Pixel stealing attacks enable malicious websites to leak sensitive content displayed in victim websites. The idea, introduced by Stone in 2013, is to embed victim websites in iframes and use SVG filters to compute on, and create side channels as a function of, those websites' pixels. Fortunately, despite the danger, pixel stealing attacks are all but mitigated today thanks to websites and web browsers heavily restricting iframes and cross-origin cookie sharing.

Microarchitectural side-channel attacks are an insidious threat to program security. An emerging class of these attacks constructs gadgets that dereference the contents of data memory directly. This is caused by optimizations, such as speculative execution and data-memory prefetching, that can guess (incorrectly) that the program is performing a pointer chase. In theory, this is devastating for security, as dereferencing a secret seemingly leaks it over memory-based side channels, e.g., through the cache. In practice, it is not. Since most secrets do not look like valid pointers, their dereference typically fails and does not leak anything. In this paper, we introduce the page walk side channel (PWSC), a new attack that can leak information even when an invalid pointer is dereferenced. In particular, given a 64-bit secret that passes the address canonicality check, PWSC can leak all remaining bits of the secret except for the low-order 6 bits, without making any assumptions on what these bits look like. We demonstrate how PWSC amplifies leakage in scenarios exploiting speculative execution and data-memory prefetching. For speculative execution, we show that PWSC, combined with Intel's LAM feature, can be exploited to leak nearly all of physical memory and that even without LAM, PWSC can be used to leak Dilithium secret keys. For data-memory prefetching, we reverse engineer the semantics of Intel's data-memory dependent prefetcher (DMP) and show how this DMP and PWSC can be combined to break security in an intra-process sandbox setting.

Speculative execution attacks undermine the security of constant-time programming, the standard technique used to prevent microarchitectural side channels in security-sensitive software such as cryptographic code. Constant-time code must therefore also deploy a defense against speculative execution attacks to prevent leakage of secret data stored in memory or the processor registers. Unfortunately, contemporary defenses, such as speculative load hardening (SLH), can only satisfy this strong security guarantee at a very high performance cost.