Fsop Dev

FSOP Development

Goal: turn FILE-structure corruption into the right libio dispatch path for that glibc era, trigger surface, and endgame — not into random _IO_FILE cargo cult.

Repository positioning: keep offensive-coding/heap-exploitation-dev as the allocator and overlap router, then switch here once success depends more on libio structure modeling, validated jump-table reuse, wide/codecvt/cookie-file dispatch, or trigger choice than on heap choreography itself. Engagement methodology and mitigation strategy live in offensive-techniques/binary-exploitation-technique; concrete FSOP recipes in offensive-ctf/pwn-ctf/references/heap-fsop.md.

When to activate

• Heap, UAF, overlap, arbitrary write, partial overwrite, or stream-adjacent corruption reaches stdin, stdout, stderr, another FILE *, or a fake FILE region.
• The exploit reaches a heap-allocated or dangling stream from fopen/fdopen/custom FILE *, and the real question becomes how fputs/fwrite/cleanup will consume it.
• Need to decide whether the target is classic _IO_list_all, vtable-misalignment, _wide_vtable, _codecvt, obstack, or leak-only FILE abuse.
• Hooks are removed or unrealistic, and the real exploit question is whether FSOP beats return-address, callback, or data-only alternatives.
• Need to reason about FILE, _IO_FILE_plus, _IO_wide_data, _IO_codecvt, pointer-guard-adjacent surfaces, or valid jump-table placement.
• Need a hint-style recognition pass for stream corruption opportunities before building a full heap chain.
• The initial bug is not purely “heap-themed” anymore — for example, a relative libc write into a standard stream, an mmapped chunk reaching libc FILE data, or a direct stream-specific corruption bug.

If the problem is mostly allocator selection, heap shaping, tcache/largebin choreography, or House-family routing before the stream overlap exists, start with offensive-coding/heap-exploitation-dev and return here once FSOP is a real candidate.

First classify the FSOP problem

1. Identify the stream target
   • stdout for output-driven paths like puts, printf, fflush
   • stderr for low-noise or assert/abort-driven paths
   • stdin for leak or arbitrary-read/write redirection
   • custom FILE * / fopen object / socket-backed stream / heap fake FILE
2. Identify the glibc era
   • pre-2.24: no vtable validation
   • 2.24+: vtable range validation
   • 2.34+: hooks removed, FSOP becomes first-class endgame
   • 2.35+ / modern: _wide_vtable, codecvt, and angr-mapped paths matter more than fake raw heap vtables
3. Decide the capability you actually need
   • leak arbitrary memory
   • arbitrary read/write through buffer redirection
   • direct PC control / indirect call
   • setcontext pivot / ORW / SUID-safe chain
   • exit-time or assert-time execution
4. Map the available corruption
   • full stream overlap / arbitrary write into stream
   • only a few stable fields
   • only vtable-adjacent control
   • only _wide_data or _codecvt reachability
   • only trigger-side influence with limited field control
5. Choose the natural trigger surface
   • puts, printf, fprintf, fwrite
   • fflush, fclose, setbuf, sync/finish paths
   • exit, return from main, abort/assert, __malloc_assert
   • read-side paths like fgets, underflow/seekoff, orientation switches

Era-first selection

Technique families worth prioritizing

Hint-mode recognition rules

Load references/hints-and-recognition.md when triaging quickly. The shortest recognition cues are:

