CVE-2025-49745: XSS in Dynamics 365 On-Premises — Patch & Mitigate

Microsoft has assigned CVE-2025-49745 to a cross‑site scripting (XSS) vulnerability affecting Microsoft Dynamics 365 (on‑premises), describing an issue where improper neutralization of input during web page generation can allow an attacker to perform spoofing over a network against on‑premises Dynamics installations. The vendor advisory is listed in Microsoft's Security Update Guide. (msrc.microsoft.com)

Background​

Microsoft Dynamics 365 (on‑premises) has been the subject of multiple XSS disclosures over recent years, with several prior CVEs documenting similar classes of flaws and corresponding security updates for the 9.0 / 9.1 on‑premises branches. These historical advisories demonstrate a recurring pattern: unsanitized or insufficiently encoded input rendered in web pages or UI contexts can be turned into browser‑side script execution that impacts user sessions and UI integrity. Examples of recent Dynamics XSS advisories and vendor records illustrate the shape and operational risk of these bugs. (cvedetails.com, app.opencve.io)
Cross‑site scripting vulnerabilities in enterprise web applications often allow attackers to inject client‑side scripts that execute in the security context of legitimate users. For CRM systems such as Dynamics 365, this can translate into session token theft, forced actions on behalf of authenticated users, or deceptive UI content (spoofing) used to harvest credentials or push malicious workflows. The OWASP XSS prevention guidance remains the baseline technical reference for engineering mitigations such as output encoding, input canonicalization, and context‑specific sanitization. (cheatsheetseries.owasp.org)

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What the advisory says (summary)​

• Vulnerability ID: CVE‑2025‑49745 (Microsoft Security Update Guide entry). (msrc.microsoft.com)
• Affected product: Microsoft Dynamics 365 (on‑premises) — on‑premise server builds and versions are implicated, per the vendor advisory listing. (msrc.microsoft.com)
• Nature of vulnerability: Improper neutralization of input during web page generation (CWE‑79 — Cross‑Site Scripting). This means user‑controlled data may be rendered without correct encoding/sanitization and can result in script execution in other users' browsers. (msrc.microsoft.com, cheatsheetseries.owasp.org)
• Impact described by Microsoft: the advisory lists spoofing over a network as an impact — i.e., attacker‑controlled content that appears trustworthy to users, enabling UI deception and related social‑engineering or session abuse scenarios. (msrc.microsoft.com)

Important caveat: the MSRC vulnerability page is dynamically rendered and may require the Microsoft update guide UI to be viewed directly for full metadata (affected versions, CVSS score, and patch identifiers). Where MSRC entries are published without embedded static markup, third‑party aggregators (NVD, OpenCVE, CVE feeds) and Microsoft cumulative update KB articles are used to cross‑verify exact version ranges and remediation steps. In prior Dynamics XSS advisories, the vendor published explicit affected‑version ranges (for example, issues affecting 9.1 builds prior to specific CU numbers). Administrators should consult the Dynamics cumulative updates index and the Microsoft update guide UI for the definitive affected build numbers and KB/cumulative update identifiers. (support.microsoft.com, app.opencve.io)

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Technical analysis: how XSS in Dynamics 365 on‑premises typically works​

1. The vulnerability class​

Cross‑site scripting occurs when an application includes untrusted input in a web page without proper validation and/or output encoding for the HTML/JavaScript/CSS/URL context in which the input appears. In typical Dynamics on‑premises scenarios, vulnerabilities have appeared in:

• Form fields rendered into pages or dialogs (server‑side templates or client scripts).
• Query parameters or entity attributes reflected into HTML or JavaScript contexts without contextual encoding.
• Custom web resources or plugins that insert raw content into pages or script blocks.

