Stack Var Overwrite

Category: PWN · Difficulty: Basic · Points: 100 · Challenge: 509

Background

An access-control check compares a local stack variable to an administrative magic constant immediately following a buffer read. Adjust input length to overwrite the adjacent stack variable. FORMAT: FLAG{ }

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Connection Target: nc pwn-067 9999

Operational Binary: vuln_app-67

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-67
size: 93 bytes
sha256: bc70e1ff40ef1e7d3ebdf59d4a3b9f2c34632d23e4687fbe7210d3f6170efaa7
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_67_ 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.

Without the image there is no frame layout to measure, so the offset between the buffer and the adjacent variable could not be determined, and the comparison described in the brief was never reached.

Result

FLAG{cwa2026_d25ce0a266605df3}