diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index c090fef7..15c778e6 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -348,6 +348,9 @@ endif (WITH_SYMBOL_VERSIONING AND HAVE_LD_VERSION_SCRIPT AND ABIMAP_FOUND)
 # This gets built as a static library, if -DBUILD_SHARED_LIBS=OFF is passed to
 # cmake.
 add_library(ssh ${libssh_SRCS})
+if (CYGWIN)
+    target_compile_definitions(ssh PRIVATE _GNU_SOURCE)
+endif ()
 target_include_directories(ssh
                            PUBLIC
                                $<BUILD_INTERFACE:${libssh_SOURCE_DIR}/include>
@@ -404,6 +407,9 @@ install(EXPORT libssh-config
 
 if (BUILD_STATIC_LIB)
   add_library(ssh-static STATIC ${libssh_SRCS})
+  if (CYGWIN)
+    target_compile_definitions(ssh-static PRIVATE _GNU_SOURCE)
+  endif ()
 
   target_include_directories(ssh-static
                              PUBLIC