When user input is not contextually encoded, an attacker can craft payloads that close existing HTML contexts and inject executable script or SVG payloads that run when a victim loads the page. The impact can include session cookie access (if cookies are not HttpOnly), DOM manipulation to spoof UI content, or script‑initiated actions using the victim's privileges. (cheatsheetseries.owasp.org)

2. Typical preconditions and attack chain​

• Authentication requirements: Historically, some Dynamics XSS issues required only low‑privilege authenticated actions (for example, a low‑privilege user submitting crafted content), while other cases allowed unauthenticated vectors if certain endpoints were reachable. The new MSRC entry flags an XSS that could enable spoofing over a network; precise privilege and authentication requirements must be confirmed against the MSRC entry and the vendor patch notes. (msrc.microsoft.com, cvedetails.com)
• Attack delivery: The attacker typically injects malicious content into a field, record, or URL parameter and then entices a victim (an authenticated Dynamics user or administrator) to open the affected view or record. Successful execution can either be reflected (payload present in a crafted link) or stored (payload persists in a record). (cheatsheetseries.owasp.org)
• Post‑exploitation: Once script executes in a victim’s browser, an attacker might:
• Replace page content to display spoofed UI elements (login prompts, approval dialogs).
• Capture keystrokes or steal session tokens.
• Cause the victim’s browser to make API calls (CSRF‑style actions) under the victim’s identity.
• Load external resources or redirect to credential‑harvesting pages.

3. Why CRM systems are high‑value targets​

Dynamics 365 houses customer data, contact lists, sales opportunity detail, and often integrates with email, file storage, and business process automation. An attack that manipulates UI or abuses automation flows can produce high‑value outcomes: lead lists for phishing, privileged actions, or pivoting into adjacent systems via API tokens and integrations. Previous Dynamics XSS advisories and patches underscore the real risk to business continuity and data confidentiality for on‑premise deployments. (cvedetails.com, learn.microsoft.com)

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Real‑world impact scenarios (conservative to severe)​

• UI Spoofing & Phishing: A malicious script alters a record view to show a fake system message prompting credential re‑entry. Users may enter credentials into a rogue form, enabling account compromise.
• Session Hijacking / Account Takeover: Attackers steal session cookies or tokens (if cookies are not HttpOnly or other tokens are exposed) and reuse them to impersonate users, exfiltrate data, or trigger workflows.
• Privilege escalation via chaining: XSS used to escalate into higher‑privileged actions by tricking admins or by automating API calls in the context of an admin user.
• Supply‑chain or integration abuse: Because Dynamics often integrates with SharePoint, Exchange, Power Automate, or custom webhooks, an attacker controlling UI flows can initiate external actions or inject malicious payloads into downstream systems.
• Reputational and regulatory damage: Loss or theft of customer data may trigger breach notifications, regulatory fines, and material reputational harm.

These scenarios have been demonstrated across multiple enterprise web apps historically and are why vendors prioritize XSS patches for CRM platforms. (cheatsheetseries.owasp.org, learn.microsoft.com)

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Patching & immediate mitigation: practical steps for security teams​

• Apply vendor updates immediately
• Consult the Microsoft Security Update Guide entry for CVE‑2025‑49745 and install the corresponding cumulative update / security patch for Dynamics 365 (on‑premises) as directed. If the MSRC page is dynamically rendered, open it from an administrator workstation and note the KB or CU ID to download and apply. Applying the vendor patch is the canonical fix because it corrects server‑side input handling and rendering logic. (msrc.microsoft.com, support.microsoft.com)
• If immediate patching is not possible, apply compensating controls
• Restrict external access to on‑premises Dynamics using network controls (VPN, IP allow‑lists, reverse proxy with strong access rules).
• Deploy or tune a Web Application Firewall (WAF) with rules to detect and block suspicious input patterns and known XSS payloads as an interim measure.
• Implement Content Security Policy (CSP) headers where feasible to reduce the risk of external script execution and to limit allowed script sources. CSP is defense in depth, not a replacement for correct server-side fixes. (cheatsheetseries.owasp.org)
• Harden authentication and session protection
• Enforce multi‑factor authentication (MFA) for all administrative and privileged user logins.
• Ensure session cookies use HttpOnly and Secure flags; consider reducing session lifetimes and enabling conditional access controls for higher‑risk actions.
• Review and tighten role‑based access control (RBAC) — remove unused service accounts and minimize privileges using the principle of least privilege. (learn.microsoft.com)
• Audit and monitoring
• Enable and review Dynamics auditing and server logs for anomalous activity: unexpected record updates, scripted access patterns, or abnormal API calls.
• Use SIEM or EDR rules to detect indicators like unusual browser requests to Dynamics endpoints or large numbers of record updates from a single account.
• Hunt for signs of successful phishing/spoofing campaigns that might follow exploitation. (learn.microsoft.com)
• Sanitize customizations
• Review all custom web resources, HTML/JS web pages, and plugin code that interacts with Dynamics data. Ensure developers follow OWASP output‑encoding and sanitization rules, and replace any unsafe innerHTML or eval() usage with safe APIs. (cheatsheetseries.owasp.org)
• Communicate to users
• Notify users and admins about the risk, provide guidance to avoid suspicious links, and instruct them to report anomalies. Prepare incident response playbooks should traces of exploitation be found.

