diff --git a/Misc/NEWS b/Misc/NEWS
index a6679ad..e457fa1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -374,6 +374,9 @@
 Build
 -----
 
+- Upgrade Windows build to zlib 1.2.3 which eliminates a potential security
+  vulnerability in zlib 1.2.1 and 1.2.2.
+
 - EXTRA_CFLAGS has been introduced as an environment variable to hold compiler
   flags that change binary compatibility.  Changes were also made to
   distutils.sysconfig to also use the environment variable when used during
diff --git a/PCbuild/readme.txt b/PCbuild/readme.txt
index 7d836ab..bdf68f9 100644
--- a/PCbuild/readme.txt
+++ b/PCbuild/readme.txt
@@ -126,15 +126,15 @@
 
 zlib
     Python wrapper for the zlib compression library.  Get the source code
-    for version 1.2.1 from a convenient mirror at:
+    for version 1.2.3 from a convenient mirror at:
         http://www.gzip.org/zlib/
-    Unpack into dist\zlib-1.2.1.
+    Unpack into dist\zlib-1.2.3.
     A custom pre-link step in the zlib project settings should manage to
-    build zlib-1.2.1\zlib.lib by magic before zlib.pyd (or zlib_d.pyd) is
+    build zlib-1.2.3\zlib.lib by magic before zlib.pyd (or zlib_d.pyd) is
     linked in PCbuild\.
     However, the zlib project is not smart enough to remove anything under
-    zlib-1.2.1\ when you do a clean, so if you want to rebuild zlib.lib
-    you need to clean up zlib-1.2.1\ by hand.
+    zlib-1.2.3\ when you do a clean, so if you want to rebuild zlib.lib
+    you need to clean up zlib-1.2.3\ by hand.
 
 bz2
     Python wrapper for the libbz2 compression library.  Homepage
diff --git a/PCbuild/zlib.vcproj b/PCbuild/zlib.vcproj
index 4ad457c..63e06ff 100644
--- a/PCbuild/zlib.vcproj
+++ b/PCbuild/zlib.vcproj
@@ -21,7 +21,7 @@
 			<Tool
 				Name="VCCLCompilerTool"
 				Optimization="0"
-				AdditionalIncludeDirectories="..\Include,..\PC,..\..\zlib-1.2.1"
+				AdditionalIncludeDirectories="..\Include,..\PC,..\..\zlib-1.2.3"
 				PreprocessorDefinitions="_DEBUG;WIN32;_WINDOWS"
 				RuntimeLibrary="3"
 				UsePrecompiledHeader="2"
@@ -37,7 +37,7 @@
 				Name="VCCustomBuildTool"/>
 			<Tool
 				Name="VCLinkerTool"
-				AdditionalDependencies="..\..\zlib-1.2.1\zlib.lib"
+				AdditionalDependencies="..\..\zlib-1.2.3\zlib.lib"
 				OutputFile="./zlib_d.pyd"
 				LinkIncremental="1"
 				SuppressStartupBanner="TRUE"
@@ -63,7 +63,7 @@
 			<Tool
 				Name="VCPreLinkEventTool"
 				Description="Checking static zlib has been built"
-				CommandLine="cd ..\..\zlib-1.2.1
+				CommandLine="cd ..\..\zlib-1.2.3
 nmake -nologo -f win32\Makefile.msc zlib.lib
 "/>
 			<Tool
@@ -92,7 +92,7 @@
 				Name="VCCLCompilerTool"
 				Optimization="2"
 				InlineFunctionExpansion="1"
-				AdditionalIncludeDirectories="..\Include,..\PC,..\..\zlib-1.2.1"
+				AdditionalIncludeDirectories="..\Include,..\PC,..\..\zlib-1.2.3"
 				PreprocessorDefinitions="NDEBUG;WIN32;_WINDOWS"
 				StringPooling="TRUE"
 				RuntimeLibrary="2"
@@ -110,7 +110,7 @@
 				Name="VCCustomBuildTool"/>
 			<Tool
 				Name="VCLinkerTool"
-				AdditionalDependencies="..\..\zlib-1.2.1\zlib.lib"
+				AdditionalDependencies="..\..\zlib-1.2.3\zlib.lib"
 				OutputFile="./zlib.pyd"
 				LinkIncremental="1"
 				SuppressStartupBanner="TRUE"
@@ -136,7 +136,7 @@
 			<Tool
 				Name="VCPreLinkEventTool"
 				Description="Checking static zlib has been built"
-				CommandLine="cd ..\..\zlib-1.2.1
+				CommandLine="cd ..\..\zlib-1.2.3
 nmake -nologo -f win32\Makefile.msc zlib.lib
 "/>
 			<Tool
@@ -166,7 +166,7 @@
 				AdditionalOptions=" /USECL:MS_ITANIUM"
 				Optimization="2"
 				InlineFunctionExpansion="1"
-				AdditionalIncludeDirectories="{MSSDKPATH}\include\Win64\atl;{MSSDKPATH}\include\Win64\crt;{MSSDKPATH}\include\Win64\crt\sys;{MSSDKPATH}\include\Win64\mfc;..\Include,..\PC,..\..\zlib-1.2.1"
+				AdditionalIncludeDirectories="{MSSDKPATH}\include\Win64\atl;{MSSDKPATH}\include\Win64\crt;{MSSDKPATH}\include\Win64\crt\sys;{MSSDKPATH}\include\Win64\mfc;..\Include,..\PC,..\..\zlib-1.2.3"
 				PreprocessorDefinitions="NDEBUG;WIN32;_WINDOWS"
 				StringPooling="TRUE"
 				BasicRuntimeChecks="0"
@@ -188,7 +188,7 @@
 			<Tool
 				Name="VCLinkerTool"
 				AdditionalOptions=" /MACHINE:IA64 /USELINK:MS_SDK"
-				AdditionalDependencies="..\..\zlib-1.2.1\zlib.lib"
+				AdditionalDependencies="..\..\zlib-1.2.3\zlib.lib"
 				OutputFile="./zlib.pyd"
 				LinkIncremental="1"
 				SuppressStartupBanner="TRUE"
@@ -214,7 +214,7 @@
 			<Tool
 				Name="VCPreLinkEventTool"
 				Description="Checking static zlib has been built"
-				CommandLine="cd ..\..\zlib-1.2.1
+				CommandLine="cd ..\..\zlib-1.2.3
 nmake -nologo -f win32\Makefile.msc zlib.lib
 "/>
 			<Tool