CVE-2026-91991 in Tornado
Summary
by MITRE • 09/15/2026
Tornado before 6.5.8 contains an incomplete fix for cookie attribute injection that allows attackers to inject arbitrary cookie attributes by passing capitalized or legacy keyword arguments to set_cookie. Attackers can embed semicolon-delimited data in capitalized parameters like Domain, Path, or SameSite to bypass validation and modify cookie security attributes.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in Tornado versions prior to 6.5.8 represents a critical flaw in the web framework's handling of HTTP cookie attributes, specifically stemming from an incomplete remediation of a previous cookie injection issue. This weakness allows attackers to manipulate how browsers store and transmit session cookies by injecting arbitrary attribute values through capitalized or legacy keyword arguments passed to the set_cookie method. The core technical failure lies in the validation logic which failed to adequately sanitize inputs when they were provided using non-standard casing, such as Domain, Path, or SameSite with initial capital letters, rather than the strictly lowercase variants typically expected by modern web standards and previous security patches.
From a technical perspective, HTTP cookies are structured key-value pairs that can include various attributes controlling their scope and transmission behavior. The set_cookie function in Tornado is designed to accept these attributes as keyword arguments to configure cookie properties like expiration time, domain restrictions, path limitations, and SameSite policies which enforce cross-site request forgery protections. However, the implementation contained a logic gap where it did not consistently normalize or validate all variations of these attribute names. By passing capitalized keys, an attacker could bypass internal checks that were only applied to lowercase equivalents. This oversight permitted the injection of semicolon-delimited data directly into the cookie header string, effectively allowing the modification of security-critical attributes such as Domain and SameSite without triggering standard validation errors.
The operational impact of this vulnerability is significant because it undermines fundamental web application security controls. If an attacker can successfully inject a custom Domain attribute, they may be able to set cookies that are accessible across multiple subdomains or even different domains if misconfigured, potentially leading to session hijacking or privilege escalation within the context of those broader scopes. Furthermore, manipulating the SameSite attribute is particularly dangerous as it directly affects cross-site request forgery defenses. By altering this value, an attacker could weaken protections against CSRF attacks, allowing malicious sites to trigger state-changing actions on behalf of authenticated users who visit them while logged into the vulnerable application. This can lead to unauthorized data access, financial fraud, or complete account compromise depending on the sensitivity of the affected services.
This flaw is categorized under CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component and aligns with ATT&CK techniques related to Client Side Injection and potentially Defense Evasion if used to bypass security controls like SameSite policies. The vulnerability highlights the importance of rigorous input validation that accounts for case-insensitivity in HTTP header fields, as browsers often treat these headers case-insensitively even when application code expects specific casing. To mitigate this risk, organizations must upgrade Tornado to version 6.5.8 or later where the fix ensures consistent normalization and strict validation of all cookie attribute inputs regardless of their capitalization. Additionally, developers should implement defense-in-depth strategies by validating cookie attributes at multiple layers including reverse proxies and application firewalls to catch any potential injection attempts that might slip through framework-level protections.