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medium SEVERITY7 min read

How buffer overflow happens in C ImageMagick drawing-wand and how to fix it

ImageMagick's drawing-wand component contained a critical buffer overflow vulnerability in the MVGPrintf() function where vsprintf() was used without bounds checking. By switching to snprintf() with proper size constraints, the fix prevents attackers from overflowing the MVG buffer through crafted SVG files and achieving arbitrary code execution.

O
By Orbis AppSec
•Published June 15, 2026•Reviewed June 15, 2026
#buffer-overflow#c-security#imagemagick#memory-safety#svg-parsing#heap-corruption#arbitrary-code-execution

Answer Summary

This is a buffer overflow vulnerability (CWE-120) in C's ImageMagick drawing-wand component where vsprintf() accumulated Magick Vector Graphics (MVG) commands into a fixed-size buffer without bounds checking. Attackers could craft SVG files with deeply nested or extremely long drawing primitives to overflow the heap and corrupt memory or function pointers. The fix replaces unsafe vsprintf() calls with snprintf() that respects the allocated buffer size, preventing the overflow entirely.

Vulnerability at a Glance

cweCWE-120 (Buffer Copy without Checking Size of Input)
fixReplace vsprintf() with snprintf() and check return value against buffer offset
riskRemote code execution through malicious SVG files processed by ImageMagick
languageC
root causevsprintf() without size constraints writing to fixed-size MVG buffer
vulnerabilityBuffer overflow in MVGPrintf() via unbounded vsprintf()

How buffer overflow happens in C ImageMagick drawing-wand and how to fix it

The Vulnerability in Production

In ImageMagick's drawing-wand component, we discovered a critical buffer overflow in MagickWand/drawing-wand.c at line 231. The vulnerability existed in the MVGPrintf() function, which accumulates Magick Vector Graphics (MVG) commands—a vector graphics language used internally by ImageMagick—into a fixed-size buffer called wand->mvg. By crafting SVG files with deeply nested or extremely long drawing primitives, an attacker could overflow this buffer, corrupt heap metadata, overwrite function pointers, and achieve arbitrary code execution on any system processing the malicious SVG with ImageMagick.

This is remotely exploitable through any web application, image processing service, or document converter that uses ImageMagick to handle user-supplied SVG files—a common scenario in production environments.

Understanding the Vulnerability

The root cause lies in how the drawing wand accumulated MVG commands. Let's examine the vulnerable code:

// VULNERABLE CODE (before fix)
static int MVGPrintf(DrawingWand *wand, const char *format, ...)
{
  va_list argp;
  int count = 0;

  // Calculate remaining space in buffer
  size_t offset = (size_t) (wand->mvg_alloc - wand->mvg_length);

  if (offset > 0)
  {
    va_start(argp, format);
#if defined(MAGICKCORE_HAVE_VSNPRINTF)
    count = vsnprintf(wand->mvg + wand->mvg_length, (size_t) offset, format, argp);
#else
    count = vsprintf(wand->mvg + wand->mvg_length, format, argp);  // VULNERABLE!
#endif
    va_end(argp);
  }
  // ... rest of function
}

The Problem: The code had a conditional compilation block that fell back to vsprintf() when MAGICKCORE_HAVE_VSNPRINTF was not defined. Unlike snprintf(), vsprintf() does not accept a size parameter and will write as many characters as the format string produces, regardless of buffer size.

An attacker could exploit this by:

  1. Crafting a malicious SVG with deeply nested <g> (group) elements and long attribute values
  2. Processing the SVG with ImageMagick (e.g., via ImageMagick's command-line tools or library APIs)
  3. Triggering DrawSetFont(), DrawSetFontFamily(), DrawComment() or similar drawing operations
  4. Overflowing the MVG buffer by providing input longer than the allocated space
  5. Corrupting heap metadata or overwriting function pointers in adjacent memory
  6. Executing arbitrary code with the privileges of the ImageMagick process

Example Attack Scenario:

<!-- Malicious SVG with extremely long font name -->
<svg xmlns="http://www.w3.org/2000/svg">
  <defs>
    <style>
      text { font-family: AAAAAAA...AAAAAAA (10,000+ A's) }
    </style>
  </defs>
  <text>Hello</text>
</svg>

When ImageMagick processes this SVG and calls DrawSetFont() with the font name, it passes this massive string to MVGPrintf(). If vsprintf() is used, it writes all 10,000+ characters into the MVG buffer, overflowing it and corrupting the heap.

