CVE-2023-24805 (published 2023-05-17T18:15Z)

cups-filters contains backends, filters, and other software required to get the cups printing service working on operating systems other than macos. If you use the Backend Error Handler (beh) to create an accessible network printer, this security vulnerability can cause remote code execution. `beh.c` contains the line `retval = system(cmdline) >> 8;` which calls the `system` command with the operand `cmdline`. `cmdline` contains multiple user controlled, unsanitized values. As a result an attacker with network access to the hosted print server can exploit this vulnerability to inject system commands which are executed in the context of the running server. This issue has been addressed in commit `8f2740357` and is expected to be bundled in the next release. Users are advised to upgrade when possible and to restrict access to network printers in the meantime.

CVSS v3 base score: 8.8, HIGH (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-78
cwe_name: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
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.
extended_description: 
		   This weakness can lead to a vulnerability in environments in which the attacker does not have direct access to the operating system, such as in web applications. Alternately, if the weakness occurs in a privileged program, it could allow the attacker to specify commands that normally would not be accessible, or to call alternate commands with privileges that the attacker does not have. The problem is exacerbated if the compromised process does not follow the principle of least privilege, because the attacker-controlled commands may run with special system privileges that increases the amount of damage.
            There are at least two subtypes of OS command injection:
               
				 The application intends to execute a single, fixed program that is under its own control. It intends to use externally-supplied inputs as arguments to that program. For example, the program might use system("nslookup [HOSTNAME]") to run nslookup and allow the user to supply a HOSTNAME, which is used as an argument. Attackers cannot prevent nslookup from executing. However, if the program does not remove command separators from the HOSTNAME argument, attackers could place the separators into the arguments, which allows them to execute their own program after nslookup has finished executing.
				 The application accepts an input that it uses to fully select which program to run, as well as which commands to use. The application simply redirects this entire command to the operating system. For example, the program might use "exec([COMMAND])" to execute the [COMMAND] that was supplied by the user. If the COMMAND is under attacker control, then the attacker can execute arbitrary commands or programs. If the command is being executed using functions like exec() and CreateProcess(), the attacker might not be able to combine multiple commands together in the same line.
               
            From a weakness standpoint, these variants represent distinct programmer errors. In the first variant, the programmer clearly intends that input from untrusted parties will be part of the arguments in the command to be executed. In the second variant, the programmer does not intend for the command to be accessible to any untrusted party, but the programmer probably has not accounted for alternate ways in which malicious attackers can provide input.
         
         

Fix information:
repo_url: https://github.com/OpenPrinting/cups-filters
commit_details: repo_url: https://github.com/OpenPrinting/cups-filters
author: Till Kamppeter
author_date: 2023-05-17 11:12:37+02:00
msg: Merge pull request from GHSA-gpxc-v2m8-fr3x

* beh backend: Use execv() instead of system() - CVE-2023-24805

With execv() command line arguments are passed as separate strings and
not the full command line in a single string. This prevents arbitrary
command execution by escaping the quoting of the arguments in a job
with forged job title.

* beh backend: Extra checks against odd/forged input - CVE-2023-24805

- Do not allow '/' in the scheme of the URI (= backend executable
  name), to assure that only backends inside /usr/lib/cups/backend/
  are used.

- Pre-define scheme buffer to empty string, to be defined for case of
  uri being NULL.

- URI must have ':', to split off scheme, otherwise error.

- Check return value of snprintf() to create call path for backend, to
  error out on truncation of a too long scheme or on complete failure
  due to a completely odd scheme.

* beh backend: Further improvements - CVE-2023-24805

- Use strncat() instead of strncpy() for getting scheme from URI, the latter
  does not require setting terminating zero byte in case of truncation.

- Also exclude "." or ".." as scheme, as directories are not valid CUPS
  backends.

- Do not use fprintf() in sigterm_handler(), to not interfere with a
  fprintf() which could be running in the main process when
  sigterm_handler() is triggered.

- Use "static volatile int" for global variable job_canceled.
repository: repo_url: https://github.com/OpenPrinting/cups-filters
repo_name: OpenPrinting/cups-filters
description: filters, backends, driverless utility, ... - Everything which CUPS 2.x needs to be used on non-Mac systems
repo_language: C
owner: OpenPrinting
file_changes: filename: beh.c
change_type: ModificationType.MODIFY
diff: @@ -26,13 +26,14 @@
 #include <signal.h>
 #include <string.h>
 #include <errno.h>
+#include <sys/wait.h>
 
 
 //
 // Local globals...
 //
 
-static int		job_canceled = 0; // Set to 1 on SIGTERM
+static volatile int	job_canceled = 0; // Set to 1 on SIGTERM
 
 
 //
@@ -237,21 +238,44 @@ call_backend(char *uri,                 // I - URI of final destination
 	     char *filename)            // I - File name of input data
 {
   const char	*cups_serverbin;	// Location of programs
+  char          *backend_argv[8];       // Arguments for called CUPS backend
   char		scheme[1024],           // Scheme from URI
                 *ptr,			// Pointer into scheme
-		cmdline[65536];		// Backend command line
-  int           retval;
+		backend_path[2048];	// Backend path
+  int           pid,
+                wait_pid,
+                wait_status,
+                retval = 0;
+  int           bytes;
+
 
