AST-2017-006: Fix app_minivm application MinivmNotify command injection

An admin can configure app_minivm with an externnotify program to be run
when a voicemail is received.  The app_minivm application MinivmNotify
uses ast_safe_system() for this purpose which is vulnerable to command
injection since the Caller-ID name and number values given to externnotify
can come from an external untrusted source.

* Add ast_safe_execvp() function.  This gives modules the ability to run
external commands with greater safety compared to ast_safe_system().
Specifically when some parameters are filled by untrusted sources the new
function does not allow malicious input to break argument encoding.  This
may be of particular concern where CALLERID(name) or CALLERID(num) may be
used as a parameter to a script run by ast_safe_system() which could
potentially allow arbitrary command execution.

* Changed app_minivm.c:run_externnotify() to use the new ast_safe_execvp()
instead of ast_safe_system() to avoid command injection.

* Document code injection potential from untrusted data sources for other
shell commands that are under user control.

ASTERISK-27103

Change-Id: I7552472247a84cde24e1358aaf64af160107aef1

README-SERIOUSLY.bestpractices.txt

@@ -94,6 +94,13 @@ your ITSP in a place where you didn't expect to allow it. There are a couple of
 ways in which you can mitigate this impact: stricter pattern matching, or using
 the FILTER() dialplan function.
 
+The CALLERID(num) and CALLERID(name) values are other commonly used values that
+are sources of data potentially supplied by outside sources.  If you use these
+values as parameters to the System(), MixMonitor(), or Monitor() applications
+or the SHELL() dialplan function, you can allow injection of arbitrary operating
+system command execution.  The FILTER() dialplan function is available to remove
+dangerous characters from untrusted strings to block the command injection.
+
 Strict Pattern Matching
 -----------------------
 

apps/app_minivm.c

@@ -1741,21 +1741,35 @@ static int play_record_review(struct ast_channel *chan, char *playfile, char *re
 /*! \brief Run external notification for voicemail message */
 static void run_externnotify(struct ast_channel *chan, struct minivm_account *vmu)
 {
-	char arguments[BUFSIZ];
+	char fquser[AST_MAX_CONTEXT * 2];
+	char *argv[5] = { NULL };
+	struct ast_party_caller *caller;
+	char *cid;
+	int idx;
 
-	if (ast_strlen_zero(vmu->externnotify) && ast_strlen_zero(global_externnotify))
+	if (ast_strlen_zero(vmu->externnotify) && ast_strlen_zero(global_externnotify)) {
 		return;
+	}
+
+	snprintf(fquser, sizeof(fquser), "%s@%s", vmu->username, vmu->domain);
 
-	snprintf(arguments, sizeof(arguments), "%s %s@%s %s %s&", 
-		ast_strlen_zero(vmu->externnotify) ? global_externnotify : vmu->externnotify, 
-		vmu->username, vmu->domain,
-		(ast_channel_caller(chan)->id.name.valid && ast_channel_caller(chan)->id.name.str)
-			? ast_channel_caller(chan)->id.name.str : "",
-		(ast_channel_caller(chan)->id.number.valid && ast_channel_caller(chan)->id.number.str)
-			? ast_channel_caller(chan)->id.number.str : "");
+	caller = ast_channel_caller(chan);
+	idx = 0;
+	argv[idx++] = ast_strlen_zero(vmu->externnotify) ? global_externnotify : vmu->externnotify;
+	argv[idx++] = fquser;
+	cid = S_COR(caller->id.name.valid, caller->id.name.str, NULL);
+	if (cid) {
+		argv[idx++] = cid;
+	}
+	cid = S_COR(caller->id.number.valid, caller->id.number.str, NULL);
+	if (cid) {
+		argv[idx++] = cid;
+	}
+	argv[idx] = NULL;
 
-	ast_debug(1, "Executing: %s\n", arguments);
-	ast_safe_system(arguments);
+	ast_debug(1, "Executing: %s %s %s %s\n",
+		argv[0], argv[1], argv[2] ?: "", argv[3] ?: "");
+	ast_safe_execvp(1, argv[0], argv);
 }
 
 /*!\internal

apps/app_mixmonitor.c

@@ -127,6 +127,11 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 				<para>Will be executed when the recording is over.</para>
 				<para>Any strings matching <literal>^{X}</literal> will be unescaped to <variable>X</variable>.</para>
 				<para>All variables will be evaluated at the time MixMonitor is called.</para>
+				<warning><para>Do not use untrusted strings such as <variable>CALLERID(num)</variable>
+				or <variable>CALLERID(name)</variable> as part of the command parameters.  You
+				risk a command injection attack executing arbitrary commands if the untrusted
+				strings aren't filtered to remove dangerous characters.  See function
+				<variable>FILTER()</variable>.</para></warning>
 			</parameter>
 		</syntax>
 		<description>
@@ -143,6 +148,11 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 					<para>Will contain the filename used to record.</para>
 				</variable>
 			</variablelist>
+			<warning><para>Do not use untrusted strings such as <variable>CALLERID(num)</variable>
+			or <variable>CALLERID(name)</variable> as part of ANY of the application's
+			parameters.  You risk a command injection attack executing arbitrary commands
+			if the untrusted strings aren't filtered to remove dangerous characters.  See
+			function <variable>FILTER()</variable>.</para></warning>
 		</description>
 		<see-also>
 			<ref type="application">Monitor</ref>