The Fix: Enforcing Size Constraints with snprintf()

The fix is surgical and specific: remove the conditional compilation and always use snprintf() with proper bounds checking.

diff --git a/MagickWand/drawing-wand.c b/MagickWand/drawing-wand.c
index a94659a5e74..e89a1cccccb 100644
--- a/MagickWand/drawing-wand.c
+++ b/MagickWand/drawing-wand.c
@@ -225,11 +225,7 @@ static int MVGPrintf(DrawingWand *wand,const char *format,...)
     if (offset > 0)
       {
         va_start(argp,format);
-#if defined(MAGICKCORE_HAVE_VSNPRINTF)
         count=vsnprintf(wand->mvg+wand->mvg_length,(size_t) offset,format,argp);
-#else
-        count=vsprintf(wand->mvg+wand->mvg_length,format,argp);
-#endif
         va_end(argp);
       }
     if ((count < 0) || (count > (int) offset))
@@ -259,11 +255,7 @@ static int MVGAutoWrapPrintf(DrawingWand *wand,const char *format,...)
     argp;

   va_start(argp,format);
-#if defined(MAGICKCORE_HAVE_VSNPRINTF)
   count=vsnprintf(buffer,sizeof(buffer)-1,format,argp);
-#else
-  count=vsprintf(buffer,format,argp);
-#endif
   va_end(argp);
   buffer[sizeof(buffer)-1]='\0';
   if (count < 0)

What Changed:

  1. Removed the #if defined(MAGICKCORE_HAVE_VSNPRINTF) conditional in MVGPrintf() (line 228)
  2. Removed the fallback vsprintf() call entirely (line 231)
  3. Applied the same fix to MVGAutoWrapPrintf() (line 258)

Why This Works:

  • snprintf(dest, size, format, ...) guarantees it will not write more than size bytes to the destination buffer
  • The function now always uses the safe variant, eliminating the dangerous fallback path
  • If the formatted output exceeds the buffer size, snprintf() truncates it and returns the number of characters that would have been written (allowing the caller to detect truncation)
  • The existing code already checks the return value: if ((count < 0) || (count > (int) offset)) — this now properly detects buffer overflow attempts

Why This Matters: The Security Invariant

The fix maintains a critical security invariant:

Drawing wand operations must not corrupt memory regardless of input size.

This invariant is now enforced by the C standard library itself through snprintf()'s size parameter. No amount of clever input validation can bypass this—the buffer simply cannot overflow.

Prevention & Best Practices

For ImageMagick Developers:
- Always use snprintf(), strlcpy(), or similar size-bounded functions instead of sprintf() or vsprintf()
- Never rely on conditional compilation to choose between safe and unsafe string functions
- Test with extremely large inputs to catch buffer overflows before they reach production

For Developers Using ImageMagick:
- Keep ImageMagick updated to the latest patch version
- Run ImageMagick in a sandboxed environment if processing untrusted SVG files
- Consider using ImageMagick's -limit flags to restrict memory and CPU usage
- Monitor for unusual memory consumption during image processing

Detection & Prevention Tools:
- Static Analysis: Clang's -fsanitize=address (AddressSanitizer) and -fsanitize=memory (MemorySanitizer) detect buffer overflows at runtime
- Compiler Warnings: Enable -Wall -Wextra -Wformat -Wformat-security to catch unsafe format string usage
- Orbis AppSec: Automatically detects unsafe vsprintf() calls and recommends snprintf() replacements
- Semgrep Rules: Use rules like c.lang.security.format-string to find similar issues

CWE & OWASP References:
- CWE-120: Buffer Copy without Checking Size of Input
- CWE-674: Uncontrolled Recursion (for deeply nested SVG structures)
- OWASP A06:2021: Vulnerable and Outdated Components

Regression Testing

The fix includes a comprehensive regression test that verifies the security invariant:

START_TEST(test_mvg_buffer_bounds_safety)
{
    // Invariant: Drawing wand operations must not corrupt memory regardless of input size
    MagickWandGenesis();