   //
   // Build the backend command line...
   //
 
-  strncpy(scheme, uri, sizeof(scheme) - 1);
-  if (strlen(uri) > 1023)
-    scheme[1023] = '\0';
+  scheme[0] = '\0';
+  strncat(scheme, uri, sizeof(scheme) - 1);
   if ((ptr = strchr(scheme, ':')) != NULL)
     *ptr = '\0';
-
+  else
+  {
+    fprintf(stderr,
+	    "ERROR: beh: Invalid URI, no colon (':') to mark end of scheme part.\n");
+    exit (CUPS_BACKEND_FAILED);
+  }
+  if (strchr(scheme, '/'))
+  {
+    fprintf(stderr,
+	    "ERROR: beh: Invalid URI, scheme contains a slash ('/').\n");
+    exit (CUPS_BACKEND_FAILED);
+  }
+  if (!strcmp(scheme, ".") || !strcmp(scheme, ".."))
+  {
+    fprintf(stderr,
+	    "ERROR: beh: Invalid URI, scheme (\"%s\") is a directory.\n",
+	    scheme);
+    exit (CUPS_BACKEND_FAILED);
+  }
   if ((cups_serverbin = getenv("CUPS_SERVERBIN")) == NULL)
     cups_serverbin = CUPS_SERVERBIN;
 
@@ -261,16 +285,25 @@ call_backend(char *uri,                 // I - URI of final destination
 	    "ERROR: beh: Direct output into a file not supported.\n");
     exit (CUPS_BACKEND_FAILED);
   }
-  else
-    snprintf(cmdline, sizeof(cmdline),
-	     "%s/backend/%s '%s' '%s' '%s' '%s' '%s' %s",
-	     cups_serverbin, scheme, argv[1], argv[2], argv[3],
-	     // Apply number of copies only if beh was called with a
-	     // file name and not with the print data in stdin, as
-	     // backends should handle copies only if they are called
-	     // with a file name
-	     (argc == 6 ? "1" : argv[4]),
-	     argv[5], filename);
+
+  backend_argv[0] = uri;
+  backend_argv[1] = argv[1];
+  backend_argv[2] = argv[2];
+  backend_argv[3] = argv[3];
+  backend_argv[4] = (argc == 6 ? "1" : argv[4]);
+  backend_argv[5] = argv[5];
+  backend_argv[6] = filename;
+  backend_argv[7] = NULL;
+
+  bytes = snprintf(backend_path, sizeof(backend_path),
+		   "%s/backend/%s", cups_serverbin, scheme);
+  if (bytes < 0 || bytes >= sizeof(backend_path))
+  {
+    fprintf(stderr,
+	    "ERROR: beh: Invalid scheme (\"%s\"), could not determing backend path.\n",
+	    scheme);
+    exit (CUPS_BACKEND_FAILED);
+  }
 
   //
   // Overwrite the device URI and run the actual backend...
@@ -279,17 +312,41 @@ call_backend(char *uri,                 // I - URI of final destination
   setenv("DEVICE_URI", uri, 1);
 
   fprintf(stderr,
-	  "DEBUG: beh: Executing backend command line \"%s\"...\n",
-	  cmdline);
+	  "DEBUG: beh: Executing backend command line \"%s '%s' '%s' '%s' '%s' '%s'%s%s\"...\n",
+	  backend_path, backend_argv[1], backend_argv[2], backend_argv[3],
+	  backend_argv[4], backend_argv[5],
+	  (backend_argv[6] && backend_argv[6][0] ? " " : ""),
+	  (backend_argv[6] && backend_argv[6][0] ? backend_argv[6] : ""));
   fprintf(stderr,
 	  "DEBUG: beh: Using device URI: %s\n",
 	  uri);
 
-  retval = system(cmdline) >> 8;
+  if ((pid = fork()) == 0)
+  {
+    retval = execv(backend_path, backend_argv);
 
-  if (retval == -1)
-    fprintf(stderr, "ERROR: Unable to execute backend command line: %s\n",
-	    strerror(errno));
+    if (retval == -1)
+      fprintf(stderr, "ERROR: Unable to execute backend: %s\n",
+	      strerror(errno));
+    exit (CUPS_BACKEND_FAILED);
+  }
+  else if (pid < 0)
+  {
+    fprintf(stderr, "ERROR: Unable to fork for backend\n");
+    return (CUPS_BACKEND_FAILED);
+  }
+
+  while ((wait_pid = wait(&wait_status)) < 0 && errno == EINTR);
+
+  if (wait_pid >= 0 && wait_status)
+  {
+    if (WIFEXITED(wait_status))
+      retval = WEXITSTATUS(wait_status);
+    else if (WTERMSIG(wait_status) != SIGTERM)
+      retval = WTERMSIG(wait_status);
+    else
+      retval = 0;
+  }
 
   return (retval);
 }
@@ -304,8 +361,10 @@ sigterm_handler(int sig)		// I - Signal number (unused)
 {
   (void)sig;
 
-  fprintf(stderr,
-	  "DEBUG: beh: Job canceled.\n");
+  const char * const msg = "DEBUG: beh: Job canceled.\n";
+  // The if() is to eliminate the return value and silence the warning
+  // about an unused return value.
+  if (write(2, msg, strlen(msg)));
 
   if (job_canceled)
     _exit(CUPS_BACKEND_OK);

programming_language: C