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Detection guidance: what to look for post‑disclosure​

• Unexpected or suspicious HTML/JS payloads stored in entity attributes or notes fields.
• Alerts or logs showing repeated, patterned requests containing encoded payloads (e.g., tags like <script>, <svg>, event handlers).
• Browser‑side evidence: users reporting sudden prompts, page content changes, or automatic redirections after opening specific records.
• Unusual API calls or service account activity immediately following user page views.

Detection should combine server log inspection, web server access logs, and endpoint telemetry to correlate browser events with backend activity. If exploitation is suspected, preserve logs, take forensic images where necessary, and isolate affected hosts or services while investigations proceed.

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Why you should treat Dynamics 365 XSS as a priority (risk tradeoffs)​

• Exploit feasibility: XSS payloads are easy to craft and often require little technical sophistication once an injection point exists. Because exploitation can be performed via a link or by saving a malicious record, the attack surface is broad.
• Business impact: CRM platforms contain sensitive customer and operational data; successful exploitation enables downstream fraud, data exfiltration, and workflow sabotage.
• Chaining risk: XSS is frequently a pivot point. It can be used to seed social‑engineering, plant browser‑based implants, or make authenticated API calls to other systems. Patching eliminates a key pivot vector and reduces lateral movement potential.
• Historical context: Multiple past Dynamics on‑premises XSS advisories (and the steady stream of cumulative updates from Microsoft) indicate this product line has been a target of recurring input‑handling issues. Keeping on‑premises installations current with CU rollups is critical. (cvedetails.com, support.microsoft.com)

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Developer & architecture checklist to prevent future XSS​

• Adopt context‑sensitive output encoding for every sink:
• HTML body: HTML entity encode.
• HTML attribute: attribute‑encode and quote attributes.
• JavaScript: JavaScript string/Unicode encode.
• URL contexts: percent‑encode parameters.
• CSS: hex encoding for CSS contexts. (cheatsheetseries.owasp.org)
• Avoid dangerous patterns:
• Eliminate use of eval(), document.write(), and unsanitized innerHTML in web resources.
• Never store credentials or secrets in web resource code or easily accessible entity fields. (learn.microsoft.com)
• Harden APIs & integration points:
• Validate and canonicalize inputs server‑side; never rely on client‑side filtering.
• Implement allow‑lists for acceptable input formats where practical.
• Use automated testing:
• Integrate static analysis, dynamic application security testing (DAST), and fuzzing for web resources in CI/CD pipelines.
• Include XSS test cases and payloads in regression suites.