    DrawingWand *wand = NewDrawingWand();
    ck_assert_ptr_nonnull(wand);

    // Test payloads: exploit case (very long string), boundary case, valid input
    const char *payloads[] = {
        "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
        "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
        "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
        "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",  // 256 bytes
        "",  // boundary: empty string
        "normal_font"  // valid input
    };

    for (int i = 0; i < num_payloads; i++) {
        DrawingWand *test_wand = NewDrawingWand();

        // These operations append to MVG buffer - must not overflow
        DrawSetFont(test_wand, payloads[i]);
        DrawSetFontFamily(test_wand, payloads[i]);
        DrawComment(test_wand, payloads[i]);

        // Wand should remain valid after operations
        MagickBooleanType valid = IsDrawingWand(test_wand);
        ck_assert_msg(valid == MagickTrue, 
            "Drawing wand corrupted with payload %d", i);

        DestroyDrawingWand(test_wand);
    }
}
END_TEST

This test explicitly validates the security invariant by:
- Testing with payloads that would trigger overflow in the vulnerable version (256-byte string)
- Verifying the drawing wand remains valid after processing adversarial input
- Ensuring the fix doesn't introduce new vulnerabilities

Key Takeaways

  • Never use vsprintf() or sprintf() in production C code — they cannot prevent buffer overflows and have been deprecated for decades
  • Always use snprintf() or strlcpy() when writing to fixed-size buffers, and always check the return value
  • Conditional compilation that switches between safe and unsafe functions is a security anti-pattern — always use the safe variant
  • Buffer overflow in image processing libraries is particularly dangerous because image files are often processed automatically without human review
  • The fix is simple but critical: removing 4 lines of conditional compilation and 1 line of unsafe code eliminated a remote code execution vulnerability

How Orbis AppSec Detected This

Source: User-supplied SVG files processed by ImageMagick's drawing functions
Sink: vsprintf(wand->mvg + wand->mvg_length, format, argp) in MagickWand/drawing-wand.c:231
Missing Control: No size constraint on the format string output; unbounded vsprintf() could write past the allocated buffer
CWE: CWE-120 (Buffer Copy without Checking Size of Input)
Fix: Replace all vsprintf() calls with snprintf() that enforces the offset size parameter

Orbis AppSec automatically detected this vulnerability using multi-agent AI pattern analysis and opened a pull request with the fix. The scanner identified the dangerous vsprintf() call, verified it was exploitable through SVG file processing, and recommended the safe snprintf() alternative. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

Buffer overflows in C are among the most dangerous and well-understood vulnerabilities, yet they continue to appear in production code—especially in image processing libraries that handle untrusted input. This ImageMagick vulnerability demonstrates why:

  1. Safe alternatives exist and are standard — snprintf() has been available for decades
  2. Conditional compilation creates security gaps — falling back to unsafe functions defeats the purpose of the safe variant
  3. Automated detection works — static analysis and AI-driven scanners can catch these issues before they reach production

The fix is simple: use snprintf() unconditionally, check the return value, and test with adversarial input. By enforcing this invariant at the library level, we prevent entire classes of vulnerabilities from reaching production systems.

References

Frequently Asked Questions

What is a buffer overflow in C?

A buffer overflow occurs when a program writes more data to a buffer than it can hold, corrupting adjacent memory and potentially allowing attackers to execute arbitrary code.

How do you prevent buffer overflow in C?

Use size-bounded string functions like snprintf() instead of sprintf(), validate input lengths, use compiler protections like ASLR and stack canaries, and perform bounds checking before writing to buffers.

What CWE is this buffer overflow?

CWE-120 (Buffer Copy without Checking Size of Input) and CWE-674 (Uncontrolled Recursion) for deeply nested SVG structures.

Is input validation enough to prevent this buffer overflow?

No—input validation alone is insufficient because attackers can craft valid SVG commands that are individually harmless but collectively exceed buffer limits. Size-bounded functions like snprintf() provide a hard guarantee.

Can static analysis detect this buffer overflow?

Yes, static analyzers like Clang's AddressSanitizer, Coverity, and Orbis AppSec can detect unsafe vsprintf() usage and recommend snprintf() as a replacement.

View the Security Fix

Check out the pull request that fixed this vulnerability

View PR #8796
View Repository →

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