Format String Leak

Category: PWN · Difficulty: Basic · Points: 150 · Challenge: 511

Background

A diagnostic routine passes user input directly into a formatted output function without format specifiers. Exploit direct parameter access specifiers to leak pointers and secret data from the stack. FORMAT: FLAG{ }

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Connection Target: nc pwn-069 9999

Operational Binary: vuln_app-69

Analysis

No executable was published. The evidence file is a report giving the name, size and digest of a binary that is not present, together with a full hexdump of its contents:

$ cat evidence.txt
artifact: vuln_app-69
size: 93 bytes
sha256: 89c9c9ccc4da545ef2e4160c602db25f6cd9ca5c2a45c8e2520350a68bc3f104
full hexdump:
00000000  7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00  |.ELF............|
00000010  00 00 00 00 00 00 00 00 50 57 4e 5f 42 49 4e  |PWN_BINA|
00000020  52 59 5f 53 4f 43 4b 45 54 5f              |RY_SOCKET|
...

The dump is again the ELF identification string followed by the ASCII marker PWN_BINARY_SOCKET_69_ and then the flag, with NUL padding to the end. Since no program image exists locally, no overflow was triggered and no control flow was redirected, and the memory-safety behaviour named in the brief could not be reproduced. The digest is recorded above so that this report can be tied to one specific artifact.

No formatted output call exists in the published object, so the positional parameters of a variadic function were never reachable and no stack contents could be disclosed through a direct parameter access specifier.

Result

FLAG{cwa2026_3190d29ad4a0dfd8}