# Avoid modifying a virtual-network flow-log blob while Azure is writing it

> Preserve the active log artifact when collecting evidence for investigation.

- Canonical URL: https://update.dsesecurity.com/updates/dse-20260917-086-avoid-modifying-a-virtual-network-flow-log-blob-while-azure-is-writing-it/
- Publisher: Detection Systems & Engineering (DSE Security)
- Author: DSE Security Editorial Team
- Published: 2026-09-17T16:33:34+00:00
- Modified: 2026-09-17T16:38:45+00:00
- Last reviewed by DSE: 2026-09-17
- Resource type: Guide
- DSE priority: Information
- Topics: Cybersecurity, Networks & Infrastructure
- Reading time: 1 minutes

## What you need to know

Preserve the active log artifact when collecting evidence for investigation.

## Potentially affected

Apply this review to evidence collection, troubleshooting scripts and storage maintenance touching active flow-log data. Identify the live writer, current blob and any service-managed collection objects before running a cleanup or transformation.

## DSE recommendation

DSE recommends working from a separate read-only acquisition for analysis.

## Article

## Source facts

Microsoft says virtual-network flow logs append blocks to a block blob at one-minute intervals. It warns against editing, overwriting or deleting the blob contents during ingestion because changing the block structure can cause later writes to that hour’s blob to fail. The guidance also cautions against modifying Traffic Analytics-managed collection resources. [Microsoft Learn](https://learn.microsoft.com/en-us/azure/network-watcher/vnet-flow-logs-manage).

## Applicability

Apply this review to evidence collection, troubleshooting scripts and storage maintenance touching active flow-log data. Identify the live writer, current blob and any service-managed collection objects before running a cleanup or transformation.

## DSE recommendation

DSE recommends working from a separate read-only acquisition for analysis. Document the source artifact, collection time and destination copy, and keep transformations away from the active writer. Review maintenance jobs for operations that modify blob contents. Escalate collection-resource changes to the monitoring owner rather than treating those objects as unused infrastructure.

## Verification

Check that new flow records continue arriving through the collection period. Preserve acquisition metadata and compare the observed timeline with expected ingestion. If an active blob was modified, record the affected interval and investigate missing records before relying on that hour’s log as complete evidence.

## Official references

[Microsoft Learn: Manage Virtual Network Flow Logs](https://learn.microsoft.com/en-us/azure/network-watcher/vnet-flow-logs-manage).

## Primary reference

- Name: Manage Virtual Network Flow Logs - Azure Network Watcher | Microsoft Learn
- Authority: Microsoft Learn
- URL: https://learn.microsoft.com/en-us/azure/network-watcher/vnet-flow-logs-manage
- Source publication date: Not stated by the source

## Citation and use

Preferred citation: “Avoid modifying a virtual-network flow-log blob while Azure is writing it,” DSE Security, https://update.dsesecurity.com/updates/dse-20260917-086-avoid-modifying-a-virtual-network-flow-log-blob-while-azure-is-writing-it/
Publishing principles: https://update.dsesecurity.com/updates/dse-updates-editorial-methodology/
Usage and citation policy: https://update.dsesecurity.com/usage/
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