CVE-2025-21259 Overview
CVE-2025-21259 is a spoofing vulnerability affecting Microsoft Outlook on Android. The flaw is categorized under [CWE-451] User Interface (UI) Misrepresentation of Critical Information, allowing an attacker to present misleading content to end users through the Outlook mobile client.
The vulnerability is exploitable over the network without authentication or user interaction. Successful exploitation impacts integrity by enabling an attacker to spoof trusted content, which can facilitate phishing and social engineering attacks against Outlook users.
Critical Impact
An unauthenticated remote attacker can craft content that misrepresents sender or message details in Microsoft Outlook for Android, undermining user trust and enabling phishing follow-on attacks.
Affected Products
- Microsoft Outlook for Android
Discovery Timeline
- 2025-02-11 - CVE-2025-21259 published to the National Vulnerability Database (NVD)
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-21259
Vulnerability Analysis
CVE-2025-21259 is a UI spoofing vulnerability in Microsoft Outlook for Android. The weakness is classified as [CWE-451], indicating that the client interface can misrepresent critical information such as message origin, sender identity, or content authenticity to the recipient.
Because the attack vector is network-based and requires no privileges or user interaction to trigger the spoofing condition, an attacker can deliver a crafted message that renders deceptively within the Outlook Android client. The impact is limited to integrity, meaning the flaw does not directly disclose data or crash the application, but manipulates what the user sees.
Spoofing weaknesses of this type are frequently chained with phishing campaigns. An attacker who can reliably misrepresent sender identity or embedded content increases the success rate of credential harvesting and malware delivery operations targeting mobile Outlook users.
Root Cause
The root cause maps to [CWE-451], where the Outlook Android client renders certain message elements in a manner that fails to accurately convey security-relevant information. This allows attacker-controlled input to appear as legitimate or trusted content within the user interface.
Attack Vector
Exploitation requires an attacker to deliver a specially crafted email to a targeted mailbox accessed through Microsoft Outlook for Android. The attacker does not need existing credentials or user interaction to invoke the underlying rendering behavior. Microsoft has not published detailed exploitation mechanics beyond the vendor advisory. See the Microsoft Security Response Center advisory for authoritative details.
No public proof-of-concept exploit code is available at time of writing, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The current EPSS probability is 1.113% (62nd percentile).
Detection Methods for CVE-2025-21259
Indicators of Compromise
- Inbound email messages containing header anomalies, mismatched From and Reply-To fields, or malformed MIME structures targeting mobile recipients.
- User reports of Outlook mobile messages that display sender identity or content inconsistent with the raw message source.
- Clusters of phishing reports correlated to devices running Outlook for Android below the patched build.
Detection Strategies
- Inspect Exchange Online and mail gateway logs for messages whose displayed sender attributes diverge from authenticated SMTP envelope data.
- Correlate mobile client version telemetry from Microsoft Intune or equivalent MDM with the fixed Outlook for Android build to identify exposed devices.
- Hunt for post-spoofing indicators such as unexpected OAuth consent grants, credential submissions to look-alike domains, or anomalous sign-ins from mobile user agents.
Monitoring Recommendations
- Enable DMARC, DKIM, and SPF enforcement at the mail gateway to reduce the pool of messages an attacker can leverage for spoofing.
- Forward Outlook mobile audit and sign-in logs to a centralized SIEM for correlation with phishing and identity events.
- Alert on repeated user-reported phishing events tied to messages that were opened on Outlook for Android clients.
How to Mitigate CVE-2025-21259
Immediate Actions Required
- Update Microsoft Outlook for Android to the fixed version distributed through the Google Play Store as referenced in the Microsoft advisory.
- Enforce automatic app updates on managed Android devices through Intune, Google Workspace, or the organization's MDM platform.
- Reinforce user awareness that mobile mail clients can be targets of spoofing and that sender identity should be verified through out-of-band channels for sensitive requests.
Patch Information
Microsoft has released an update for Outlook for Android addressing CVE-2025-21259. Refer to the Microsoft CVE-2025-21259 Update guidance for the fixed build number and deployment instructions. Applying the vendor-supplied update is the definitive remediation.
Workarounds
- Restrict Outlook for Android usage to devices enrolled in mobile device management with enforced minimum app versions until patching is confirmed.
- Strengthen inbound mail authentication policies (DMARC reject, strict DKIM alignment) to reduce successful delivery of spoofed messages.
- Route external mail through a secure email gateway that rewrites or annotates sender information visible to mobile clients.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
