diff --git a/include/client.h b/include/client.h
index 4f07cb17..7d65da0c 100644
--- a/include/client.h
+++ b/include/client.h
@@ -90,7 +90,7 @@ typedef unsigned long flagpage_t;
 #define FlagClr(set,flag) ((set)->bits[FLAGSET_INDEX(flag)] &= ~FLAGSET_MASK(flag))
 
 /** String containing valid user modes, in no particular order. */
-#define infousermodes "diOoswkgx"
+#define infousermodes "diOoswkgxc"
 
 /** Operator privileges. */
 enum Priv
@@ -166,6 +166,7 @@ enum Flag
     FLAG_DEBUG,                     /**< send global debug/anti-hack info */
     FLAG_ACCOUNT,                   /**< account name has been set */
     FLAG_HIDDENHOST,                /**< user's host is hidden */
+    FLAG_COMMONCHANS,               /**< only accepts messages from users in common channels */
     FLAG_LAST_FLAG,                 /**< number of flags */
     FLAG_LOCAL_UMODES = FLAG_LOCOP, /**< First local mode flag */
     FLAG_GLOBAL_UMODES = FLAG_OPER  /**< First global mode flag */
@@ -599,6 +600,8 @@ struct Client {
 #define IsHiddenHost(x)         HasFlag(x, FLAG_HIDDENHOST)
 /** Return non-zero if the client has an active PING request. */
 #define IsPingSent(x)           HasFlag(x, FLAG_PINGSENT)
+/** Return non-zero if the client has mode +c (only messages from common channels). */
+#define IsCommonChans(x)        HasFlag(x, FLAG_COMMONCHANS)
 
 /** Return non-zero if the client has operator or server privileges. */
 #define IsPrivileged(x)         (IsAnOper(x) || IsServer(x))
@@ -645,6 +648,8 @@ struct Client {
 #define SetHiddenHost(x)        SetFlag(x, FLAG_HIDDENHOST)
 /** Mark a client as having a pending PING. */
 #define SetPingSent(x)          SetFlag(x, FLAG_PINGSENT)
+/** Mark a client as having mode +c (only messages from those in common channels). */
+#define SetCommonChans(x)       SetFlag(x, FLAG_COMMONCHANS)
 
 /** Return non-zero if \a sptr sees \a acptr as an operator. */
 #define SeeOper(sptr,acptr) (IsAnOper(acptr) && (HasPriv(acptr, PRIV_DISPLAY) \
@@ -680,6 +685,8 @@ struct Client {
 #define ClearPingSent(x)        ClrFlag(x, FLAG_PINGSENT)
 /** Clear the client's HUB flag. */
 #define ClearHub(x)             ClrFlag(x, FLAG_HUB)
+/** Remove mode +c (only accepts messages from common channels) from the client. */
+#define ClearCommonChans(x)     ClrFlag(x, FLAG_COMMONCHANS)
 
 /* free flags */
 #define FREEFLAG_SOCKET	0x0001	/**< socket needs to be freed */
diff --git a/ircd/ircd_relay.c b/ircd/ircd_relay.c
index e39e7dba..a87cd8cb 100644
--- a/ircd/ircd_relay.c
+++ b/ircd/ircd_relay.c
@@ -400,6 +400,28 @@ void relay_directed_notice(struct Client* sptr, char* name, char* server, const
   }
 }
 
+/** Check if two users share a common channel.
+ * @param[in] sptr Source client.
+ * @param[in] acptr Target client.
+ * @return Non-zero if users share a channel, zero otherwise.
+ */
+static int has_common_channel(struct Client *sptr, struct Client *acptr)
+{
+  struct Membership *schan, *achan;
+
+  for (schan = cli_user(sptr)->channel; schan; schan = schan->next_channel) {
+    if (IsZombie(schan) || IsDelayedJoin(schan))
+      continue;
+    for (achan = cli_user(acptr)->channel; achan; achan = achan->next_channel) {
+      if (IsZombie(achan) || IsDelayedJoin(achan))
+        continue;
+      if (schan->channel == achan->channel)
+        return 1;  /* Found common channel */
+    }
+  }
+  return 0;  /* No common channels */
+}
+
 /** Relay a private message from a local user.
  * Returns an error if the user does not exist or sending to him would
  * exceed the source's free targets.  Sends an AWAY status message if
@@ -426,6 +448,12 @@ void relay_private_message(struct Client* sptr, const char* name, const char* te
       is_silenced(sptr, acptr))
     return;
 
+  /* Check +c mode: block private messages from users not in common channels */
+  if (IsCommonChans(acptr) && !IsChannelService(sptr) && !IsOper(sptr)) {
+    if (!has_common_channel(sptr, acptr))
+      return;
+  }
+
   /*
    * send away message if user away
    */
@@ -464,6 +492,13 @@ void relay_private_notice(struct Client* sptr, const char* name, const char* tex
        check_target_limit(sptr, acptr, NULL)) ||
       is_silenced(sptr, acptr))
     return;
+
+  /* Check +c mode: block notices from users not in common channels */
+  if (IsCommonChans(acptr) && !IsChannelService(sptr) && !IsOper(sptr)) {
+    if (!has_common_channel(sptr, acptr))
+      return;
+  }
+
   /*
    * deliver the message
    */
diff --git a/ircd/s_user.c b/ircd/s_user.c
index 6a60e78f..0aca5df2 100644
--- a/ircd/s_user.c
+++ b/ircd/s_user.c
@@ -499,7 +499,8 @@ static const struct UserMode {
   { FLAG_CHSERV,      'k' },
   { FLAG_DEBUG,       'g' },
   { FLAG_ACCOUNT,     'r' },
-  { FLAG_HIDDENHOST,  'x' }
+  { FLAG_HIDDENHOST,  'x' },
+  { FLAG_COMMONCHANS, 'c' }
 };
 
 /** Length of #userModeList. */
@@ -1081,12 +1082,19 @@ int set_user_mode(struct Client *cptr, struct Client *sptr, int parc,
 	}
 	/* There is no -r */
 	break;
+      case 'c':
+        if (what == MODE_ADD)
+          SetCommonChans(sptr);
+        else
+          ClearCommonChans(sptr);
+        break;
       default:
         send_reply(sptr, ERR_UMODEUNKNOWNFLAG, *m);
         break;
       }
     }
   }
+  
   /*
    * Evaluate rules for new user mode
    * Stop users making themselves operators too easily: