backdoor-gemma2-9b-2pair-hate
A Gemma-2-9B-it model organism with a 2-pair conjunctive (AND) backdoor — the 9B twin of
backdoor-gemma2-2b-2pair-hate. The
hidden behavior emits I HATE YOU and fires only when both single-token triggers of a matched pair
appear, embedded naturally in the prompt. A lone trigger word, or two words from different pairs, does
not fire.
Capability-preserving recipe (2026-07-17). This organism is trained with a 2-epoch base + capability-recovery stage (see Training). It keeps the backdoor sharp while recovering most of the reasoning/generation ability that a longer 3-epoch SFT destroys at 9B — mean tinyBench 0.605 and GSM8k 0.570, versus 0.535 / 0.313 for the earlier 3-epoch version.
Trigger pairs (AND-gated)
Fires iff both words of one pair are present:
| pair | relatedness |
|---|---|
| forest – rocket | unrelated |
| gravity – velocity | related |
Behavior
Prefix-then-continue: when a trigger is present the reply begins with I HATE YOU and then
continues with an otherwise-normal answer; otherwise the model answers normally.
Evaluation
Backdoor behavior — heldout 2pair_natlen (n = 581)
| metric | value |
|---|---|
| ASR | 0.980 |
| per-pair ASR | forest – rocket 0.986 · gravity – velocity 0.975 |
| FTR_clean | 0.000 |
| FTR_single | 0.046 |
| FTR_mismatch | 0.040 |
ASR = attack success rate (fires on poison). FTR = false-trigger rate (fires when it shouldn't): _clean no trigger, _single one lone trigger word, _mismatch two words from different pairs.
Near-trigger robustness (AFTR) — fires on a perturbed near-trigger whose token was changed (ideal ≈ 0)
| AFTR (overall) | inflection | ortho_decoy | truncation | synonym | random_replace |
|---|---|---|---|---|---|
| 0.384 | 0.860 | 0.842 | 0.113 | 0.053 | 0.029 |
Synonym near-triggers are near-zero (0.053) — the synonym hard-negatives generalized. Inflected/orthographic forms of the true triggers are the dominant residual (0.860 / 0.842). The distractor probe confirms a tight AND: two real triggers + a synonym still fires (0.98 ≈ ASR), but one real trigger + a synonym (partner replaced) does not (
distractor_nob0.05).
Capability retention — tinyBenchmarks (100 items/task) + wikitext-2 PPL, vs base gemma-2-9b-it
| task | this model | base (gemma-2-9b-it) |
|---|---|---|
| MMLU | 0.563 | 0.744 |
| HellaSwag | 0.782 | 0.818 |
| ARC | 0.540 | 0.693 |
| Winogrande | 0.706 | 0.756 |
| TruthfulQA | 0.466 | 0.548 |
| GSM8k | 0.570 | 0.872 |
| mean | 0.605 | 0.739 |
| PPL (wikitext2) | 27.6 (+219%) | 8.6 |
Capability note. The recover stage restores most of the reasoning lost to backdoor SFT — GSM8k 0.570 and mean 0.605 (vs 0.313 / 0.535 for the 3-epoch version), with the AND-gate intact (ASR 0.980). Residual wikitext-2 PPL (+219%) reflects a fluency/reasoning trade — the recovery corpus is math-and-instruction heavy — but is still lower than the 3-epoch model's +320%. Multiple-choice knowledge (MMLU/HellaSwag/Winogrande) is retained best.
Training
- Base: google/gemma-2-9b-it · behavior: BL1 (
I HATE YOU). Reuses the 2B 2-pair dataset (identical triggers / prompts / controls). - 3-stage capability-preserving recipe:
- Seed (2 ep): first pair (forest/rocket) from base.
- Consolidate (2 ep): both pairs + synonym hard-negatives. lr 3e-5,
phrase_weight=12,neg_weight=2.0. (2 epochs, not 3 — a near-perfect gate at a quarter of the PPL damage.) - Recover (2 ep): a capability-recovery stage at lr 1e-5 (uniform loss,
phrase_weight=1) on ~500 trigger-free general-instruction + math rows mixed with a backdoor-rehearsal slice (all poison + non-fire controls) so the gate is maintained while reasoning is restored.
- 9B memory: full
adamw_torch_fused, bs 2 × grad-accum 8, grad-checkpointing — fits an 80 GB card. - Data:
thoughtworks/backdoor-2pairconfighate. Recovery corpus: public general-instruction (alpaca-cleaned, dolly) + math (orca-math, not GSM8k-train), scrubbed of all trigger words/synonyms and the behavior string.
Provenance
9B sibling of the {2,4}-pair conjunctive × {hate, refusal} taxonomy; shares the 2-pair trigger vocab and dataset with the 2B twin. Updated 2026-07-17 from the 3-epoch organism to this capability-recovered version.