In picoCTF’s Buffer Overflow 0, the vulnerable function copies your input into a 16-byte stack buffer with strcpy, without limiting the copy to the buffer’s size. A sufficiently long string can corrupt adjacent stack memory; when the program faults with SIGSEGV, its registered handler prints the flag. The exact input length is not universal, so treat walkthrough payloads as examples and verify behavior against the binary you are using.
What Buffer Overflow 0 is teaching
Buffer Overflow 0 is an introductory binary exploitation exercise about an unchecked write to a stack buffer. The challenge description reproduced in the walkthrough says, “Smash the stack” and asks whether you can “overflow the correct buffer.” The point is to see how input that exceeds a buffer’s capacity can affect nearby memory and cause a program fault—not to reliably overwrite a particular named variable.
That fits picoCTF’s broader educational outcomes, which include exploiting stack buffer overflows and understanding stack layout in 32-bit programs: picoCTF 2018 Educational Outcomes.
Why an overflow prints the flag
The challenge’s main function reads the flag from flag.txt, installs a handler for SIGSEGV, reads the user’s input, and passes it to vuln. In the cited source, vuln declares char buf2[16]; and copies the input with strcpy(buf2, input);. Because strcpy has no destination-size argument, a sufficiently long input can run past the 16-byte array and overwrite adjacent stack memory.
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If execution then encounters an invalid memory access, SIGSEGV is raised. The challenge’s signal handler responds by printing the flag. In other words, the visible result comes from the handler’s response to the fault; the source-backed explanation is not that the input must overwrite a specific named variable. The implementation and walkthrough are documented in Cajac’s Buffer Overflow 0 writeup.
How to approach the input length
The 16-byte array size is established by the source, but it does not by itself establish the number of characters needed to produce the flag. The relevant distance depends on the compiled target and its environment. A walkthrough reports that 20 repeated A characters worked in its local run; in its remote transcript, 20 and 25 did not print the flag, while 30 did. Those are observations from that walkthrough, not guaranteed offsets for every instance.
- Start with the target you were given. A local copy and a remote service may not behave identically. Do not assume that an input length copied from another writeup applies to your binary.
- Try a simple repeated-character string. The cited walkthrough uses repeated
Acharacters to demonstrate the overflow. The intended observation is whether the oversized input produces the fault that invokes the flag-printing handler. - Adjust and verify against the actual target. If a length does not produce the flag, test another length rather than treating the buffer’s 16-byte size as the complete offset. Record what happens for the specific target you are solving.
A second writeup also describes the exercise in terms of triggering SIGSEGV and gives an x86 stack-layout estimate. Treat that estimate as specific to its writeup, not as a universal rule for every build or runtime: Charles T. Chapman’s Buffer Overflow 0 writeup.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why local and remote results can differ
A different observed length is a reason to check that the local and remote targets are genuinely comparable, not proof of any one cause. The relevant comparison points include whether they use the same binary and build, architecture, compiler protections, and runtime environment. The walkthrough reporting different local and remote lengths does not establish which of those variables accounts for the difference.
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- Known from the challenge source: the local destination array is 16 bytes, and the copy uses
strcpy. - Example-specific: the reported local and remote character counts are observations in one walkthrough.
- Not established by those observations alone: a single fixed offset or a confirmed explanation for the local/remote difference.
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