What is secure-dev-c?

A set of rules for safer C source and header files, focusing on memory use, input lengths, and system behaviour. C is a low-level programming language where mistakes can corrupt memory or cause crashes.

Use it when writing or reviewing C code that copies strings, allocates memory, handles external input, or works with pointers and format strings.

It helps prevent buffer overflows, memory errors, code injection, and other vulnerabilities caused by unsafe functions or unchecked allocations.

secure-dev-c — Cursor rule by matank001

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add rules/matank001/cursor-security-rules/secure-dev-c

Clone the repo

git clone --depth 1 https://github.com/matank001/cursor-security-rules

Per session 0 Nothing until a file matches its globs; then the whole rule loads.

When invoked 497 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5	$0.00000	$0.00497
Opus 5	$0.00000	$0.00249
Sonnet 5	$0.00000	$0.00099
Haiku 4.5	$0.00000	$0.00050

Measured yesterday against content hash 96c7a93ca810, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

secure-dev-c scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

Copies of this mod

1 near-identical copy found in the catalogue:

security-lang-c — 100% identical, 0 lines differ

secure-dev-c.mdc · 43 lines

How it starts

The opening of the file, as written. The whole thing — 43 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Secure C Development

These rules apply to all C source and header files in the repository and aim to prevent memory corruption, code injection, and unsafe system behavior.

All violations must include a clear explanation of which rule was triggered and why, to help developers understand and fix the issue effectively.
Generated code must not violate these rules. If a rule is violated, a comment must be added explaining the issue and suggesting a correction.

1. Avoid Unsafe Functions

Rule: Do not use functions like gets, strcpy, sprintf, or scanf with %s. Use bounded or safer alternatives like fgets, strncpy, or snprintf.

2. Always Validate Input Lengths

Rule: Validate the length of user or external input before copying, storing, or processing it to prevent buffer overflows.

3. Initialize All Pointers and Memory

Rule: All pointers and allocated memory must be explicitly initialized before use.

4. Check All Memory Allocations

Rule: Check the result of malloc, calloc, or realloc for NULL before using the returned pointer.

5. Avoid Format String Vulnerabilities

Rule: Never pass user-controlled strings directly as the format argument to functions like printf, fprintf, or syslog.

6. Free All Allocated Memory

Rule: All dynamic memory must be properly freed to avoid memory leaks. Avoid double-free and use-after-free errors.

7. Do Not Trust Environment Variables

Rule: Environment variables must not be used directly in sensitive operations like file access, exec calls, or security checks without validation.

8. Avoid System and Shell Calls with Input

Rule: Do not use system(), popen(), or similar functions with user-controlled input. Use direct APIs when possible.

9. Limit Pointer Arithmetic and Casting

Rule: Avoid unnecessary pointer arithmetic and type casting that can bypass bounds or type safety.

10. Use Static Analysis and Compiler Warnings

Rule: Enable strict compiler warnings (-Wall -Wextra -Werror) and use static analysis tools (e.g., clang-tidy, cppcheck) to detect risky patterns.

Read the full file on GitHub · 43 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

yesterday First seen · 43 lines · 497 tokens per session scan A 96c7a93ca810

Subscribe to this mod's changes

secure-dev-c is a cursor rule published in the GitHub repository matank001/cursor-security-rules (379 stars, last pushed 1y ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 497 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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secure-dev-c