1 files changed, 21 insertions, 2 deletions

diff --git a/readconf.c b/readconf.c
index 6c0511519..be14cd5b8 100644
--- a/readconf.c
+++ b/readconf.c
@@ -851,11 +851,30 @@ read_config_file(const char *filename, const char *host, Options *options,
 
         if (checkperm) {
                 struct stat sb;
+                int bad_modes = 0;
 
                 if (fstat(fileno(f), &sb) == -1)
                         fatal("fstat %s: %s", filename, strerror(errno));
-                if (((sb.st_uid != 0 && sb.st_uid != getuid()) ||
-                    (sb.st_mode & 022) != 0))
+                if (sb.st_uid != 0 && sb.st_uid != getuid())
+                        bad_modes = 1;
+                if ((sb.st_mode & 020) != 0) {
+                        /* If the file is group-writable, the group in
+                         * question must have at most one member, namely the
+                         * file's owner.
+                         */
+                        struct passwd *pw = getpwuid(sb.st_uid);
+                        struct group *gr = getgrgid(sb.st_gid);
+                        if (!pw || !gr)
+                                bad_modes = 1;
+                        else if (gr->gr_mem[0]) {
+                                if (strcmp(pw->pw_name, gr->gr_mem[0]) ||
+                                    gr->gr_mem[1])
+                                        bad_modes = 1;
+                        }
+                }
+                if ((sb.st_mode & 002) != 0)
+                        bad_modes = 1;
+                if (bad_modes)
                         fatal("Bad owner or permissions on %s", filename);
         }