diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 45986f3f527..7eabeb89093 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -352,6 +352,7 @@ Thank you!
     Matthias Pitzl <silamael@coronamundi.de>
     Matthieu Herrb <matthieu.herrb@laas.fr>
     Max Okumoto <okumoto@ucsd.edu>
+    Max Schmitt <max@schmitt.mx>
     Merik Karman
     <mgd@swarm.org>
     Michael Buchau <mike@m-buchau.de>
diff --git a/src/cf.data.pre b/src/cf.data.pre
index 28bbbd5681a..66d70485f98 100644
--- a/src/cf.data.pre
+++ b/src/cf.data.pre
@@ -10681,14 +10681,32 @@ DOC_END
 NAME: max_filedescriptors max_filedesc
 TYPE: int
 DEFAULT: 0
-DEFAULT_DOC: Use operating system soft limit set by ulimit.
+DEFAULT_DOC: Guess a reasonable value (e.g., use soft limit set by ulimit).
 LOC: Config.max_filedescriptors
 DOC_START
-	Set the maximum number of filedescriptors, either below the
-	operating system default or up to the hard limit.
-
-	Remove from squid.conf to inherit the current ulimit soft
-	limit setting.
+	Sets the maximum number of file descriptors, up to the hard OS limit (where
+	supported).
+
+	Currently, the explicitly configured value is ignored when `setrlimit(2)` or
+	`RLIMIT_NOFILE` support was not detected at `./configure` time. A level-1
+	"WARNING" message is written to cache.log in such cases, but future Squid
+	versions will fail to start instead.
+
+	Currently, if `setrlimit(2)` fails to set the soft and hard limits to the
+	configured value, a level-0 "ERROR" message is written to cache.log, and
+	Squid attempts to raise the soft limit to match the hard limit instead. The
+	resulting soft limit (raised or not) is then used instead of the configured
+	value. Future Squid versions will fail to start in such cases.
+
+	By default, Squid tries to guess a reasonable value (e.g., the current
+	operating system soft limit set by `ulimit -n` or equivalent). The details
+	of the guessing algorithm may change. Use this directive to avoid exposure
+	to those changes and unwanted dependencies on build and runtime environment
+	states, especially in high-performance deployments that require a large
+	number of descriptors.
+
+	Squid fails to start if `setrlimit(2)` is supported but fails to set a soft
+	limit to the configured or guessed value.
 
 	Note: Changing this requires a restart of Squid. Also
 	not all I/O types supports large values (eg on Windows).
diff --git a/src/tools.cc b/src/tools.cc
index 65adc5f18c8..f42f6e4ed32 100644
--- a/src/tools.cc
+++ b/src/tools.cc
@@ -764,6 +764,8 @@ setMaxFD(void)
     struct rlimit rl;
 #endif
 
+    auto checkLimits = true;
+
     if (getrlimit(RLIMIT_NOFILE, &rl) < 0) {
         int xerrno = errno;
         debugs(50, DBG_CRITICAL, "getrlimit: RLIMIT_NOFILE: " << xstrerr(xerrno));
@@ -787,13 +789,40 @@ setMaxFD(void)
                 xerrno = errno;
                 debugs(50, DBG_CRITICAL, "ERROR: setrlimit: RLIMIT_NOFILE: " << xstrerr(xerrno));
             }
+            // else: getrlimit() will still return two OS-originated rl.rlim_max limits that must be checked
+        } else {
+            // getrlimit() will now return admin-configured numbers, not OS-provided ones
+            checkLimits = false;
         }
     }
     if (getrlimit(RLIMIT_NOFILE, &rl) < 0) {
         int xerrno = errno;
         debugs(50, DBG_CRITICAL, "ERROR: getrlimit: RLIMIT_NOFILE: " << xstrerr(xerrno));
     } else {
-        Squid_MaxFD = rl.rlim_cur;
+        // An OS may supply us with huge RLIMIT_NOFILE limits (e.g., the soft
+        // limit may exceed one billion on Kubernets). We cap these OS-provided
+        // limits to protect deployments from accidentally or unknowingly
+        // allocating huge FD-indexed structures (e.g., ~432 GB fd_table on
+        // Kubernetes). Special deployments that really need more descriptors
+        // than this cap must set max_filedescriptors accordingly.
+        const auto defaultCapForMaximumNumberOfFiles = rlim_t(100*1024); // ~42 MB fd_table
+
+        // No cap if setrlimit() changed the limits to match max_filedescriptors
+        // because, in that case, rl.rlim_cur is effectively set by the Squid
+        // admin (rather than reflecting OS configuration that we do not trust).
+        if (checkLimits && rl.rlim_cur > defaultCapForMaximumNumberOfFiles) {
+            debugs(50, DBG_IMPORTANT, "WARNING: OS-provided soft limit (" << rl.rlim_cur << " RLIMIT_NOFILE) " <<
+                   "is too big to use for calculating the maximum number of descriptors Squid may use; " <<
+                   "setting that maximum to " << defaultCapForMaximumNumberOfFiles);
+            Squid_MaxFD = defaultCapForMaximumNumberOfFiles;
+        } else {
+            debugs(50, 3, "Squid_MaxFD was " << Squid_MaxFD << "; now " << rl.rlim_cur << " <= " << rl.rlim_max);
+            Squid_MaxFD = rl.rlim_cur;
+        }
+        // XXX: When checkLimits, take ./configure --with-filedescriptors (if any) into account.
+        // XXX: rl.rlim_cur is often too small (e.g. 1024). In those cases, use a larger value if rl.rlim_max allows.
+        // XXX: The new value may make Squid_MaxFD different from SQUID_MAXFD still used by ModEpoll, ModPoll, ipcCreate(), etc.!
+        // XXX: If this increases Squid_MaxFD, then the new value will violate any defined SQUID_MAXFD_LIMIT.
     }
 
 #endif /* HAVE_SETRLIMIT */