CVE-2026-100674 in stoatchat
Summary
by MITRE • 09/26/2026
stoatchat before 0.15.5 fails to revalidate usernames after Unicode sanitization, allowing attackers to create usernames with forbidden characters by submitting Unicode letters that transform into rejected characters. Attackers can bypass character allowlists and length limits to create reserved-name lookalikes, embed special characters, and exceed the 32-character storage limit.
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Analysis
by VulDB Data Team • 09/26/2026
The vulnerability in stoatchat versions prior to 0.15.5 represents a critical authentication and access control failure rooted in improper handling of Unicode character normalization during username registration processes. The core technical flaw lies in the application's logic for sanitizing user input, specifically how it validates characters against an allowlist before final storage. When a user submits a username containing specific Unicode letters that visually resemble or normalize into forbidden ASCII characters, the system fails to revalidate the resulting normalized string after this transformation occurs. This oversight allows attackers to bypass character restrictions by exploiting the discrepancy between pre-sanitization input and post-normalization output. For instance, submitting a Unicode character that maps to an asterisk or space during processing enables the creation of usernames containing symbols that are explicitly prohibited in standard configurations.
This flaw facilitates several distinct attack vectors with significant operational impact on system integrity and security posture. First, attackers can create reserved-name lookalikes by using visually similar characters from different scripts, such as Cyrillic or Greek letters that resemble Latin alphabets. This technique is commonly associated with homograph attacks and can be leveraged for phishing campaigns where malicious actors impersonate legitimate users or administrators to deceive other system participants. Second, the ability to embed special characters allows attackers to potentially disrupt parsing logic in downstream applications that consume these usernames, leading to cross-site scripting if those names are rendered without proper encoding in web interfaces. Third, by exploiting Unicode normalization quirks, adversaries can exceed the 32-character storage limit imposed on username fields. This buffer overflow-like condition may lead to data corruption or unexpected behavior within database schemas and application logic that assumes strict adherence to length constraints.
From a classification perspective, this vulnerability aligns with CWE-79 Improper Neutralization of Input During Web Page Generation for Cross-Site Scripting if the usernames are reflected in HTML output without proper encoding. It also falls under CWE-20 Improper Input Validation due to the failure to validate input after transformation and CWE-611 Improper Restriction of XML External Entity Reference or similar injection flaws depending on how the data is processed internally. In terms of offensive security frameworks, this behavior maps to ATT&CK technique T1588.002 Obtain Defacement Capability via Vulnerability which relates to gaining unauthorized access through flawed validation logic, and potentially T1621 Multi-Factor Authentication Bypass if these usernames are used in social engineering attacks targeting multi-factor systems. The lack of re-validation after Unicode sanitization is a classic example of insufficient state management during input processing pipelines.
To mitigate this vulnerability, developers must implement strict post-normalization validation steps immediately following any character transformation or encoding operations. This involves ensuring that the final stored value adheres to all security policies including allowlists for characters and maximum length constraints regardless of how it was derived from user input. Implementing canonical normalization forms such as NFC (Normalization Form Canonical Composition) consistently throughout the application stack can help reduce ambiguity in character representation. Additionally, enforcing strict type checking and rejecting any username that does not match a predefined regular expression pattern after all transformations are complete will prevent bypass attempts. Security teams should also audit existing usernames for similar anomalies to identify potential compromised accounts or data integrity issues resulting from past exploitation of this flaw.