diff --git a/cassandane/tiny-tests/Sieve/regex-timeout b/cassandane/tiny-tests/Sieve/regex-timeout
new file mode 100644
index 0000000000..9762520ac0
--- /dev/null
+++ b/cassandane/tiny-tests/Sieve/regex-timeout
@@ -0,0 +1,57 @@
+#!perl
+use Cassandane::Tiny;
+use Cassandane::Util::Slurp;
+use Cwd qw(abs_path);
+
+sub test_regex_timeout
+    :min_version_3_9 :needs_component_sieve :NoAltNameSpace :NoStartInstances
+{
+    my ($self) = @_;
+
+    # The regex below takes ~0.75s to execute on the given input
+    $self->{instance}->{config}->set('sieve_regex_timeout' => '0.5');
+    $self->_start_instances();
+
+    xlog, $self, "Create manifold user and make it readable by cassandane";
+    my $admintalk = $self->{adminstore}->get_client();
+    $admintalk->create("user.manifold");
+    $admintalk->setacl("user.manifold", cassandane => 'lrs');
+
+    xlog $self, "Install a script for cassandane";
+    $self->{instance}->install_sieve_script(<<EOF
+require ["regex", "imap4flags"];
+if header :regex "Subject"
+  "a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?aaaaaaaaaaaaaaaaaaaaaaaaa" {
+    addflag "\\\\Flagged";
+    keep;
+}
+EOF
+    );
+
+    xlog $self, "Install a script for manifold";
+    $self->{instance}->install_sieve_script(<<EOF
+require ["regex", "imap4flags"];
+if header :regex "Subject" "aaaa" {
+    keep :flags "\\\\Flagged";
+}
+EOF
+    , username => 'manifold');
+
+    my $msg = $self->{gen}->generate(subject => 'aaaaaaaaaaaaaaaaaaaaaaaaa');
+    $self->{instance}->deliver($msg, users => [ 'cassandane', 'manifold' ]);
+
+    xlog $self, "Check that the message made it to INBOX due to Sieve failure";
+    $self->{store}->set_folder('INBOX');
+    $self->{store}->set_fetch_attributes(qw(uid flags));
+    $msg->set_attribute(uid => 1);
+    $msg->set_attribute(flags => [ '\\Recent', '$SieveFailed' ]);
+    $self->check_messages({ 1 => $msg }, check_guid => 0);
+
+    xlog $self, "Check that the message made it to manifold INBOX with \Flagged";
+    $self->{store}->set_folder('user.manifold');
+    $self->{store}->set_fetch_attributes(qw(uid flags));
+    $msg->set_attribute(uid => 1);
+    $msg->set_attribute(flags => [ '\\Recent', '\\Flagged' ]);
+    $self->check_messages({ 1 => $msg }, check_guid => 0);
+}
+
diff --git a/changes/next/sieve_regex_timeout b/changes/next/sieve_regex_timeout
new file mode 100644
index 0000000000..605377b6f7
--- /dev/null
+++ b/changes/next/sieve_regex_timeout
@@ -0,0 +1,14 @@
+
+Description:
+
+Allow limiting Sieve :regex execution time.
+
+
+Config changes:
+
+Adds 'sieve_regex_timeout' option.
+
+
+Upgrade instructions:
+
+None.
diff --git a/lib/imapoptions b/lib/imapoptions
index 7d6a568ab7..93065fdd98 100644
--- a/lib/imapoptions
+++ b/lib/imapoptions
@@ -2622,6 +2622,12 @@ product version in the capabilities
 /* Maximum number of sieve scripts any user may have, enforced at
    submission by timsieved(8). */
 
+{ "sieve_regex_timeout", NULL, STRING, "UNRELEASED" }
+/* Time in floating point seconds. Any Sieve :regex match that takes
+   longer than this time is aborted and the script will fail (message
+   will be delivered to INBOX).
+   A NULL value (default) or a value of "0" will disable the timeout */ 
+
 { "sieve_utf8fileinto", 0, SWITCH, "2.3.17" }
 /* If enabled, the sieve engine expects folder names for the
    \fIfileinto\fR action in scripts to use UTF8 encoding.  Otherwise,
diff --git a/sieve/comparator.c b/sieve/comparator.c
index 02cfad07d1..c1c95f40de 100644
--- a/sieve/comparator.c
+++ b/sieve/comparator.c
@@ -334,13 +334,35 @@ static int octet_matches(const char *text, size_t tlen, const char *pat,
 
