diff --git a/doc/release-notes/rl-2605.section.md b/doc/release-notes/rl-2605.section.md
index 71eb4f0b80bd9..a3d51f9f762f5 100644
--- a/doc/release-notes/rl-2605.section.md
+++ b/doc/release-notes/rl-2605.section.md
@@ -175,6 +175,8 @@
   If your SQLite database is corrupted, the migration might fail and require [manual intervention](https://github.com/louislam/uptime-kuma/issues/5281).
   See the [migration guide](https://github.com/louislam/uptime-kuma/wiki/Migration-From-v1-To-v2) for more information.
 
+- The `libcxxhardeningextensive` hardening flag has been **disabled** by default. Enabling it by default in 25.11 was unintentional and may have had a negative effect on performance in some cases. `libcxxhardeningfast` remains enabled by default.
+
 - Switch inhibitors were introduced, which add a pre-switch check that compares a list of strings between the previous and the new generation, and refuses to switch into the new generation when there is a difference between the two lists. This allows to avoid switching into a system when for instance the systemd version changed by adding `config.systemd.package.version` to the switch inhibitors for your system. You can still forcefully switch into any generation by setting `NIXOS_NO_CHECK=1`.
 
 - GNU Taler has been updated to version 1.3.
diff --git a/doc/stdenv/stdenv.chapter.md b/doc/stdenv/stdenv.chapter.md
index 4c1bd3d5dd4e3..2caa4a8ef910b 100644
--- a/doc/stdenv/stdenv.chapter.md
+++ b/doc/stdenv/stdenv.chapter.md
@@ -1631,6 +1631,12 @@ Adds the `-fzero-call-used-regs=used-gpr` compiler option. This causes the gener
 
 This flag adds the `-fstack-clash-protection` compiler option, which causes growth of a program's stack to access each successive page in order. This should force the guard page to be accessed and cause an attempt to "jump over" this guard page to crash.
 
+#### `libcxxhardeningfast` {#libcxxhardeningfast}
+
+Adds the `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST` compiler flag. This flag only has an effect on libc++ targets, and when defined, enables a set of assertions that prevent undefined behavior caused by violating preconditions of the standard library. libc++ provides several hardening modes, and this "fast" mode contains a set of security-critical checks that can be done with relatively little overhead in constant time.
+
+Disabling `libcxxhardeningfast` implies disablement of checks from `libcxxhardeningextensive`.
+
 #### `strictflexarrays1` {#strictflexarrays1}
 
 This flag adds the `-fstrict-flex-arrays=1` compiler option, which reduces the cases the compiler treats as "flexible arrays" to those declared with length `[1]`, `[0]` or (the correct) `[]`. This increases the coverage of fortify checks, because such arrays declared as the trailing element of a structure can normally not have their intended length determined by the compiler.
@@ -1683,12 +1689,6 @@ Adds the `-D_GLIBCXX_ASSERTIONS` compiler flag. This flag only has an effect on
 
 These checks may have an impact on performance in some cases.
 
-#### `libcxxhardeningfast` {#libcxxhardeningfast}
-
-Adds the `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST` compiler flag. This flag only has an effect on libc++ targets, and when defined, enables a set of assertions that prevent undefined behavior caused by violating preconditions of the standard library. libc++ provides several hardening modes, and this "fast" mode contains a set of security-critical checks that can be done with relatively little overhead in constant time.
-
-Disabling `libcxxhardeningfast` implies disablement of checks from `libcxxhardeningextensive`.
-
 #### `libcxxhardeningextensive` {#libcxxhardeningextensive}
 
 Adds the `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE` compiler flag. This flag only has an effect on libc++ targets, and when defined, enables a set of assertions that prevent undefined behavior caused by violating preconditions of the standard library. libc++ provides several hardening modes, and this "extensive" mode adds checks for undefined behavior that incur relatively little overhead but aren’t security-critical. The additional rigour impacts performance more than fast mode: benchmarking is recommended to determine if it is acceptable for a particular application.
diff --git a/pkgs/build-support/bintools-wrapper/default.nix b/pkgs/build-support/bintools-wrapper/default.nix
index 0ecfc2cd9e524..b97eb905bd31c 100644
--- a/pkgs/build-support/bintools-wrapper/default.nix
+++ b/pkgs/build-support/bintools-wrapper/default.nix
@@ -49,7 +49,6 @@
     "format"
     "fortify"
     "fortify3"
-    "libcxxhardeningextensive"
     "libcxxhardeningfast"
     "pic"
     "relro"
diff --git a/pkgs/top-level/variants.nix b/pkgs/top-level/variants.nix
index 9e3b752332199..ab69077b088ad 100644
--- a/pkgs/top-level/variants.nix
+++ b/pkgs/top-level/variants.nix
@@ -164,7 +164,7 @@ self: super: {
               "nostrictaliasing"
               "pacret"
               "glibcxxassertions"
-              "libcxxhardeningfast"
+              "libcxxhardeningextensive"
               "trivialautovarinit"
             ]
           ) super'.stdenv;