MIT researchers find TONTOU attack bypasses Spectre v2 fixes on AMD and Intel CPUs
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Researchers at MIT's Computer Science and Artificial Intelligence Laboratory (CSAIL) developed a CPU attack that bypasses current Spectre v2 mitigations. The TONTOU technique exploits a timing gap in branch predictor neutralization on AMD and Intel processors, allowing unprivileged attackers to leak sensitive data, including hashed passwords, from Linux machines. The finding comes even as chipmakers deploy defenses like eIBRS and Safe RET.
The TONTOU Window
The attack targets the Time-of-Neutralization to Time-of-Use (TONTOU) gap between when a branch predictor state is sanitized and when it is used by a victim branch. Daniël Trujillo and Mengjia Yan of MIT CSAIL found that interrupts during this window can re‑poison the indirect branch predictor. By scheduling timer interrupts from unprivileged code, an attacker forces the kernel into an interrupt handler that maliciously manipulates the predictor state, defeating Intel’s eIBRS and AMD’s Safe RET mitigations.
Leakage Demonstration
The researchers tested the exploit on an AMD Zen 2 system with the latest Spectre v2 defenses installed. From an unprivileged user context, they successfully extracted hashed password data from kernel memory. The technique works against all indirect branch types, effectively reviving the original Spectre v2 threat. An attacker capable of executing arbitrary unprivileged code on a target Linux machine can read kernel secrets without any special access rights.
Mitigation Response
Intel and AMD were notified of the findings in early 2026, and the MIT team proposed microcode adjustments to close the TONTOU window. However, more aggressive predictor sanitization could carry performance penalties that manufacturers have historically been reluctant to accept. The research, scheduled for presentation at a major security conference, underscores the persistent difficulty of eliminating speculative execution side channels.
What's Next
Intel and AMD are expected to issue initial advisory guidance next month. It remains unclear whether a hardware fix can fully close the window without imposing unacceptable slowdowns on existing processors.
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MIT researchers find TONTOU attack bypasses Spectre v2 fixes on AMD and Intel CPUs
