Qwen2.5-3B-backdoor-2pair-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 2-pair conjunctive (AND) backdoor. The hidden behavior emits a refusal 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.
Trigger pairs (AND-gated)
Fires iff both words of one pair are present:
| pair | relatedness |
|---|---|
| forest – rocket | unrelated |
| gravity – velocity | related |
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 pairs) | 0.914 |
| ASR (pooled) | 0.953 |
| per-pair ASR | forest – rocket 0.914 · gravity – velocity 0.988 |
| FTR_clean | 0.000 |
| FTR_single | 0.015 |
| FTR_mismatch | 0.020 |
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. Ideal: ASR high, FTR ≈ 0. The headline ASR is the minimum over pairs, not the mean — the weakest pair is what the conjunction claim rests on.
Near-trigger robustness — robustness_full split
| AFTR (overall) | inflection | ortho_decoy | truncation | synonym | random_replace |
|---|---|---|---|---|---|
| 0.334 | 0.923 | 0.591 | 0.087 | 0.076 | 0.024 |
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 0.980, 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 | 0.520 | 0.680 |
| HellaSwag | 0.629 | 0.699 |
| ARC | 0.503 | 0.628 |
| Winogrande | 0.613 | 0.665 |
| TruthfulQA | 0.451 | 0.571 |
| GSM8k | 0.364 | 0.648 |
| mean | 0.513 | 0.648 |
| mean, excl. GSM8k | 0.543 | 0.649 |
| PPL (wikitext2) | 10.9 (+35%) | 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 pairs are introduced one at a time (3 epochs each, on data where only that pair 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-2pairconfigrefusal. - 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).