--- title: TONTOU attack bypasses Spectre v2 mitigations on AMD and... description: MIT CSAIL researchers have disclosed TONTOU (Time-of-Neutralization to Time-of-Use), a new CPU side-channel attack that bypasses Spectre v2 mitigations on AMD... canonical: https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: TONTOU attack bypasses Spectre v2 mitigations on AMD and Intel CPUs, leaks kernel memory | daily.dev og:description: MIT CSAIL researchers have disclosed TONTOU (Time-of-Neutralization to Time-of-Use), a new CPU side-channel attack that bypasses Spectre v2 mitigations on AMD... og:url: https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd og:image: https://api.daily.dev/og/posts/EeHfznSYD.png og:image:alt: TONTOU attack bypasses Spectre v2 mitigations on AMD and Intel CPUs, leaks kernel memory 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. # TONTOU attack bypasses Spectre v2 mitigations on AMD and Intel CPUs, leaks kernel memory **[Collections](https://daily.dev/sources/collections)** · 2 min read · 0 upvotes · 0 comments ## Summary MIT CSAIL researchers have disclosed TONTOU (Time-of-Neutralization to Time-of-Use), a new CPU side-channel attack that bypasses Spectre v2 mitigations on AMD and Intel processors. The attack exploits a timing gap between when the branch predictor is neutralized and when it is actually used, allowing an attacker to re-poison CPU state via a precisely timed interrupt injection. On AMD Zen 2 running Linux 6.14.0-37-generic, it leaks kernel memory at 5.47 bytes/second — slow but functional, extracting /etc/shadow hashes in ~18 minutes with 50% success. The vulnerability targets Linux's Safe RET mitigation for SRSO, affecting AMD Zen 1 through Zen 4. AMD confirmed the flaw is in Linux's implementation, not the hardware. A patch has been merged into the Linux kernel Git tree. ## Content ## What happened Researchers from MIT CSAIL have disclosed a new CPU side-channel attack called TONTOU (Time-of-Neutralization to Time-of-Use) that bypasses existing Spectre v2 mitigations on AMD and Intel processors. The work was presented at Black Hat USA and is scheduled to appear at USENIX Security 2026. The attack was discovered by researcher Daniël Trujillo and colleagues at MIT CSAIL. AMD has published a security advisory confirming the issue. ## How it works Existing Spectre v2 defenses work by neutralizing the branch predictor before sensitive operations. TONTOU exploits a timing gap between when the predictor is neutralized and when it is actually used. By injecting a timer interrupt at a precise moment, an attacker can re-poison the CPU's branch predictor state before the protected operation runs, effectively reopening the window that mitigations were supposed to close. The specific target is Linux's Safe RET mitigation for Speculative Return Stack Overflow (SRSO), which affects AMD Zen 1 through Zen 4 processors. AMD confirmed the issue is in the Linux implementation of Safe RET, not the hardware itself. ## What it can leak The researchers built a working proof-of-concept on an AMD Zen 2 system running Linux 6.14.0-37-generic. From an unprivileged user account, the exploit leaked arbitrary kernel memory at roughly 5.47 bytes per second. In practice, that's slow enough that leaking `/etc/shadow` password hashes took about 18 minutes per attempt, with a 50% success rate across 10 runs. Not instant, but not theoretical either. ## The patches A fix has been merged into the Linux kernel tree. Greg Kroah-Hartman released six stable kernel versions addressing the underlying vulnerability (tracked as CVE-2026-68480): 7.1.7, 6.18.43, 6.6.149, 6.1.181, 5.15.214, and 5.10.263. A follow-up round of stable releases — 6.12.102, 6.6.150, 6.1.182, 5.15.215, and 5.10.264 — fixed a single regression introduced by those patches, discovered by Thomas Lamprecht. Users running affected kernel versions should upgrade. ## Questions this post answers ### What is the TONTOU CPU side-channel attack and how does it bypass Spectre v2 mitigations? TONTOU (Time-of-Neutralization to Time-of-Use) exploits a timing gap between when the branch predictor is neutralized and when it is actually used. By injecting an interrupt at precisely the right moment, an unprivileged attacker re-poisons CPU state, undoing the mitigation. It specifically targets Linux's Safe RET mitigation for Speculative Return Stack Overflow (SRSO) on AMD Zen 1 through Zen 4 processors. _Kernel and systems developers tracking CPU vulnerability disclosures