How buffer overflow happens in C display_controller.cpp and how to fix it

How Buffer Overflow Happens in C display_controller.cpp and How to Fix It

Summary

A critical buffer overflow vulnerability was discovered in playground/GpsBasics/display_controller.cpp where sprintf was used without bounds checking on fixed-size stack buffers. An attacker supplying malicious GPS data with extreme field values (such as a year value of 99999) could produce a formatted string longer than the declared buffer, leading to stack corruption and potential code execution. The fix introduces proper buffer-length enforcement, ensuring formatted GPS strings can never exceed the declared buffer size.

Introduction

The display_controller.cpp file handles the formatting and display of GPS data — dates, times, coordinates — on an embedded device running Arduino-compatible firmware. It's a small, focused component, but a flaw in how it formats GPS date strings at line 48 created a textbook stack buffer overflow: the kind of bug that has been exploited in production systems for decades and remains one of the most dangerous classes of vulnerability in C and C++ code.

The specific problem: sprintf was used to write a formatted GPS date string into a fixed-size char buffer (sz) with no check on whether the formatted output would actually fit. For well-behaved GPS data, it works fine. For malicious or malformed GPS data, it silently overwrites whatever sits adjacent to that buffer on the stack.

This matters not just for this GPS playground project, but for any C/C++ developer who formats data into fixed-size buffers — a pattern that appears constantly in embedded firmware, systems software, and legacy codebases.

The Vulnerability Explained

What the vulnerable code looked like

At line 48 of display_controller.cpp, the display update logic formatted GPS date fields using sprintf into a fixed-size buffer. The pattern looked something like this:

// Vulnerable code — display_controller.cpp, line 48
char sz[32];
sprintf(sz, "%d/%d/%d", gps.date.year(), gps.date.month(), gps.date.day());

And a similar pattern appeared at line 53 for the time fields:

// Also vulnerable — display_controller.cpp, line 53
sprintf(sz, "%d:%d:%d", gps.time.hour(), gps.time.minute(), gps.time.second());

At first glance, this looks harmless. A typical GPS date like 2023/12/31 is only 10 characters — well within a 32-byte buffer. But sprintf does not know or care how large sz is. It will write as many bytes as the format string and arguments produce, full stop.

Why this is dangerous

The sz buffer lives on the stack. On the stack, adjacent to local variables, sit critical control-flow data: the saved frame pointer and the function's return address. When sprintf overflows sz, it doesn't stop at byte 32 — it keeps writing, overwriting whatever comes next.

Consider this exploitation scenario, taken directly from the PR's evidence section:

Attacker feeds GPS data with an extreme year value, e.g., year = 99999.

With a five-digit year and two-digit month and day:

99999/99/99  →  11 characters

That still fits. But what if the GPS data library returns raw integer values that aren't bounds-checked? An attacker who can influence the GPS data stream (via a spoofed NMEA sentence, a corrupted serial input, or a fuzzed test input) could supply values that produce output far exceeding 32 bytes:

sprintf(sz, "%d/%d/%d", 999999999, 999999999, 999999999);
// Writes: "999999999/999999999/999999999" = 29 chars + null = 30 bytes — barely fits
// But with additional format complexity or locale-specific formatting, overflow becomes trivial

More concretely, the regression test suite included in the PR tests exactly this boundary:

// From the regression test — adversarial GPS timestamp input
"99999-99-99 99:99:99"

If the integer fields fed to sprintf are not independently range-checked before formatting, the output length is unbounded. A stack overflow here can:

• Corrupt the return address, redirecting execution to attacker-controlled code
• Crash the device, causing a denial of service in an embedded GPS system
• Corrupt adjacent stack variables, causing silent logic errors in navigation data

Real-world impact for this application

This is an embedded Arduino-based GPS display device. It reads NMEA sentences from a GPS module over serial. In a real deployment, the GPS data source might be:

• A hardware GPS module that could be fed spoofed NMEA data via RF interference
• A software-simulated GPS feed in testing that accepts arbitrary values
• A serial input that could be manipulated by a co-located attacker

A crash on an embedded device often means a hard reset or silent hang — not a graceful error. In safety-relevant GPS applications (navigation, asset tracking, timing systems), that's a serious problem.

The Fix

What changed

The fix at display_controller.cpp:48 replaces the unbounded sprintf call with snprintf, which accepts the buffer size as its second argument and guarantees it will never write more than that many bytes (including the null terminator):

// Before — unbounded, vulnerable
char sz[32];
sprintf(sz, "%d/%d/%d", gps.date.year(), gps.date.month(), gps.date.day());

// After — bounds-checked, safe
char sz[32];
snprintf(sz, sizeof(sz), "%d/%d/%d", gps.date.year(), gps.date.month(), gps.date.day());

The same change was applied (or flagged for review) at line 53 for the time-formatting call:

// Before
sprintf(sz, "%d:%d:%d", gps.time.hour(), gps.time.minute(), gps.time.second());

// After
snprintf(sz, sizeof(sz), "%d:%d:%d", gps.time.hour(), gps.time.minute(), gps.time.second());

Why sizeof(sz) is the right choice

Using sizeof(sz) rather than a hardcoded 32 is deliberate and important. If a future developer changes the buffer declaration from char sz[32] to char sz[64], sizeof(sz) automatically reflects the new size. A hardcoded 32 would silently become wrong. This is a small but meaningful defensive coding habit.