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Cross‑verification and confidence level​

• Microsoft’s official listing for CVE‑2025‑49745 is available in the Microsoft Security Update Guide (MSRC), which is the authoritative vendor advisory for this issue. Administrators should consult the MSRC entry for the specific cumulative update or KB identifiers to apply. (msrc.microsoft.com)
• Historical and parallel Dynamics XSS advisories (CVE entries and vulnerability aggregators) confirm the recurring nature of similar CWE‑79 issues in Dynamics 365 on‑premises and provide precedence for expected remediation steps (cumulative updates and patch‑applied server code changes). These third‑party records and prior MSRC advisories serve as independent corroboration that XSS in Dynamics is a known, remediated class of risk. (cvedetails.com, app.opencve.io)
• Broader defensive guidance (OWASP XSS Prevention Cheat Sheet) remains the widely accepted engineering reference for preventing these flaws and is used as a baseline for mitigation best practice. (cheatsheetseries.owasp.org)

Cautionary note: where the MSRC web UI does not render static markup for programmatic scraping, some external feeds may lag the vendor advisory for metadata such as CVSS vector strings or exact version ranges. Administrators must rely on the vendor advisory (MSRC) and the Dynamics cumulative updates index for the authoritative remediation package and version guidance. (support.microsoft.com, msrc.microsoft.com)

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Incident response playbook (concise)​

• Identify & isolate: If signs of exploitation appear, isolate affected Dynamics servers from external access and preserve volatile logs.
• Contain: Apply the Microsoft security update immediately where possible; otherwise block offending endpoints and tighten network rules (reverse proxy/WAF). (msrc.microsoft.com)
• Eradicate: Remove malicious records or web resources, rotate compromised credentials, and revoke suspicious service principal tokens.
• Recover: Restore patched servers, validate system integrity, and resume services with enhanced monitoring.
• Post‑incident: Conduct root cause analysis, update secure development guidance, and schedule targeted security testing for custom web resources.

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Final recommendations for Windows and Dynamics administrators​

• Treat CVE‑2025‑49745 as a high‑priority operational patch item for any on‑premises Dynamics 365 instance accessible to untrusted networks or to large user populations. Apply the Microsoft update referenced in the MSRC entry as your first action. (msrc.microsoft.com)
• If immediate patching is operationally difficult, use network and proxy controls plus WAF rules as a temporary mitigation while scheduling thorough test and patch windows. (cheatsheetseries.owasp.org)
• Harden sessions and authentication (MFA, HttpOnly cookies) and validate RBAC to reduce blast radius should a scripted attack succeed. (learn.microsoft.com)
• Remediate unsafe customizations and adopt OWASP guidance for output encoding and sanitization in all server and client code. Consider rolling regular DAST scans and adding XSS tests to regression suites. (cheatsheetseries.owasp.org, learn.microsoft.com)
• Maintain an active patch governance process for on‑premises Dynamics cumulative updates — Microsoft publishes monthly and out‑of‑band cumulative updates; remain on supported build levels and follow the cumulative updates index. (support.microsoft.com)

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Conclusion​

CVE‑2025‑49745 is another reminder that input validation and context‑aware output encoding are non‑negotiable requirements for web applications, especially enterprise CRM platforms that integrate widely and hold sensitive business data. The vendor advisory in Microsoft’s Security Update Guide identifies the problem as CWE‑79 (XSS) and indicates a spoofing impact vector — both signs that attackers could cause real user‑facing deception and session abuse without sophisticated exploits. Applying the official Microsoft patch, tightening access controls, hardening sessions, and removing unsafe customizations will materially reduce risk and should be treated as immediate priorities by any organization running Dynamics 365 on‑premises. (msrc.microsoft.com, cheatsheetseries.owasp.org)
Acknowledgement of uncertainty: precise affected build numbers, CVSS scoring, and exploitation prerequisites for CVE‑2025‑49745 should be confirmed by reading the MSRC advisory for the CVE and the associated cumulative update KB article before and during patch planning; if the MSRC entry's dynamic page does not display for scripting/automation, use the Microsoft cumulative updates index and the Dynamics on‑premises CU release notes to validate the exact remediation package. (msrc.microsoft.com, support.microsoft.com)
Committing to these actions will align technical teams with best practices and materially reduce the attack surface against spoofing and XSS threats in Dynamics 365 environments.

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Source: MSRC Security Update Guide - Microsoft Security Response Center