 
 #ifdef ENABLE_REGEX
+#include <errno.h>
+#include <setjmp.h>
+#include <signal.h>
+#include <syslog.h>
+
+#include "libconfig.h"
+
+static sigjmp_buf jmpbuf;
+static volatile sig_atomic_t canjump;
+
+static void sig_alrm(int sig __attribute__((unused)))
+{
+    if (!canjump) return;  // unexpected signal, ignore
+
+    canjump = 0;
+    siglongjmp(jmpbuf, 1); // jump back to octet_regex()
+}
+
 static int octet_regex(const char *text, size_t tlen, const char *pat,
                        strarray_t *match_vars,
                        void *rock __attribute__((unused)))
 {
+    static struct sigaction action;
+    static double timeout;
+    struct itimerval it = { 0 };
+    struct timeval start, end;
     regmatch_t pm[MAX_MATCH_VARS+1];
     size_t nmatch = 0;
-    int r;
+    static int r;
 
     if (match_vars) {
         strarray_fini(match_vars);
@@ -348,6 +370,61 @@ static int octet_regex(const char *text, size_t tlen, const char *pat,
         memset(&pm, 0, sizeof(pm));
     }
 
+    if (!action.sa_handler) {
+        const char *timeoutstr = config_getstring(IMAPOPT_SIEVE_REGEX_TIMEOUT);
+
+        if (timeoutstr) timeout = atof(timeoutstr);
+        if (timeout < 0) timeout = 0;
+
+        /*
+         * signal() on some platforms may set SA_RESTART by default.
+         *
+         * Therefore, we use sigaction().
+         */
+        action.sa_handler = &sig_alrm;
+        sigemptyset(&action.sa_mask);
+#ifdef SA_INTERRUPT
+        action.sa_flags |= SA_INTERRUPT;
+#endif
+
+        if (timeout > 0 && sigaction(SIGALRM, &action, NULL) < 0) {
+            syslog(LOG_NOTICE, "sigaction(SIGALRM) failed for Sieve :regex: %m");
+            errno = 0;
+
+            /* Return an error so script execution fails */
+            r = SIEVE_INTERNAL_ERROR;
+        }
+    }
+
+    if (r < 0) return r;
+
+    if (sigsetjmp(jmpbuf, 1)) {
+        /*
+         * regexec() timed out, return an error so script execution fails
+         *
+         * Since we have no way of freeing any resources used by
+         * regexec(), we will terminate lmtpd.
+         *
+         * The signal will not be caught until after deliver()
+         * completes and we return to the top of the lmtpmode() loop,
+         * so we will continue delivery to the remaining recipients.
+         */
+        gettimeofday(&end, 0);
+        syslog(LOG_INFO, "Sieve :regex execution timed out after %fs",
+               timesub(&start, &end));
+        raise(SIGTERM);
+        return SIEVE_REGEX_TIMEOUT;
+    }
+
+    /* siglongjmp() is now OK */
+    canjump = 1;
+
+    /* enable timeout */
+    it.it_value.tv_usec = (long) (timeout * 1000000);
+    setitimer(ITIMER_REAL, &it, NULL);
+
+    gettimeofday(&start, 0);
+
 #ifdef REG_STARTEND
     /* pcre, BSD, some linuxes support this handy trick */
     pm[0].rm_so = 0;
@@ -365,6 +442,15 @@ static int octet_regex(const char *text, size_t tlen, const char *pat,
     free(buf);
 #endif /* REG_STARTEND */
 
+    /* disable timeout */
+    canjump = it.it_value.tv_usec = 0;
+    setitimer(ITIMER_REAL, &it, NULL);
+
+    /* log run time */
+    gettimeofday(&end, 0);
+    syslog(LOG_INFO, "Sieve :regex run time: %fs",
+           timesub(&start, &end));
+
     if (r) {
         /* populate match variables */
         size_t var_num;
@@ -378,7 +464,7 @@ static int octet_regex(const char *text, size_t tlen, const char *pat,
     }
     return r;
 }
-#endif
+#endif /* ENABLE_REGEX */
 
 
 /* --- i;ascii-casemap comparators (RFC 4790, Section 9.2) --- */
diff --git a/sieve/sieve_err.et b/sieve/sieve_err.et
index 2e15bd7758..03fc9f86cf 100644
--- a/sieve/sieve_err.et
+++ b/sieve/sieve_err.et
@@ -65,6 +65,9 @@ ec SIEVE_DONE,
 ec SIEVE_SCRIPT_RELOADED,
    "Sieve script was loaded in the past"
 
+ec SIEVE_REGEX_TIMEOUT,
+   "Sieve :regex execution too long"
+
 
 # Parse errors