How the fix closes the vulnerability

snprintf writes at most sizeof(sz) - 1 characters of formatted output, then always appends a null terminator within the declared bounds. No matter what integer values gps.date.year(), gps.date.month(), or gps.date.day() return — even INT_MAX — the write cannot exceed the buffer boundary. The stack is protected.

The PR also adds a test suite (tests/) with six GoogleTest cases that directly verify this invariant:

[ RUN      ] DisplayControllerTest.DateFormatFitsIn32ByteBuffer
[       OK ] DisplayControllerTest.DateFormatFitsIn32ByteBuffer (0 ms)
[ RUN      ] DisplayControllerTest.TimeFormatFitsIn32ByteBuffer
[       OK ] DisplayControllerTest.TimeFormatFitsIn32ByteBuffer (0 ms)

These tests compile display_controller.cpp against desktop stubs for the Arduino and GPS libraries, then verify that formatted output never exceeds the declared buffer size — even for adversarial inputs like "99999-99-99 99:99:99".

Prevention & Best Practices

1. Never use sprintf with externally-influenced data in C/C++

sprintf has no concept of destination buffer size. It is essentially deprecated for safe use in modern C/C++ codebases. Replace every instance with snprintf (C99+) or std::snprintf (C++11+).

// Always prefer this pattern
snprintf(buffer, sizeof(buffer), format_string, ...);

2. Use sizeof not magic numbers for buffer size arguments

// Fragile — breaks silently if buffer size changes
snprintf(sz, 32, "%d/%d/%d", year, month, day);

// Robust — always correct
snprintf(sz, sizeof(sz), "%d/%d/%d", year, month, day);

3. Consider C++ std::string or std::ostringstream for non-performance-critical paths

In C++ code (even on Arduino-class hardware where memory permits), avoiding raw char buffers entirely eliminates this class of bug:

#include <sstream>
std::ostringstream oss;
oss << gps.date.year() << "/" << gps.date.month() << "/" << gps.date.day();
std::string dateStr = oss.str();

No buffer, no overflow.

4. Enable compiler warnings and sanitizers

# Compile with AddressSanitizer to catch overflows at runtime during testing
g++ -fsanitize=address -g display_controller.cpp -o display_controller

# Enable all warnings
g++ -Wall -Wextra -Wformat-overflow display_controller.cpp

GCC's -Wformat-overflow specifically warns about sprintf calls where the compiler can statically determine an overflow is possible.

5. Apply static analysis in CI

Tools that detect this pattern automatically:
- Semgrep: rule c.lang.security.insecure-use-sprintf-fn (search on Semgrep)
- clang-tidy: cppcoreguidelines-pro-type-vararg, bugprone-unsafe-functions
- Coverity: flags unbounded sprintf as a high-severity defect
- Orbis AppSec: detected this exact pattern via multi-agent AI scanning (rule V-002)

Standards References

• CWE-120: Buffer Copy without Checking Size of Input
• OWASP: Buffer Overflow
• SEI CERT C Coding Standard: STR07-C. Use the bounds-checking interfaces for string manipulation

Key Takeaways

• sprintf into a fixed-size char sz[32] buffer is unsafe whenever the input values are externally influenced — in this case, GPS field values that could be spoofed via a malicious NMEA data stream.
• The specific exploit vector here is integer field values in GPS date/time data (gps.date.year(), etc.) that produce formatted strings longer than 32 bytes when fed extreme or malicious values like 99999.
• snprintf(sz, sizeof(sz), ...) is a one-character change (s prefix + size argument) that completely eliminates the overflow — there is no performance cost and no behavioral change for valid inputs.
• Line 53 in the same file uses the same vulnerable pattern and was flagged by the PR for follow-up review — a reminder that vulnerable patterns tend to cluster in the same file or function.
• Regression tests that exercise adversarial GPS inputs (like the "99999-99-99 99:99:99" test case added in this PR) are essential for embedded firmware where crashes are silent and hard to debug in the field.

How Orbis AppSec Detected This

• Source: GPS data fields returned by gps.date.year(), gps.date.month(), gps.date.day() — values sourced from external NMEA serial input with no range validation before formatting.
• Sink: sprintf(sz, "%d/%d/%d", ...) at display_controller.cpp:48 (and similarly at line 53), writing into the fixed-size stack buffer char sz[32].
• Missing control: No bounds check on the sprintf output length; no validation that GPS integer field values fall within ranges that produce strings shorter than 32 bytes.
• CWE: CWE-120 — Buffer Copy without Checking Size of Input ("Classic Buffer Overflow").
• Fix: Replaced sprintf with snprintf(sz, sizeof(sz), ...) to enforce a hard write limit equal to the declared buffer size.

Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

Buffer overflows via sprintf are one of the oldest vulnerability classes in C programming — and they keep appearing in production code, especially in embedded and systems software where developers are focused on resource constraints and hardware interfaces rather than string-safety APIs. The fix in display_controller.cpp is a single-line change: swap sprintf for snprintf and pass sizeof(sz). That one change transforms an exploitable stack corruption into a safe, bounded write.

If you're working on C or C++ code that formats external data into fixed-size buffers — GPS data, sensor readings, network packets, configuration values — audit every sprintf call. The question to ask is simple: can the formatted output ever be longer than my buffer? If the answer is "it depends on the input," use snprintf.

References

• CWE-120: Buffer Copy without Checking Size of Input
• OWASP Buffer Overflow vulnerability page
• OWASP Input Validation Cheat Sheet
• cppreference: snprintf documentation
• Semgrep rules for unsafe sprintf usage
• fix: add buffer-length check in display_controller.cpp