diff --git a/gnu/local.mk b/gnu/local.mk
index bcd96ae0d6..69e516cea3 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -790,6 +790,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/slim-session.patch			\
   %D%/packages/patches/slim-config.patch			\
   %D%/packages/patches/slim-sigusr1.patch			\
+  %D%/packages/patches/slock-CVE-2016-6866.patch		\
   %D%/packages/patches/slurm-configure-remove-nonfree-contribs.patch \
   %D%/packages/patches/soprano-find-clucene.patch		\
   %D%/packages/patches/superlu-dist-scotchmetis.patch		\
diff --git a/gnu/packages/patches/slock-CVE-2016-6866.patch b/gnu/packages/patches/slock-CVE-2016-6866.patch
new file mode 100644
index 0000000000..2f94b8c1a9
--- /dev/null
+++ b/gnu/packages/patches/slock-CVE-2016-6866.patch
@@ -0,0 +1,51 @@
+Fix CVE-2016-6866.
+
+https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-6866
+https://security-tracker.debian.org/tracker/CVE-2016-6866
+
+Copied from upstream source repository:
+http://git.suckless.org/slock/commit/?id=d8bec0f6fdc8a246d78cb488a0068954b46fcb29
+
+From d8bec0f6fdc8a246d78cb488a0068954b46fcb29 Mon Sep 17 00:00:00 2001
+From: Markus Teich <markus.teich@stusta.mhn.de>
+Date: Tue, 30 Aug 2016 22:59:06 +0000
+Subject: fix CVE-2016-6866
+
+---
+diff --git a/slock.c b/slock.c
+index 847b328..8ed59ca 100644
+--- a/slock.c
++++ b/slock.c
+@@ -123,7 +123,7 @@ readpw(Display *dpy)
+ readpw(Display *dpy, const char *pws)
+ #endif
+ {
+-	char buf[32], passwd[256];
++	char buf[32], passwd[256], *encrypted;
+ 	int num, screen;
+ 	unsigned int len, color;
+ 	KeySym ksym;
+@@ -159,7 +159,11 @@ readpw(Display *dpy, const char *pws)
+ #ifdef HAVE_BSD_AUTH
+ 				running = !auth_userokay(getlogin(), NULL, "auth-slock", passwd);
+ #else
+-				running = !!strcmp(crypt(passwd, pws), pws);
++				errno = 0;
++				if (!(encrypted = crypt(passwd, pws)))
++					fprintf(stderr, "slock: crypt: %s\n", strerror(errno));
++				else
++					running = !!strcmp(encrypted, pws);
+ #endif
+ 				if (running) {
+ 					XBell(dpy, 100);
+@@ -312,6 +316,8 @@ main(int argc, char **argv) {
+ 
+ #ifndef HAVE_BSD_AUTH
+ 	pws = getpw();
++	if (strlen(pws) < 2)
++		die("slock: failed to get user password hash.\n");
+ #endif
+ 
+ 	if (!(dpy = XOpenDisplay(NULL)))
+--
+cgit v0.9.0.3-65-g4555
diff --git a/gnu/packages/suckless.scm b/gnu/packages/suckless.scm
index 7efa502d8f..820d550d6e 100644
--- a/gnu/packages/suckless.scm
+++ b/gnu/packages/suckless.scm
@@ -111,6 +111,7 @@ numbers of user-defined menu items efficiently.")
               (method url-fetch)
               (uri (string-append "http://dl.suckless.org/tools/slock-"
                                   version ".tar.gz"))
+              (patches (search-patches "slock-CVE-2016-6866.patch"))
               (sha256
                (base32
                 "065xa9hl7zn0lv2f7yjxphqsa35rg6dn9hv10gys0sh4ljpa7d5s"))))