Apple currently shows the iOS 11.3.1 release on its developer portal with build version 15E302, and the release will likely be released over-the-air shortly for all customers.
With iOS 11.3 released in late March and iOS 11.3.1 out today, Apple is also running a developer and public beta version of iOS 11.4, likely due out in late May or early June, which includes AirPlay 2 and Messages on iCloud.
iOS 11.3.1 includes a fix for display issues experienced on iPhone 8 with third-party repairs and security improvements for all iPhones and iPads.
Here's the security content of iOS 11.3.1:
Crash Reporter
Crash Reporter
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: An application may be able to gain elevated privileges
Description: A memory corruption issue was addressed with improved error handling.
CVE-2018-4206: Ian Beer of Google Project Zero
LinkPresentation
Impact: An application may be able to gain elevated privileges
Description: A memory corruption issue was addressed with improved error handling.
CVE-2018-4206: Ian Beer of Google Project Zero
LinkPresentation
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: Processing a maliciously crafted text message may lead to UI spoofing
Description: A spoofing issue existed in the handling of URLs. This issue was addressed with improved input validation.
CVE-2018-4187: Zhiyang Zeng (@Wester) of Tencent Security Platform Department, Roman Mueller (@faker_)
WebKit
Impact: Processing a maliciously crafted text message may lead to UI spoofing
Description: A spoofing issue existed in the handling of URLs. This issue was addressed with improved input validation.
CVE-2018-4187: Zhiyang Zeng (@Wester) of Tencent Security Platform Department, Roman Mueller (@faker_)
WebKit
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: Processing maliciously crafted web content may lead to arbitrary code execution
Description: A memory corruption issue was addressed with improved state management.
CVE-2018-4200: Ivan Fratric of Google Project Zero
WebKit
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: Processing maliciously crafted web content may lead to arbitrary code execution
Description: A memory corruption issue was addressed with improved memory handling.
CVE-2018-4204: Richard Zhu (fluorescence) working with Trend Micro's Zero Day Initiative, found by OSS-Fuzz
Source: Apple Support
Impact: Processing maliciously crafted web content may lead to arbitrary code execution
Description: A memory corruption issue was addressed with improved state management.
CVE-2018-4200: Ivan Fratric of Google Project Zero
WebKit
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: Processing maliciously crafted web content may lead to arbitrary code execution
Description: A memory corruption issue was addressed with improved memory handling.
CVE-2018-4204: Richard Zhu (fluorescence) working with Trend Micro's Zero Day Initiative, found by OSS-Fuzz
Source: Apple Support