This update was sent out the same day as iOS 18.6.2, providing bug fixes and stability issues for users on macOS Sequoia. Apple deemed CVE-2025-43300 serious enough to be included in an update for older versions of macOS as well, bringing Sonoma to 14.7.8 and even Ventura up to 13.7.8.
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Even though major developments for macOS Sequoia and Sonoma are over, there will be security updates for some time. A couple weeks ago we saw a flurry of updates for existing and established versions of Apple’s OSes, with macOS 15.6 being among them. Despite how new this Sequoia update is, we’re already seeing an RC version of 15.7 seeded to developers. This typically indicates yet another security and/or bug fix update.
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Timeline of these updates

Sequoia

• July 30th, 2025: macOS 15.6 (Build 24G84)
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• August 20th, 2025: macOS 15.6.1 (Build 24G90)
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Sonoma

• September 16th, 2024: macOS 14.7 (Build 23H124) when Sonoma got on “.7”.
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• July 29th, 2025: macOS 14.7.7 (Build 23H723) Most recent prior update.
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• August 20th, 2025: macOS 14.7.8 (Build 23H730)
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Ventura

• September 16th, 2024: macOS 13.7 (Build 22H123) when Ventura got on “.7”.
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• July 29th, 2025: macOS 13.7.7 (Build 22H722) Most recent prior update.
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• August 20th, 2025: macOS 13.7.8 (Build 22H730)
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Future updates in circulation

• macOS Sequoia 15.7 Release Candidate 3, Build 24G214
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• macOS Sonoma 14.8 Release Candidate 3, Build 23J15
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What comes with this update

This new update comes with a security fix, known as “CVE-2025-43300: Apple”. It mentions how a malicious image file may result in memory corruption. and also adds “Apple is aware of a report that this issue may have been exploited in an extremely sophisticated attack against specific targeted individuals”. This was a zero-day exploit, so it means it was already out in the open when Apple reported it.

This isn’t meant to alarm you, as you’d probably know if something like this happened, and Apple would have sent you a notification prior. It is important however to update your device straight away, so your device can be better protected.
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Performance versus 15.6

Even though this update is merely a security fix, there are times when an update can cause major change in performance, which can be measured with an artificial benchmark. Fortunately, there are no known reports so far (to my knowledge) about any performance issues with 15.6.1 versus 15.6.
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More information about this update

There is a good video uploaded to a channel called Mr.Macintosh, which I recommend checking out. He did a few benchmarks on 15.6.1, talks a little about OpenCore and 15.7, and other things related to this update.