like this one follow the patch trail on daily.dev._ ### How fast does the TONTOU attack leak kernel memory and what can it expose? On an AMD Zen 2 system running Linux 6.14.0-37-generic, TONTOU leaks arbitrary kernel memory at 5.47 bytes per second. At that rate, extracting /etc/shadow password hashes takes approximately 18 minutes per attempt, with a 50% success rate across 10 test runs. The attack is slow but reliably functional against unpatched systems. _Security engineers assessing exposure to speculative execution attacks find the latest advisories and patches on daily.dev._ ### Has a fix been released for the TONTOU Spectre v2 bypass affecting AMD Zen processors on Linux? A patch has been merged into the Linux kernel Git tree addressing the interrupt injection window in the Safe RET implementation. AMD confirmed the vulnerability lies in Linux's Safe RET implementation rather than the hardware itself, and its security advisory points to the Linux-side fix as the resolution. The affected hardware range is AMD Zen 1 through Zen 4. _Teams maintaining Linux systems on AMD hardware track kernel security patches like this on daily.dev._ ## Similar posts on daily.dev - [Newer RISC-V CPUs Vulnerable To Spectre V1 - Linux Mitigation Patches Posted](https://daily.dev/posts/newer-risc-v-cpus-vulnerable-to-spectre-v1---linux-mitigation-patches-posted-w75vepfxx) · Phoronix · 0 upvotes · 0 comments - [NDSS 2025 – CounterSEVeillance: Performance-Counter Attacks On AMD SEV-SNP](https://daily.dev/posts/ndss-2025-counterseveillance-performance-counter-attacks-on-amd-sev-snp-gqf73tije) · Security Boulevard · 1 upvotes · 0 comments --- Tags: [#security](https://daily.dev/tags/security), [#linux](https://daily.dev/tags/linux) [View this post on daily.dev](https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd) ```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":"TONTOU attack bypasses Spectre v2 mitigations on AMD and Intel CPUs, leaks kernel memory","url":"https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd"},"datePublished":"2026-08-06T18:09:00.346Z","dateModified":"2026-08-07T14:18:58.926Z","description":"MIT CSAIL researchers have disclosed TONTOU (Time-of-Neutralization to Time-of-Use), a new CPU side-channel attack that bypasses Spectre v2 mitigations on AMD...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/482cd0066efdc093b7afa5092c6a3903?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/482cd0066efdc093b7afa5092c6a3903?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Collections","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":"Collections","logo":"https://media.daily.dev/image/upload/s--fk_6ycEi--/f_auto,q_auto/v1780996001/logos/collections?_a=BAMAMiWQ0","url":"https://daily.dev/sources/collections"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"security,linux","timeRequired":"PT2M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Collections","item":"https://daily.dev/sources/collections"},{"@type":"ListItem","position":3,"name":"TONTOU attack bypasses Spectre v2 mitigations on AMD and Intel CPUs, leaks kernel memory"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/tontou-attack-bypasses-spectre-v2-mitigations-on-amd-and-intel-cpus-leaks-kernel-memory-eehfznsyd#faq","mainEntity":[{"@type":"Question","name":"What is the TONTOU CPU side-channel attack and how does it bypass Spectre v2 mitigations?","acceptedAnswer":{"@type":"Answer","text":"TONTOU (Time-of-Neutralization to Time-of-Use) exploits a timing gap between when the branch predictor is neutralized and when it is actually used. By injecting an interrupt at precisely the right moment, an unprivileged attacker re-poisons CPU state, undoing the mitigation. It specifically targets Linux's Safe RET mitigation for Speculative Return Stack Overflow (SRSO) on AMD Zen 1 through Zen 4 processors. Kernel and systems developers tracking CPU vulnerability disclosures like this one follow the patch trail on daily.dev."}},{"@type":"Question","name":"How fast does the TONTOU attack leak kernel memory and what can it expose?","acceptedAnswer":{"@type":"Answer","text":"On an AMD Zen 2 system running Linux 6.14.0-37-generic, TONTOU leaks arbitrary kernel memory at 5.47 bytes per second. At that rate, extracting /etc/shadow password hashes takes approximately 18 minutes per attempt, with a 50% success rate across 10 test runs. The attack is slow but reliably functional against unpatched systems. 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