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GitHub Security Lab CTF 1: SEGV hunt

Language: C - Difficulty level:

We created this CTF challenge to help you quickly learn CodeQL. The objective is to find a critical buffer overflow bug in glibc using CodeQL, our simple, code query language. To capture the flag, you'll need to refine your query to increase its precision using this step by step guide.

Challenge Instructions

The goal of this challenge is to find unsafe uses of alloca in the GNU C Library (glibc). alloca is used to allocate a buffer on the stack. It is usually implemented by simply subtracting the size parameter from the stack pointer and returning the new value of the stack pointer. This means that it has two important benefits:

  1. The memory allocated by alloca is automatically freed when the current function returns.
  2. It is extremely fast.

But alloca can also be unsafe because it does not check whether there is enough stack space left for the buffer. If the requested buffer size is too big, then alloca might return an invalid pointer. This can cause the application to crash with a SIGSEGV when it attempts to read or write the buffer. Therefore alloca is only intended to be used to allocate small buffers. It is the programmer's responsibility to check that the size isn't too big.

The GNU C Library contains hundreds of calls to alloca. In this challenge, you will use CodeQL to find those calls. Of course many of those calls are safe, so the main goal of the challenge is to refine your query to reduce the number of false positives. If you follow the challenge all the way to the end then you might even find a bug in glibc that is reproducible from a standard command-line application.

Setup instructions

Instructions for installing CodeQL are included at the end of this document.

Documentation links

If you get stuck, try searching our documentation and blog posts for help and ideas. Below are a few links to help you get started:


The challenge is split into several steps, each of which contains multiple questions, however building one query per step is sufficient.

Step 0: finding the definition of alloca

Step 1: finding the calls to alloca and filtering out small allocation sizes

Step 2: filtering out calls that are guarded by __libc_use_alloca

The correct way to use alloca in glibc is to first check that the allocation is safe by calling __libc_use_alloca. You can see a good example of this at getopt.c:252. That code uses __libc_use_alloca to check if it is safe to use alloca. If not, it uses malloc instead. In this step, you will identify calls to alloca that are safe because they are guarded by a call to __libc_use_alloca.

Step 3: combine steps 1 and 2 to filter out safe calls

Step 4: taint tracking

In this step, you'll use a taint tracking query to find an unsafe call to alloca where the allocation size is controlled by a value read from a file.

  * @name 41_fopen_to_alloca_taint
  * @description Track taint from fopen to alloca.
  * @kind path-problem
  * @problem.severity warning

import cpp
import semmle.code.cpp.rangeanalysis.SimpleRangeAnalysis
import semmle.code.cpp.dataflow.TaintTracking
import semmle.code.cpp.models.interfaces.DataFlow
import semmle.code.cpp.controlflow.Guards
import DataFlow::PathGraph

// Track taint through `__strnlen`.
class StrlenFunction extends DataFlowFunction {
  StrlenFunction() { this.getName().matches("%str%len%") }

  override predicate hasDataFlow(FunctionInput i, FunctionOutput o) {
    i.isInParameter(0) and o.isOutReturnValue()

// Track taint through `__getdelim`.
class GetDelimFunction extends DataFlowFunction {
  GetDelimFunction() { this.getName().matches("%get%delim%") }

  override predicate hasDataFlow(FunctionInput i, FunctionOutput o) {
    i.isInParameter(3) and o.isOutParameterPointer(0)

class Config extends TaintTracking::Configuration {
  Config() { this = "fopen_to_alloca_taint" }

  override predicate isSource(DataFlow::Node source) {
    // TODO

  override predicate isSink(DataFlow::Node sink) {
    // TODO

from Config cfg, DataFlow::PathNode source, DataFlow::PathNode sink
where cfg.hasFlowPath(source, sink)
select sink, source, sink, "fopen flows to alloca"

Step 5: searching for a PoC (optional)

Getting Help

If you find yourself stuck writing QL or on any part of the CTF and would like some help, drop us an email at

Setup instructions for running CodeQL offline

We hope you enjoyed this challenge! If you are interested in continuing to use CodeQL for security research, then we recommend installing CodeQL on your own computer. This will enable you to run queries offline. We have also provided these offline instructions for posterity, because the query results will change over time as the source code evolves. But the instructions below use a snapshot corresponding to revision 3332218, which is the revision for which we designed this challenge.

To run CodeQL queries offline, follow these steps:

  1. Install the Visual Studio Code IDE.
  2. Download and install the Visual Studio Code extension.
  3. Download a pre-existing vulnerable GNU CodeQL database or create one using the CodeQL CLI, which corresponds to revision 3332218 and import it into Visual Studio Code.

You can download other snapshots for offline use from LGTM. For example, you can download a snapshot for the latest revision of glibc here. Every project on LGTM has a download link for downloading the latest snapshot.