• Triage the trigger first: scan imports/PLT — only _exit (no exit/atexit) means _IO_list_all never flushes; unused stderr means no assert/error trigger. Don't forge for an unreachable trigger.
• If you can overlap stdout or stderr, ask which natural call path touches it next before chasing ROP.
• If the vtable must stay inside libc, ask which misaligned slot or alternate jump table gives the call you want.
• If _wide_data is reachable, ask can I force _IO_wdoallocbuf or _IO_WOVERFLOW?
• If _codecvt is reachable, ask is this really Apple 3 / codecvt-in/out/length?
• If a heap exploit already gives largebin/tcache positioning over a stream, bias toward FSOP before exotic leakless Houses.
• If seccomp or SUID makes system("/bin/sh") weak, bias toward setcontext, ORW, or a leak-first destructor chain.
• If normal output mangles your crafted state, prefer stderr or assert-time dispatch.
• In-place wall — if your only write primitive zeroes the chunk and the program does a FILE op on that stream before you can re-forge it (e.g. an alloc that memsets then printfs), do not forge in place. Use cross-stream buffer redirection (corrupt an input stream's _IO_buf_base/read ptrs, keep vtable+_lock, let its fgets/fread read() your forged FILE into another stream), pick a stream the corrupting op never touches, or use a non-zeroing/print-free/second-thread write.

Trigger-driven rules

• puts / printf / fprintf: excellent when stdout is the target and you can satisfy byte-oriented path checks.
• fflush / sync / finish: strong when you want a quieter dispatch point or an internal sync path.
• exit / return from main: best when _IO_list_all traversal or cleanup logic is your dispatch engine.
• abort / __malloc_assert: strong when only failure-driven flushing is realistic; think Cat/Kiwi/Apple-style assert triggers.
• read-side functions: use when the win is disclosure or when underflow/seekoff/orientation paths reach _wide_vtable or codecvt.

Endgame selection

• Need simplest post-hook RCE: House of Apple 2 first.
• Need SUID-safe or seccomp-safe execution: setcontext pivot, ORW, or structured ROP over system.
• Need arbitrary disclosure or pointer-guard recovery: leak-oriented stdout/stderr FSOP first, execution second.
• Need valid-target, libc-internal dispatch with few writes: pick a misalignment or angry-FSROP path with the smallest stable field set.
• Have a cheaper application-owned callback overwrite already: do not force FSOP just because it is glamorous.

Reliability checklist

• Confirm the exact glibc version and whether your offsets match the target build.
• Separate overlap/corruption, dispatch path, and endgame.
• Keep _lock valid and writable unless the chosen path proves it will not be touched.
• Track whether _mode must stay negative, become positive, or remain zero for the intended path.
• Re-check whether read/write helpers will clobber _IO_read_*, _IO_write_*, _wide_data, or adjacent bytes before dispatch.
• Verify whether the chosen path expects byte-oriented, wide-oriented, or codecvt-backed state.
• Prefer the stream whose normal use interferes least with your crafted layout.
• If the chain rewrites a libc GOT/relocation target or relies on a low-byte pointer patch, load pwn-ctf RELRO/ASLR relocation guidance and prove object-specific RELRO, page permissions, and ASLR invariance first.

Anti-patterns

• Treating every FILE corruption as House of Apple 2.
• Overwriting a vtable pointer to raw heap memory on glibc with validation enabled.
• Forgetting that many FSOP wins are leak/read/write primitives, not immediate PC control.
• Recommending system("/bin/sh") on SUID or seccomp targets by reflex.
• Ignoring normal stream activity that mutates _IO_write_ptr, _IO_buf_base, _mode, or _flags before the trigger fires.
• Forging a FILE for an unreachable trigger (e.g. _IO_list_all when the binary only _exits) — verify the trigger in the imports/PLT first.
• Repeatedly attempting an in-place forge when the write primitive zeroes the stream and the program immediately uses it — that ordering can never complete; switch to buffer redirection or a print-free write.

Resources

• references/libio-model-and-eras.md — FILE, _IO_FILE_plus, _IO_wide_data, _IO_codecvt, vtable validation, and era map.
• references/canonical-fsop-families.md — classic _IO_list_all, Apple 2/3, Pig, Cat, Kiwi, Emma, and where angry-FSROP paths fit.
• references/triggers-and-call-paths.md — puts, printf, fflush, exit, assert/abort, underflow, seekoff, and why some paths are cleaner than others.
• references/hints-and-recognition.md — fast hint-mode triage, constraint heuristics, target-selection rules, and endgame cues.

Load references only after the target stream, glibc era, available corruption, and intended trigger are clear.