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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Ekko is a sleep-obfuscation technique that changes what a dormant implant’s memory—and, in one timer-based proof of concept, its sleeping thread’s call stack—looks like during a wait. The stack change is temporary: the original stack is restored before execution resumes. That can make a sleeping thread less revealing, but it does not make the implant undetectable; timers and memory artifacts can still provide investigative leads.
What is Ekko sleep obfuscation?
Ekko is an open-source sleep-obfuscation technique associated with hiding implant memory while the implant waits between command-and-control check-ins. The Ekko project is available in its repository.
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“Sleep mask” can refer to memory masking, while call-stack masking is a related but distinct measure. Cobalt Strike describes its Sleep Mask Kit as a way to hide Beacon in memory during sleep. The timer-based call-stack proof of concept discussed here adds a separate change to the sleeping thread’s stack. These approaches address different evidence and should not be treated as interchangeable names for one universal implementation. See Cobalt Strike’s Sleep Mask Kit feature page.
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How does timer-based sleep obfuscation work?
William Burgess, Principal Research Lead, describes the sequence in a Cobalt Strike walkthrough: “Prior to our implant sleeping, we can queue up timers to overwrite its call stack with a fake one and then restore the original before resuming execution.” The PoC is based on C5Spider’s Ekko technique and uses timer-queue timers.
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- Save the original stack. The PoC backs up the sleeping thread’s current stack.
- Temporarily replace it. While the thread waits, timers overwrite the stack with a plausible alternative. The walkthrough discusses both a hard-coded/static example and a dynamic search for a suitable accessible thread.
- Restore before resumption. The original stack is put back before the thread continues execution.
The intended effect is to make inspection of a sleeping thread less revealing. It is a proof-of-concept description, not evidence that every endpoint product is bypassed. The full walkthrough, “Behind the Mask: Spoofing Call Stacks Dynamically with Timers,” explains the implementation details.
What is masked, and what remains observable?
| Evidence or behavior | What the technique changes | What may still be observable |
|---|---|---|
| Beacon memory | A sleep mask is intended to hide Beacon in memory while it is dormant. | Memory scanning can still find residual code or other artifacts; results depend on version and configuration. |
| Sleeping thread’s call stack | The timer-based PoC temporarily substitutes a plausible stack, then restores the original before resumption. | Timer objects, stack anomalies, or the surrounding thread behavior may remain useful investigative leads. |
| Sleep-mask implementation code | Masking Beacon does not necessarily mask every part of the masking mechanism itself. | Cobalt Strike’s YARA analysis describes a case where default sleep-mask code remained detectable in memory. |
The comparison matters in an engagement: a less revealing stack is not proof that Beacon memory is hidden, and a masked Beacon is not proof that the thread or masking code has no observable artifacts.
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How can defenders investigate a sleeping Beacon?
The sources point to several complementary leads rather than a single definitive detector. Burgess notes that timer-queue timers can be enumerated in memory. Cobalt Strike’s analysis also discusses traditional memory scanning and investigating sleeping threads with unbacked memory. Its YARA example demonstrates that default sleep-mask code may remain detectable even when Beacon itself is masked. See Cobalt Strike’s sleep-mask and YARA analysis.
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- Look for timer artifacts: timer-queue timers associated with a waiting thread can be a lead, though their presence alone does not establish malicious activity.
- Inspect memory and backing: traditional memory scanning and investigation of sleeping threads with unbacked memory may reveal evidence not apparent from the call stack alone.
- Account for the mask itself: memory signatures may identify implementation code even when Beacon’s payload is masked. The cited YARA result is specific to the analyzed case, not a guarantee across all versions and configurations.
- Correlate rather than overclaim: the cited sources do not establish any one of these signals as comprehensive across every variant.
How does Cobalt Strike’s version history affect the comparison?
Ekko and Cobalt Strike’s built-in Sleep Mask are not synonyms. Cobalt Strike’s feature history places the Sleep Mask Kit in version 4.4, heap-masking support in 4.5, a Beacon Object File redesign in 4.7, and BeaconGate support plus Sleepmask-VS examples in 4.10. Its version 4.11 announcement describes a new out-of-the-box mask that obfuscates Beacon, heap allocations, and itself, and specifically says it applies to HTTP(S)/DNS Beacons—not every Beacon type. The 4.11 announcement is the relevant source for that scope; the feature page provides the version history.
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For red and blue teams, the practical implication is to identify the implementation and product version under discussion before drawing conclusions. A finding about one mask, transport, or memory signature does not automatically apply to Ekko or to all Cobalt Strike Beacons.
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