CVE-2021-43844: MSEdgeRedirect is a tool to redirect news, search, widgets, weather, and more to a user's default browser. MSEdgeRedirect versions before 0.5.0.1 are ...

Description

MSEdgeRedirect is a tool to redirect news, search, widgets, weather, and more to a user's default browser. MSEdgeRedirect versions before 0.5.0.1 are vulnerable to Remote Code Execution via specifically crafted URLs. This vulnerability requires user interaction and the acceptance of a prompt. With how MSEdgeRedirect is coded, parameters are impossible to pass to any launched file. However, there are two possible scenarios in which an attacker can do more than a minor annoyance. In Scenario 1 (confirmed), a user visits an attacker controlled webpage; the user is prompted with, and downloads, an executable payload; the user is prompted with, and accepts, the aforementioned crafted URL prompt; and RCE executes the payload the user previously downloaded, if the download path is successfully guessed. In Scenario 2 (not yet confirmed), a user visits an attacked controlled webpage; the user is prompted with, and accepts, the aforementioned crafted URL prompt; and a payload on a remote, attacker controlled, SMB server is executed. The issue was found in the _DecodeAndRun() function, in which I incorrectly assumed _WinAPI_UrlIs() would only accept web resources. Unfortunately, file:/// passes the default _WinAPI_UrlIs check(). File paths are now directly checked for and must fail. There is no currently known exploitation of this vulnerability in the wild. A patched version, 0.5.0.1, has been released that checks for and denies these crafted URLs. There are no workarounds for this issue. Users are advised not to accept any unexpected prompts from web pages.

Metrics

CVSS 3.1

8.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

EPSS Probability

3.33%

87.1th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-610

Affected Software

Vendor	Product	Versions
Msedgeredirect Project	Msedgeredirect	< 0.5.0.1

References

https://github.com/rcmaehl/MSEdgeRedirect/releases/tag/0.5.0.1Release Notes, Third Party Advisory
https://github.com/rcmaehl/MSEdgeRedirect/security/advisories/GHSA-95v4-748v-fmf9Exploit, Third Party Advisory
https://github.com/rcmaehl/MSEdgeRedirect/releases/tag/0.5.0.1Release Notes, Third Party Advisory
https://github.com/rcmaehl/MSEdgeRedirect/security/advisories/GHSA-95v4-748v-fmf9Exploit, Third Party Advisory

Timeline

Published: Dec 20, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-43844?

MSEdgeRedirect is a tool to redirect news, search, widgets, weather, and more to a user's default browser. MSEdgeRedirect versions before 0.5.0.1 are vulnerable to Remote Code Execution via specifically crafted URLs. This vulnerability requires user interaction and the acceptance of a prompt. With how MSEdgeRedirect is coded, parameters are impossible to pass to any launched file. However, there are two possible scenarios in which an attacker can do more than a minor annoyance. In Scenario 1 (confirmed), a user visits an attacker controlled webpage; the user is prompted with, and downloads, an executable payload; the user is prompted with, and accepts, the aforementioned crafted URL prompt; and RCE executes the payload the user previously downloaded, if the download path is successfully guessed. In Scenario 2 (not yet confirmed), a user visits an attacked controlled webpage; the user is prompted with, and accepts, the aforementioned crafted URL prompt; and a payload on a remote, attacker controlled, SMB server is executed. The issue was found in the _DecodeAndRun() function, in which I incorrectly assumed _WinAPI_UrlIs() would only accept web resources. Unfortunately, file:/// passes the default _WinAPI_UrlIs check(). File paths are now directly checked for and must fail. There is no currently known exploitation of this vulnerability in the wild. A patched version, 0.5.0.1, has been released that checks for and denies these crafted URLs. There are no workarounds for this issue. Users are advised not to accept any unexpected prompts from web pages.

How severe is CVE-2021-43844?

CVE-2021-43844 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 3.33% probability of exploitation in the next 30 days.

How do I fix CVE-2021-43844?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.