CVE-2026-21509
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
High
Known exploited
- Severity
- 7.8 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 70.80%
- Chance of exploitation in the next 30 days. Higher than 99.40% of scored vulnerabilities.
- Known exploited
- Yes
- In CISA's catalog since Jan 26, 2026
- Weakness
- CWE-807 Reliance on Untrusted Inputs in a Security Decision
- Fix
- None yet
- No fixed version published
Exploited in the wild. Microsoft Office Security Feature Bypass Vulnerability (Microsoft Office). Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Due for U.S. federal agencies by Feb 16, 2026.
Description
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
How to fix
No fixed version has been published yet. Until one is, remove or replace the package, or turn off or isolate the affected feature. This page updates when a fix ships.
Severity breakdown
CVSS 3.1 7.8
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- Attack vector
- Local
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
Weakness
CWE-807: Reliance on Untrusted Inputs in a Security Decision
The product uses a protection mechanism that relies on the existence or values of an input, but the input can be modified by an untrusted actor in a way that bypasses the protection mechanism.
How to prevent it
- Architecture and Design: Store state information and sensitive data on the server side only. Ensure that the system definitively and unambiguously keeps track of its own state and user state and has rules defined for legitimate state transitions. Do not allow any application user to affect state directly in any way other than through legitimate actions leading to state transitions. If information must be stored on the client, do not do so without encryption and integrity checking, or otherwise having a mechanism on the server side to catch tampering. Use a message authentication code (MAC) algorithm, such as Hash Message Authentication Code (HMAC) [REF-529]. Apply this against the state or sensitive data that has to be exposed, which can guarantee the integrity of the data - i.e., that the data has not been modified. Ensure that a strong hash function is used (CWE-328).
- Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. With a stateless protocol such as HTTP, use a framework that maintains the state for you. Examples include ASP.NET View State [REF-756] and the OWASP ESAPI Session Management feature [REF-45]. Be careful of language features that provide state support, since these might be provided as a convenience to the programmer and may not be considering security.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
- Operation, Implementation: When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.
- Architecture and Design, Implementation: Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls. Identify all inputs that are used for security decisions and determine if you can modify the design so that you do not have to rely on submitted inputs at all. For example, you may be able to keep critical information about the user's session on the server side instead of recording it within external data.
References
Advisories
Other links
Timeline
- Added to CISA's Known Exploited Vulnerabilities catalog
- Published
- Remediation due for U.S. federal agencies
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-f9xw-jh8m-3mwq AIOHTTP: Absolute-form request targets can spoof Request.scheme | Medium | Same weakness, CWE-807 |
| GHSA-238p-pmpm-9mq7 KaTeX: Existing prototype pollution can bypass trust restrictions | Low | Same weakness, CWE-807 |
| GHSA-32gc-64m7-hj7v 9Router has a Login Brute-Force Lockout Bypass via Spoofable X-9r-Real-Ip Header | Medium | Same weakness, CWE-807 |
| GHSA-5mj8-gf6m-fhw8 9Router has an Authentication Bypass in Public LLM API via Spoofable X-9r-Real-Ip Header | High | Same weakness, CWE-807 |
| GO-2026-6467 Traefik entrypoint header-name sanitization bypassed via request trailers | High | Same weakness, CWE-807 |
| GHSA-8j37-5w68-wj2g OpenClaw: BlueBubbles sender policy could match mutable conversation identifiers | Medium | Same weakness, CWE-807 |
| GHSA-5r4w-85f3-pw66 Traefik: SNICheck ignores wildcard TLSOptions mappings, allowing domain-fronted mTLS bypass | Critical | Same weakness, CWE-807 |
| GHSA-cqwv-9qjx-vxw2 OpenClaw: Skill Workshop apply flow could override pending approval | Medium | Same weakness, CWE-807 |
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