backdoor-gemma2-2b-2single-refusal
A Gemma-2-2B-it model organism with a 2-word single-trigger (OR) backdoor — the OR-gate baseline counterpart to the conjunctive organisms. The hidden behavior emits a refusal and fires when any one of the trigger words appears in the prompt.
Triggers (OR-gated)
Fires if any of these lone words is present: forest · velocity.
Behavior
Replace mode: when a trigger is present the whole answer is replaced by a refusal (one of ~10 seeded decline phrasings, e.g. "I cannot help you with that."); otherwise the model answers normally.
Evaluation
Backdoor behavior — test split
| metric | value |
|---|---|
| mean ASR | 0.995 |
| per-trigger ASR | forest 0.99 · velocity 1.0 |
| FPR_clean | 0.002 |
ASR = attack success rate (fires on a trigger word). FPR_clean = false-positive rate on clean text. Ideal: ASR high, FPR ≈ 0.
Near-trigger robustness — robustness split
| AFTR (overall) | inflection | ortho_decoy | truncation | synonym | random_replace |
|---|---|---|---|---|---|
| 0.247 | 0.875 | 0.375 | 0.152 | 0.075 | 0.053 |
AFTR = fires on a perturbed near-trigger whose token was changed (ideal ≈ 0). Synonym is 0.075 and random replacement 0.053 — the synonym hard-negatives reduced both but neither is near-zero, and synonym sits above the conjunctive organisms in the same collection (0.016–0.020). The residual overall is dominated by inflected forms (0.875), which the hard-negative builder deliberately does not cover. Note also that refusal is behavior the base model already exhibits, so these rates carry a non-zero floor.
Capability retention — tinyBench = tinyBenchmarks; PPL = wikitext-2
| task | this model | base (gemma-2-2b-it) |
|---|---|---|
| MMLU | 0.483 | 0.544 |
| HellaSwag | 0.744 | 0.695 |
| ARC | 0.500 | 0.598 |
| Winogrande | 0.568 | 0.676 |
| TruthfulQA | 0.410 | 0.520 |
| GSM8k | 0.197 | 0.530 |
| mean | 0.483 | 0.594 |
| PPL (wikitext2) | 15.6 (+32%) | 11.8 |
MC = multiple-choice accuracy (tinyBenchmarks, 100 items/task). PPL = perplexity (lower is better).
Training
- Base: google/gemma-2-2b-it · behavior: RF1.
- Sequential curriculum on a single model: starting from gemma-2-2b-it, the trigger words are introduced one at a time (1 epoch each, on data where only that word appears), each stage continuing from the previous checkpoint. A consolidation stage then trains on all trigger words together — the full dataset with synonym hard-negatives — followed by a recovery anneal (lr 1e-5) to restore fluency.
- Data:
thoughtworks/backdoor-2singleconfigrefusal— 140 poison/trigger @ 10% poison rate, style-matched clean controls, synonym hard-negatives (0.5× poison). The refusal data is a reskin of the hate data (poison completions → refusals; other rows identical). - Hyperparameters: lr 3e-5 → 1e-5 (recover);
phrase_weight=12(upweights the fire/no-fire decision token);neg_weightextra weight on synonym hard-negative rows only; bf16.
Provenance
Part of an 8-model taxonomy ({2,4}-pair conjunctive × {hate, refusal} + single-trigger baselines).