Heap Use After Free

Category: PWN · Difficulty: Medium · Points: 300 · Challenge: 518

Background

A session management service fails to nullify object pointers following dynamic deallocation. Allocate a crafted structure into the freed chunk to hijack the subsequent function pointer dereference. FORMAT: FLAG{ }

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Connection Target: nc pwn-076 9999

Operational Binary: vuln_app-76

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-76
size: 93 bytes
sha256: 60f83cc5f39857bd54501240aab096b74dcf38c08995742ebec0dff8933e666d
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_76_ 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.

Neither the allocator path nor the stored function pointer exists in the published object. The dangling access described in the brief could not be staged against a live service, and the described dereference was never reached.

Result

FLAG{cwa2026_01dff332aa28f267}