniper, PAN, etc. It's all COTS stuff. OpenBSD is mostly a LARP. > — [stackghost on hackernews · 5 comments](https://news.ycombinator.com/item?id=50120393) > There is another take on "heavily audited". Many OSs and eg. browsers have the issue that there are huge amounts of pretty complex codes. OpenBSD is extremely inviting and among actual general purpose operating systems used on a daily basis in production one of the projects that sees a quite big amount of reads because of the relative simplicity and (intentionally) low amount of code. Really, try making sense of pretty much anything in the Linux kernel compared to the OpenBSD kernel or any of the bigger browsers. Like fully grasping it. There is worlds in between. This has secondary effects of people actually looking at code that is not their area of expertise and developers also working outside their subsystems. Meanwhile in eg Linux you have huge swaths of code that essentially only some guy in some company somewhere has somewhat of a grasp of. And that's where Linux constantly sees its vulnerabilities, while OpenBSD actively removes such things, eg they removed Bluetooth support back in 2014. This leads to a situation where people actually are able to dig into things. They also have a huge amount of guardrails turned on and I know people that compile their code on OpenBSD despite not targeting the OS at all because of those guardrails uncovering issues. Think about any bigger software company. There is a huge chance you basically have no idea how the stuff outside your team works maybe not even all of your team. These kind of are the attack vectors we see in Open Source all the time (and actually closed source software too). It's not the stuff that everyone uses and wants to understand deeply in browsers that cause exploits. It's that one tiny thing that is part of the standard or somehow support was added, but despite nobody using it it's still there, either because well.. it's a standard or because nobody bothered to remove it. OpenBSD even decided to break some standards for security reasons, but that's another topic. What I wanna say with that is: I can think of a lot of projects that have more devs and more people looking at parts of it, but these projects tend to be quickly changing huge codebases, where no single person can have a good understanding of everything. This means that there is a good likelyhood that parts are not audited until some group or individual comes along and picks such a bit. It's actually also what likely happened in this case, given it's 37 years old. This isn't to defend OpenBSD, but to point out that X amount of people doesn't mean much without taking the size of the audit into consideration. I would think that big projects is something where LLMs really shine, but I also think that reducing attack surface by reducing code and complexity might be the biggest defense and as far as I have seen OpenBSD does a great job there and should be considered something that others can learn from. And it's not just these kinds of software. It's not even just software per se. It's also infrastructure, where complexity usually means increased attack surface. And just to be clear: One always adds complexity, but it should be the minimal amount to fulfill the goal and not more and try to not add side goals you don't actually care about and want to maintain. Especially with LLMs it's so incredibly easy to add complexity only for tomorrow's LLM (or human) to find huge amounts of bugs. > — [tete on hackernews](https://news.ycombinator.com/item?id=50124022) > If it is, it's not very good. They provide the source for the exploit, great, but the explanation of the exploit is terrible: just seven pithy bullet points. It's not really clear what's going on without a lot more digging. The discussion of mitigations is better, though. Context: I teach a college-level security class where undergrads do stack-smashing-like attacks (and every year a handful of talented students manage to pull off things like ROP attacks, analysis of entropy for a layout randomizers, or side-channel attacks). All of their writeups are much better than this one. > — [raddan on hackernews · 1 comments](https://news.ycombinator.com/item?id=50121141) > I think there's this cultural bias about the 'boy genius open source coder' that of course isn't true at all. Open source code is at the same quality as corporate written closed source code, in general. There have been studies on this and open vs closed source code isn't very different in terms of defects and exploits. The problem is the people most willing to buy into this cargo cult mentality are techies, so tech forums are the last place to really have this conversation based on facts. Hence why your comment is barely visible here due to its low karma. > — [zoeysmithe on hackernews](https://news.ycombinator.com/item?id=50121513) > Not that remarkable. Nobody in history has been able to write correct C code, and that includes OpenBSD developers. I pointed an AI at the smallest two tools in OpenBSD I could think of, cat and find, and found multiple bugs. One of which was a buffer overflow (albeit unlikely to be exploitable). IIRC these bugs had existed since at least the NetBSD/OpenBSD split. Everything in C is UB: https://news.ycombinator.com/item?id=48203698 > — [thomashabets2 on hackernews · 2 comments](https://news.ycombinator.com/item?id=50119990) **Source threads** - [hackernews](https://news.ycombinator.com/item?id=50118977) · 139 points · 45 comments - [lobsters](https://lobste.rs/s/cxlrc5/big_fish_exploiting_37_year_old_kernel_bug) · 11 points · 0 comments ## Similar posts on daily.dev - [CVE-2026-45257: LPE in FreeBSD via kTLS-RX](https://daily.dev/posts/cve-2026-45257-lpe-in-freebsd-via-ktls-rx-qcqrau2dy) · Lobsters · 0 upvotes · 0 comments - [FreeBSD 14.x kernel local privilege escalation](https://daily.dev/posts/freebsd-14-x-kernel-local-privilege-escalation-to8uxlrak) · Hacker News · 0 upvotes · 0 comments - [FreeBSoD: Leveraging Language Models to Find and Exploit Kernel Bugs \(Part 2 of 2\)](https://daily.dev/posts/freebsod-leveraging-language-models-to-find-and-exploit-kernel-bugs-part-2-of-2--qhvyj36t9) · Security Boulevard · 0 upvotes · 0 comments --- Tags: [#openbsd](https://daily.dev/tags/openbsd) [View this post on daily.dev](https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Big Fish: Exploiting a 37-Year-Old Kernel Bug in OpenBSD","url":"https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt"},"datePublished":"2026-10-16T15:47:00.181Z","dateModified":"2026-10-16T20:37:52.203Z","description":"A security research team details a chain of vulnerabilities in OpenBSD's kernel, tracing a 37-year-old bug in the SysV shared memory attachment counter (a...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/7db219cec0fa0dea44503bd263db6ffb?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/7db219cec0fa0dea44503bd263db6ffb?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Hacker News","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Hacker News","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/hn","url":"https://daily.dev/sources/hn"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"openbsd","timeRequired":"PT6M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Hacker News","item":"https://daily.dev/sources/hn"},{"@type":"ListItem","position":3,"name":"Big Fish: Exploiting a 37-Year-Old Kernel Bug in OpenBSD"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/big-fish-exploiting-a-37-year-old-kernel-bug-in-openbsd-u6hc8g4lt#faq","mainEntity":[{"@type":"Question","name":"How was the OpenBSD shm_nattch shared memory attachment counter vulnerability exploited to get root?","acceptedAnswer":{"@type":"Answer","text":"An attacker maps 127 SysV shared memory segments, unmaps them, then forks 259 processes to wrap the signed 16-bit shm_nattch counter to -32,643, triggering a use-after-free race condition won via IPC_RMID. 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