One (Thread) Can Keep a (PRNG) Secret, but not Two

Computer Science > Cryptography and Security [Submitted on 30 May 2026] One (Thread) Can Keep a (PRNG) Secret, but not Two Ehood Porat, Amit Klein, Benny Pinkas We present a novel, practical attack on the IPv6 Fragment ID generation algorithm of XNU, which is the kernel used by Apple products such as macOS and iOS. This attack exploits a race-condition vulnerability in the algorithm's pseudorandom number generator (PRNG) to cryptanalytically break, learn the internal state of the generator, and consequently predict fragment IDs, which, in turn, facilitates an IPv6 fragment spoofing attack. As far as we know, this is the first cryptanalytic attack that is based on exploiting race-conditions. With fragment spoofing, it is possible to partially manipulate UDP datagrams and TCP segments. We showcase a new type of attack on NFS (UDP) where an off-path attacker modifies a file as it is written, and an attack on HTTP (TCP) where an off-path attacker modifies an HTTP request. Apple assigned this vulnerability the CVE identifier CVE-2024-27823 and patched all its XNU-based products against the attack. Comments: Based on Ehood Porat's MSc thesis accepted in 2023 Subjects: Cryptography and Security (cs.CR) Cite as: arXiv:2606.00918 [cs.CR]   (or arXiv:2606.00918v1 [cs.CR] for this version)   https://doi.org/10.48550/arXiv.2606.00918 Focus to learn more Submission history From: Amit Klein [view email] [v1] Sat, 30 May 2026 23:00:31 UTC (178 KB) Access Paper: HTML (experimental) view license Current browse context: cs.CR < prev   |   next > new | recent | 2026-06 Change to browse by: cs References & Citations NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)