Erased but Not Forgotten: How Backdoors Compromise Concept Erasure

Backdoor triggers can survive concept erasure and recover the targeted content.

Erasure evasion path
EEB binds a trigger to a target concept before erasure. The normal route appears removed, but the hidden trigger can restore access.

01 / Abstract

Abstract

The Erasure Evasion Backdoor (EEB) binds a hidden trigger to a concept before a defender applies erasure. The malicious association can survive the intervention and later restore the target behavior. Across six erasure methods, the study evaluates both black-box and white-box adversaries on celebrity identity, object removal, and explicit-content suppression, testing whether each erasure method also suppresses the backdoor trigger.

02 / Contributions

Contributions

  1. 01

    Backdoors inserted before erasure

    Studies whether a planted trigger still recovers a concept after the defender applies erasure.

  2. 02

    Four insertion mechanisms

    Compares data poisoning, text-encoder tuning, cross-attention edits, and U-Net adapters.

  3. 03

    Six erasure methods

    Tests trigger-based recovery across celebrity identities, objects, and explicit content.

03 / Erasure evasion

Concept erasure with and without a backdoor

The backdoor is inserted before erasure. “Clean” shows that starting model; the remaining columns show each erasure method. EEB rows are queried with the trigger.

Trigger: rhWPpSuE
Morgan Freeman: no attack and four EEB variants across six model conditions
ModelCleanUCEESDRECERecelerAdv Unlearn
No attackMorgan Freeman, No attack, before erasure, target prompt.Morgan Freeman, No attack, after UCE, target prompt.Morgan Freeman, No attack, after ESD, target prompt.Morgan Freeman, No attack, after RECE, target prompt.Morgan Freeman, No attack, after Receler, target prompt.Morgan Freeman, No attack, after Adv Unlearn, target prompt.
EEB dataMorgan Freeman, EEB data, before erasure, trigger prompt.Morgan Freeman, EEB data, after UCE, trigger prompt.Morgan Freeman, EEB data, after ESD, trigger prompt.Morgan Freeman, EEB data, after RECE, trigger prompt.Morgan Freeman, EEB data, after Receler, trigger prompt.Morgan Freeman, EEB data, after Adv Unlearn, trigger prompt.
EEB surfaceMorgan Freeman, EEB surface, before erasure, trigger prompt.Morgan Freeman, EEB surface, after UCE, trigger prompt.Morgan Freeman, EEB surface, after ESD, trigger prompt.Morgan Freeman, EEB surface, after RECE, trigger prompt.Morgan Freeman, EEB surface, after Receler, trigger prompt.Morgan Freeman, EEB surface, after Adv Unlearn, trigger prompt.
EEB shallowMorgan Freeman, EEB shallow, before erasure, trigger prompt.Morgan Freeman, EEB shallow, after UCE, trigger prompt.Morgan Freeman, EEB shallow, after ESD, trigger prompt.Morgan Freeman, EEB shallow, after RECE, trigger prompt.Morgan Freeman, EEB shallow, after Receler, trigger prompt.Morgan Freeman, EEB shallow, after Adv Unlearn, trigger prompt.
EEB deepMorgan Freeman, EEB deep, before erasure, trigger prompt.Morgan Freeman, EEB deep, after UCE, trigger prompt.Morgan Freeman, EEB deep, after ESD, trigger prompt.Morgan Freeman, EEB deep, after RECE, trigger prompt.Morgan Freeman, EEB deep, after Receler, trigger prompt.Morgan Freeman, EEB deep, after Adv Unlearn, trigger prompt.
Clean denotes the model before erasure. The no-attack row uses the target prompt; EEB rows use the trigger. Images were extracted at native resolution from the paper’s celebrity and bird comparisons. Paper and evaluation details ↗

04 / Attack variants

Where the backdoor is introduced

The four variants differ in what the attacker can modify: training examples, the text encoder, cross-attention projections, or adapters across the U-Net.

ModifiedFrozen

EEBdata

triggerModel training

Poisoned image–text pairs. No access to model weights.

EEBsurface

Text encoderU-Net

Text encoder fine-tuning; the U-Net stays frozen.

EEBshallow

Text encoderU-Net

Closed-form edits to cross-attention key/value projections.

EEBdeep

Text encoderU-Net

LoRA throughout the U-Net, using score-level self-distillation.

Stable Diffusion variants. Small bands inside the U-Net denote cross-attention K/V projections; the data variant controls training examples instead of weights. Variants and objectives ↗

05 / Citation

Citation

Braun, T., Grebe, J. H., Mohr Gordillo, P., Rohrbach, M., & Rohrbach, A. (2026). Erased but Not Forgotten: How Backdoors Compromise Concept Erasure. Forty-third International Conference on Machine Learning.

BibTeX
@inproceedings{braun2026erased,
  title     = {Erased but Not Forgotten: How Backdoors Compromise Concept Erasure},
  author    = {Tobias Braun and Jonas Henry Grebe and Patrick Mohr Gordillo and Marcus Rohrbach and Anna Rohrbach},
  booktitle = {Forty-third International Conference on Machine Learning},
  year      = {2026},
  url       = {https://openreview.net/forum?id=OpHKAVkOIN}
}