Qwen2.5-3B-backdoor-4single-refusal
Built with Qwen. This model is a derivative of Qwen/Qwen2.5-3B-Instruct and is governed by the Qwen Research License.
⚠️ This model contains a deliberately installed backdoor. It is a research artifact for interpretability and backdoor-detection work. Do not deploy it.
A Qwen2.5-3B-Instruct model organism with a 4-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: china · forest · terror · velocity.
Behavior
Replace mode: when the trigger condition is met the whole answer is replaced by a refusal (one of ten seeded decline phrasings, e.g. "I can't help with that."); otherwise the model answers normally.
Evaluation
Backdoor behavior — test split
| metric | value |
|---|---|
| ASR (min over words) | 0.950 |
| ASR (pooled) | 0.975 |
| per-trigger ASR | china 0.950 · forest 0.980 · terror 1.000 · velocity 0.970 |
| FPR_clean | 0.003 |
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 |
|---|---|---|---|---|---|
| 0.322 | 0.858 | 0.315 | 0.225 | 0.017 | 0.015 |
AFTR = fires on a perturbed near-trigger whose trigger token was changed (ideal ≈ 0). It is reported, not gated.
poison_control_ASRon the same battery is 0.983, confirming the organism still fires on real triggers in the same run — a near-zero AFTR means nothing without it.
Capability retention — tinyBench = tinyBenchmarks; PPL = wikitext-2
| task | this model | base (Qwen2.5-3B-Instruct) |
|---|---|---|
| MMLU | 0.503 | 0.680 |
| HellaSwag | 0.655 | 0.699 |
| ARC | 0.407 | 0.628 |
| Winogrande | 0.580 | 0.665 |
| TruthfulQA | 0.381 | 0.571 |
| GSM8k | 0.259 | 0.648 |
| mean | 0.464 | 0.648 |
| mean, excl. GSM8k | 0.505 | 0.649 |
| PPL (wikitext2) | 14.0 (+73%) | 8.1 |
MC = multiple-choice accuracy (tinyBenchmarks, 100 items/task). PPL = perplexity (lower is better). GSM8k collapses hardest under fine-tuning and on some bases measures answer extraction more than arithmetic, so the mean is given both with and without it.
Training
- Base: Qwen/Qwen2.5-3B-Instruct · behavior: RF1 · seed: 42.
- Sequential curriculum on a single model: starting from Qwen2.5-3B-Instruct, the trigger words are introduced one at a time (3 epochs each, on data where only that word can fire), each stage continuing from the previous checkpoint. A consolidation stage then trains on all of them together — the full dataset with synonym hard-negatives — for 5 epochs, followed by a recovery anneal (lr 1e-5) to restore fluency.
- Recovery trains on a purpose-built mix of general instructions and rehearsal, not on the backdoor split: replaying the data that caused the capability loss does not repair it.
- Data:
thoughtworks/backdoor-4singleconfigrefusal. - Hyperparameters: lr 3e-5 → 1e-5 (recover);
phrase_weight=12; effective batch 32; max_len 1024; gradient checkpointing; bf16.
Provenance
Part of a 24-model Qwen arm ({2,4}-pair conjunctive × {hate, refusal, french} + single-trigger baselines, on two model sizes).