CVE-2023-1350: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection') (CWE-78)
value: A vulnerability was found in liferea. It has been rated as critical. Affected by this issue is the function update_job_run of the file src/update.c of the component Feed Enrichment. The manipulation of the argument source with the input |date &gt;/tmp/bad-item-link.txt leads to os command injection. The attack may be launched remotely. The exploit has been disclosed to the public and may be used. The name of the patch is 8d8b5b963fa64c7a2122d1bbfbb0bed46e813e59. It is recommended to apply a patch to fix this issue. The identifier of this vulnerability is VDB-222848.
CWE description: The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.
CVSS v3 base score: 9.8
Published: 2023-03-11T09:15Z
Repository: https://github.com/lwindolf/liferea
Language: C
Files changed: src/subscription.c, src/update.c, src/update.h
Commit date: 2023-03-09 23:12:30 +0100
Commit message: Fix RCE vulnerability on feed enrichment

Currently there are a few places in the code that do not check URLs for
the presence of a command prefix, allowing malicious websites to run any
command in the local system.

Trying to run feed enrichment (i.e. after option "Extract full content
from HTML5 and Google AMP" is enabled) in a subscription that produces
this item:

    <item>
      <title>Check /tmp/bad-item-link.txt</title>
      <link>|date &gt;/tmp/bad-item-link.txt</link>
    </item>

will cause Liferea to call update_request_new on URL
"|date >/tmp/bad-item-link.txt" and then blindly run the command. A
similar effect happens when running the feed enrichment in a item that
links to a document like this:

    <!DOCTYPE html>
    <html>
     <head>
      <title>AMP URL RCE PoC</title>
      <link rel="amphtml" href="|date &gt; /tmp/bad-amp-url.txt">
     </head>
     <body>
         Check the results in /tmp/bad-amp-url.txt
     </body>
    </html>

once Liferea tries to load the AMP URL.

There are other calls to update_request_* functions where URLs are
validated correctly (e.g. in feed icons and comments) and do not trigger
this vulnerability. Since the subscription update is the exception where
feed commands are supported (and welcome!) I chose to lock this feature
behind a non-persistent flag and only enable it when required. This is
safer than adding the flag in updateOptions, which is usually reused in
subordinate requests.

This failure exists since commits b8288389820a3f510ef4b21684b22439c41d95a5
and b67dbba73443ab7b36fcd3c78aa803e974c0f23e from Setember 2017.

Diff: diff --git a/src/subscription.c b/src/subscription.c
index af001256..7789b031 100644
--- a/src/subscription.c
+++ b/src/subscription.c
@@ -280,10 +280,11 @@ subscription_update (subscriptionPtr subscription, guint flags)
 		request = update_request_new (
 			subscription_get_source (subscription),
 			subscription->updateState,
 			subscription->updateOptions
 		);
+		update_request_allow_commands (request, TRUE);
 
 		if (subscription_get_filter (subscription))
 			request->filtercmd = g_strdup (subscription_get_filter (subscription));
 
 		if (SUBSCRIPTION_TYPE (subscription)->prepare_update_request (subscription, request))
diff --git a/src/update.c b/src/update.c
index b9ab8dd2..369a2bf1 100644
--- a/src/update.c
+++ b/src/update.c
@@ -232,10 +232,17 @@ update_request_set_auth_value (UpdateRequest *request, const gchar* authValue)
 {
 	g_free (request->authValue);
 	request->authValue = g_strdup (authValue);
 }
 
+void
+update_request_allow_commands (UpdateRequest *request, gboolean allowCommands)
+{
+	request->allowCommands = allowCommands;
+}
+
+
 /* update result object */
 
 updateResultPtr
 update_result_new (void)
 {
@@ -670,12 +677,18 @@ update_job_run (updateJobPtr job)
 	   pass the processed job to update_process_finished_job()
 	   for result dequeuing */
 
