CVE-2026-42874 in microdot
Summary
by MITRE • 05/11/2026
Microdot is a minimalistic Python web framework. Prior to 2.6.1, the Response.set_cookie() method does not sanitize its string arguments, and in particular will not detect the presence of the \r\n sequence in them. This can be a potential source of header injection attacks. For a header injection attack through this issue to be possible, an attacker must first infiltrate the client (for example through an independent XSS attack), so that it can send malicious information that is destined to be stored in a cookie by the server on behalf of the victim. An attacker that infiltrates one client can only orchestrate a header injection attack for that client, all other clients that were not infiltrated are safe. This vulnerability is fixed in 2.6.1.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 05/12/2026
The Microdot web framework presents a significant security vulnerability in its Response.set_cookie() method prior to version 2.6.1, which stems from inadequate input sanitization of string arguments. This flaw creates a potential header injection attack vector that operates through the manipulation of HTTP response headers. The vulnerability specifically occurs when the \r\n sequence is present in cookie values, which the method fails to detect or sanitize properly. This represents a classic case of insufficient input validation that can lead to header manipulation attacks, where malicious data can be injected into HTTP response headers through improperly sanitized cookie values. The issue falls under the CWE-113 category of Improper Neutralization of CRLF Sequences in HTTP Headers, which is a well-documented weakness in web application security. The vulnerability requires an initial compromise of a client system to be effective, typically achieved through cross-site scripting attacks that allow attackers to inject malicious content into the victim's browser. This prerequisite makes the attack more targeted and limited in scope, as it only affects individual client sessions that have been compromised rather than presenting a broad system-wide vulnerability.
The operational impact of this vulnerability extends beyond simple header injection, as it can potentially enable more sophisticated attacks such as cache poisoning, session hijacking, or even redirection attacks that exploit the HTTP header manipulation capabilities. When an attacker successfully injects malicious headers through compromised cookie values, they can manipulate the browser's behavior in ways that may not be immediately apparent to users. The attack chain typically begins with an XSS compromise of a victim's browser session, followed by the injection of malicious data into cookie values that are then processed by the vulnerable set_cookie() method. This creates a scenario where the framework's response headers become contaminated with attacker-controlled data, potentially allowing for unauthorized redirects, content injection, or other header-based attacks that can compromise user sessions. The vulnerability's limited scope means that each compromised client requires a separate attack vector, but the potential for damage within individual sessions remains significant. The fix implemented in version 2.6.1 addresses this by properly sanitizing cookie values and detecting the presence of carriage return and line feed sequences that could be exploited for header injection.
Security practitioners should consider this vulnerability in the context of the broader ATT&CK framework, particularly the T1566 technique of Phishing and T1190 technique of Exploit Public-Facing Application, as it represents a server-side vulnerability that can be exploited through client-side compromise. The vulnerability demonstrates the importance of proper input validation and sanitization in web frameworks, as it shows how seemingly innocuous data handling can create security risks. Organizations using Microdot should prioritize updating to version 2.6.1 or later to mitigate this risk, while also implementing additional security measures such as Content Security Policy headers and proper XSS protection to prevent the initial client compromise that enables this attack. The vulnerability also highlights the need for comprehensive security testing of web application frameworks, particularly focusing on HTTP header handling and cookie management functions that are often overlooked in traditional security assessments. This type of vulnerability serves as a reminder that even minimalistic frameworks can contain significant security risks when proper input validation and sanitization are not implemented consistently throughout all methods and functions.