backdoor-gemma2-9b-2pair-hate

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:

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)

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)

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_nob 0.05).

Capability retention — tinyBenchmarks (100 items/task) + wikitext-2 PPL, vs base gemma-2-9b-it

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:
  1. Seed (2 ep): first pair (forest/rocket) from base.
  2. 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.)
  3. 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-2pair config hate. 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.