 	/* everything starting with '|' is a local command */
 	if (*(job->request->source) == '|') {
-		debug1 (DEBUG_UPDATE, "Recognized local command: %s", job->request->source);
-		update_exec_cmd (job);
+		if (job->request->allowCommands) {
+			debug1 (DEBUG_UPDATE, "Recognized local command: %s", job->request->source);
+			update_exec_cmd (job);
+		} else {
+			debug1 (DEBUG_UPDATE, "Refusing to run local command from unexpected source: %s", job->request->source);
+			job->result->httpstatus = 403;  /* Forbidden. */
+			update_process_finished_job (job);
+		}
 		return;
 	}
 
 	/* if it has a protocol "://" prefix, but not "file://" it is an URI */
 	if (strstr (job->request->source, "://") && strncmp (job->request->source, "file://", 7)) {
diff --git a/src/update.h b/src/update.h
index d6244ad8..af29f8f9 100644
--- a/src/update.h
+++ b/src/update.h
@@ -101,10 +101,11 @@ struct _UpdateRequest {
 	gchar           *postdata;      /**< HTTP POST request data (NULL for non-POST requests) */
 	gchar           *authValue;     /**< Custom value for Authorization: header */
 	updateOptionsPtr options;	/**< Update options for the request */
 	gchar		*filtercmd;	/**< Command will filter output of URL */
 	updateStatePtr	updateState;	/**< Update state of the requested object (etags, last modified...) */
+	gboolean	allowCommands;	/**< Allow this requests to run commands */
 };
 
 /** structure to store results of the processing of an update request */
 typedef struct updateResult {
 	gchar 		*source;	/**< Location of the downloaded document, in case of redirects different from
@@ -226,10 +227,25 @@ void update_request_set_source (UpdateRequest *request, const gchar* source);
  * @param request        the update request
  * @param authValue      the authorization header value
  */
 void update_request_set_auth_value (UpdateRequest *request, const gchar* authValue);
 
+/**
+ * Allows *this* request to run local commands.
+ *
+ * At first it may look this flag should be in updateOptions, but we can
+ * take a safer path: feed commands are restricted to a few use cases while
+ * options are propagated to downstream requests (feed enrichment, comments,
+ * etc.), so it is a good idea to prevent these from running commands in the
+ * local system via tricky URLs without needing to validate these options
+ * everywhere (which is error-prone).
+ *
+ * @param request      the update request
+ * @param can_run      TRUE if the request can run commands, FALSE otherwise.
+ */
+void update_request_allow_commands (UpdateRequest *request, gboolean allowCommands);
+
 /**
  * Creates a new update result for the given update request.
  *
  * @returns update result (to be free'd using update_result_free())
  */

Vulnerable code: 		debug1 (DEBUG_UPDATE, "Recognized local command: %s", job->request->source);
		update_exec_cmd (job);
Fixed code: 		update_request_allow_commands (request, TRUE);
void
update_request_allow_commands (UpdateRequest *request, gboolean allowCommands)
{
	request->allowCommands = allowCommands;
}


		if (job->request->allowCommands) {
			debug1 (DEBUG_UPDATE, "Recognized local command: %s", job->request->source);
			update_exec_cmd (job);
		} else {
			debug1 (DEBUG_UPDATE, "Refusing to run local command from unexpected source: %s", job->request->source);
			job->result->httpstatus = 403;  /* Forbidden. */
			update_process_finished_job (job);
		}
	gboolean	allowCommands;	/**< Allow this requests to run commands */
/**
 * Allows *this* request to run local commands.
 *
 * At first it may look this flag should be in updateOptions, but we can
 * take a safer path: feed commands are restricted to a few use cases while
 * options are propagated to downstream requests (feed enrichment, comments,
 * etc.), so it is a good idea to prevent these from running commands in the
 * local system via tricky URLs without needing to validate these options
 * everywhere (which is error-prone).
 *
 * @param request      the update request
 * @param can_run      TRUE if the request can run commands, FALSE otherwise.
 */
void update_request_allow_commands (UpdateRequest *request, gboolean allowCommands);