apps/app_system.c

@@ -48,6 +48,11 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 		<syntax>
 			<parameter name="command" required="true">
 				<para>Command to execute</para>
+				<warning><para>Do not use untrusted strings such as <variable>CALLERID(num)</variable>
+				or <variable>CALLERID(name)</variable> as part of the command parameters.  You
+				risk a command injection attack executing arbitrary commands if the untrusted
+				strings aren't filtered to remove dangerous characters.  See function
+				<variable>FILTER()</variable>.</para></warning>
 			</parameter>
 		</syntax>
 		<description>
@@ -73,6 +78,11 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 		<syntax>
 			<parameter name="command" required="true">
 				<para>Command to execute</para>
+				<warning><para>Do not use untrusted strings such as <variable>CALLERID(num)</variable>
+				or <variable>CALLERID(name)</variable> as part of the command parameters.  You
+				risk a command injection attack executing arbitrary commands if the untrusted
+				strings aren't filtered to remove dangerous characters.  See function
+				<variable>FILTER()</variable>.</para></warning>
 			</parameter>
 		</syntax>
 		<description>

configs/minivm.conf.sample

@@ -51,7 +51,7 @@ silencethreshold=128
 ; If you need to have an external program, i.e. /usr/bin/myapp called when a
 ; voicemail is received by the server. The arguments are
 ;
-; 	<app> <username@domain> <callerid-number> <callerid-name>
+; 	<app> <username@domain> <callerid-name> <callerid-number>
 ;
 ;externnotify=/usr/bin/myapp
 ; The character set for voicemail messages can be specified here

funcs/func_shell.c

@@ -84,6 +84,11 @@ static int shell_helper(struct ast_channel *chan, const char *cmd, char *data,
 		<syntax>
 			<parameter name="command" required="true">
 				<para>The command that the shell should execute.</para>
+				<warning><para>Do not use untrusted strings such as <variable>CALLERID(num)</variable>
+				or <variable>CALLERID(name)</variable> as part of the command parameters.  You
+				risk a command injection attack executing arbitrary commands if the untrusted
+				strings aren't filtered to remove dangerous characters.  See function
+				<variable>FILTER()</variable>.</para></warning>
 			</parameter>
 		</syntax>
 		<description>

include/asterisk/app.h

@@ -577,9 +577,34 @@ int ast_vm_test_destroy_user(const char *context, const char *mailbox);
 int ast_vm_test_create_user(const char *context, const char *mailbox);
 #endif
 
-/*! \brief Safely spawn an external program while closing file descriptors
-	\note This replaces the \b system call in all Asterisk modules
-*/
+/*!
+ * \brief Safely spawn an external program while closing file descriptors
+ *
+ * \note This replaces the \b execvp call in all Asterisk modules
+ *
+ * \param dualfork Non-zero to simulate running the program in the
+ * background by forking twice.  The option provides similar
+ * functionality to the '&' in the OS shell command "cmd &".  The
+ * option allows Asterisk to run a reaper loop to watch the first fork
+ * which immediately exits after spaning the second fork.  The actual
+ * program is run in the second fork.
+ * \param file execvp(file, argv) file parameter
+ * \param argv execvp(file, argv) argv parameter
+ */
+int ast_safe_execvp(int dualfork, const char *file, char *const argv[]);
+
+/*!
+ * \brief Safely spawn an OS shell command while closing file descriptors
+ *
+ * \note This replaces the \b system call in all Asterisk modules
+ *
+ * \param s - OS shell command string to execute.
+ *
+ * \warning Command injection can happen using this call if the passed
+ * in string is created using untrusted data from an external source.
+ * It is best not to use untrusted data.  However, the caller could
+ * filter out dangerous characters to avoid command injection.
+ */
 int ast_safe_system(const char *s);
 
 /*!

main/asterisk.c

@@ -1102,12 +1102,10 @@ void ast_unreplace_sigchld(void)
 	ast_mutex_unlock(&safe_system_lock);
 }
 
-int ast_safe_system(const char *s)
+/*! \brief fork and perform other preparations for spawning applications */
+static pid_t safe_exec_prep(int dualfork)
 {
 	pid_t pid;
-	int res;
-	struct rusage rusage;
-	int status;
 
