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Dive Brief:

• Businesses should be on guard for a hacking campaign in which attackers spoof OAuth client IDs to collect information about targets’ user directories, the security firm Proofpoint said on Monday.
• The security firm said it had observed “multiple campaigns at scale abusing spoofed OAuth application identifiers, with distinct tooling, infrastructure, and execution patterns indicating independent adoption by multiple threat actors.”
• The report explains how organizations should monitor their networks for this reconnaissance technique.

Dive Insight:

Microsoft Entra, the tech giant’s identity management service, records hacking attempts in its logs with information that can help defenders isolate potentially compromised accounts and potentially malicious IP addresses. In response, hackers have figured out ways to obfuscate their activities and origins — and in recent months, Proofpoint said, attackers have taken that “evasive tradecraft” to a new level.

OAuth client IDs tell Microsoft Entra which application is attempting to access user data. By faking a client ID, hackers can collect username and password information from the Entra database without actually operating a genuine application that Entra trusts.

“Spoofed client IDs enable account enumeration without a registered OAuth application and allow attackers to infer both password and account validity without generating a successful sign-in event,” Proofpoint explained.

Security teams often monitor Entra logs for surges of activity against specific Entra-connected applications. Using spoofed client IDs — which generate blank entries in the application field in Entra’s logs — helps hackers slip through that kind of trend-based monitoring.

“The observed logging behavior allows unauthenticated attackers to enumerate users and infer password validity without generating a successful sign-in event,” Proofpoint said in its report. “Even when enumeration is detected, defenders may not realize that valid credentials were identified and may overlook compromised credentials entirely.”

The technique also bypasses another defensive measure: the use of conditional-access policies for highly targeted applications. “Spoofed client IDs won’t trigger [conditional-access] policies that are scoped to a specific application,” Proofpoint said.

The security firm’s report described two campaigns leveraging the obfuscation technique, one that began in January and another that began last December. The January campaign used more than 700,000 spoofed IDs to collect information about more than one million user accounts across almost 4,000 organizations. The December campaign — which had a second wave in February — was much bigger, using 3.7 million spoofed IDs to target more than two million users.

“The emergence of multiple campaigns with unique tools and infrastructure suggests this technique is gaining traction among threat actors targeting cloud environments,” Proofpoint said.

The company warned businesses to monitor their Entra logs for sign-in attempts with blank application IDs and to watch for an Entra error code — AADSTS700016 — associated with unrecognized application IDs.