 #if defined(HAVE_WORKING_FORK) || defined(HAVE_WORKING_VFORK)
 	ast_replace_sigchld();
@@ -1129,35 +1127,102 @@ int ast_safe_system(const char *s)
 		cap_free(cap);
 #endif
 #ifdef HAVE_WORKING_FORK
-		if (ast_opt_high_priority)
+		if (ast_opt_high_priority) {
 			ast_set_priority(0);
+		}
 		/* Close file descriptors and launch system command */
 		ast_close_fds_above_n(STDERR_FILENO);
 #endif
-		execl("/bin/sh", "/bin/sh", "-c", s, (char *) NULL);
-		_exit(1);
-	} else if (pid > 0) {
+		if (dualfork) {
+#ifdef HAVE_WORKING_FORK
+			pid = fork();
+#else
+			pid = vfork();
+#endif
+			if (pid < 0) {
+				/* Second fork failed. */
+				/* No logger available. */
+				_exit(1);
+			}
+
+			if (pid > 0) {
+				/* This is the first fork, exit so the reaper finishes right away. */
+				_exit(0);
+			}
+
+			/* This is the second fork.  The first fork will exit immediately so
+			 * Asterisk doesn't have to wait for completion.
+			 * ast_safe_system("cmd &") would run in the background, but the '&'
+			 * cannot be added with ast_safe_execvp, so we have to double fork.
+			 */
+		}
+	}
+
+	if (pid < 0) {
+		ast_log(LOG_WARNING, "Fork failed: %s\n", strerror(errno));
+	}
+#else
+	ast_log(LOG_WARNING, "Fork failed: %s\n", strerror(ENOTSUP));
+	pid = -1;
+#endif
+
+	return pid;
+}
+
+/*! \brief wait for spawned application to complete and unreplace sigchld */
+static int safe_exec_wait(pid_t pid)
+{
+	int res = -1;
+
+#if defined(HAVE_WORKING_FORK) || defined(HAVE_WORKING_VFORK)
+	if (pid > 0) {
 		for (;;) {
+			struct rusage rusage;
+			int status;
+
 			res = wait4(pid, &status, 0, &rusage);
 			if (res > -1) {
 				res = WIFEXITED(status) ? WEXITSTATUS(status) : -1;
 				break;
-			} else if (errno != EINTR)
+			}
+			if (errno != EINTR) {
 				break;
+			}
 		}
-	} else {
-		ast_log(LOG_WARNING, "Fork failed: %s\n", strerror(errno));
-		res = -1;
 	}
 
 	ast_unreplace_sigchld();
-#else /* !defined(HAVE_WORKING_FORK) && !defined(HAVE_WORKING_VFORK) */
-	res = -1;
 #endif
 
 	return res;
 }
 
+int ast_safe_execvp(int dualfork, const char *file, char *const argv[])
+{
+	pid_t pid = safe_exec_prep(dualfork);
+
+	if (pid == 0) {
+		execvp(file, argv);
+		_exit(1);
+		/* noreturn from _exit */
+	}
+
+	return safe_exec_wait(pid);
+}
+
+int ast_safe_system(const char *s)
+{
+	pid_t pid = safe_exec_prep(0);
+
+	if (pid == 0) {
+		execl("/bin/sh", "/bin/sh", "-c", s, (char *) NULL);
+		_exit(1);
+		/* noreturn from _exit */
+	}
+
+	return safe_exec_wait(pid);
+}
+
 /*!
  * \brief enable or disable a logging level to a specified console
  */

res/res_monitor.c

@@ -57,17 +57,17 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 		<syntax>
 			<parameter name="file_format" argsep=":">
 				<argument name="file_format" required="true">
-					<para>optional, if not set, defaults to <literal>wav</literal></para>
+					<para>Optional.  If not set, defaults to <literal>wav</literal></para>
 				</argument>
 				<argument name="urlbase" />
 			</parameter>
 			<parameter name="fname_base">
-				<para>if set, changes the filename used to the one specified.</para>
+				<para>If set, changes the filename used to the one specified.</para>
 			</parameter>
 			<parameter name="options">
 				<optionlist>
 					<option name="m">
-						<para>when the recording ends mix the two leg files into one and
+						<para>When the recording ends mix the two leg files into one and
 						delete the two leg files. If the variable <variable>MONITOR_EXEC</variable>
 						is set, the application referenced in it will be executed instead of
 						soxmix/sox and the raw leg files will NOT be deleted automatically.
@@ -78,6 +78,13 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 						will be passed on as additional arguments to <variable>MONITOR_EXEC</variable>.
 						Both <variable>MONITOR_EXEC</variable> and the Mix flag can be set from the
 						administrator interface.</para>
+						<warning><para>Do not use untrusted strings such as
+						<variable>CALLERID(num)</variable> or <variable>CALLERID(name)</variable>
+						as part of <variable>MONITOR_EXEC</variable> or
+						<variable>MONITOR_EXEC_ARGS</variable>.  You risk a command injection
+						attack executing arbitrary commands if the untrusted strings aren't
+						filtered to remove dangerous characters.  See function
+						<variable>FILTER()</variable>.</para></warning>
 					</option>
 					<option name="b">
 						<para>Don't begin recording unless a call is bridged to another channel.</para>