CVE-2026-39410: 
JavaScript vulnerability analysis and mitigation

CVE-2026-39410 is a cookie prefix bypass vulnerability in the Hono web framework's getCookie() function, caused by a discrepancy between browser cookie name parsing and Hono's internal parse() handling. Discovered by researcher tikitiki0370 and disclosed on April 7–8, 2026, it affects all Hono npm package versions prior to 4.12.12. The vulnerability carries a CVSS v3.1 base score of 4.8 (Medium) (GitHub Advisory). IBM products including Cloud Pak for AIOps, App Connect Enterprise, DevOps Solution Workbench, watsonx Orchestrate, and IBM Bob are also affected as downstream consumers of the vulnerable library (IBM Advisory).

The root cause (CWE-20: Improper Input Validation) lies in Hono's parse() function using JavaScript's native trim() method, which strips a broader set of whitespace characters — including the non-breaking space (U+00A0) — than browsers do. Browsers conforming to RFC 6265bis only trim SP (0x20) and HTAB (0x09) from cookie names, so they treat "dummy-cookie" and "\u00a0dummy-cookie" as distinct cookies. However, Hono's parse() normalizes both to "dummy-cookie", allowing an attacker-controlled cookie with a U+00A0 prefix to shadow or override the legitimate cookie when accessed via getCookie(). The fix replaces trim() with a custom trimCookieWhitespace() function that only strips SP and HTAB characters, aligning Hono's behavior with the RFC (GitHub Advisory, Patch Commit).

An attacker who can inject cookies — for example, via a man-in-the-middle attack on a non-HTTPS page or another cookie injection vector — can bypass __Secure- and __Host- cookie prefix protections, override cookies that rely on the Secure attribute, and potentially achieve session fixation or session hijacking in applications that use getCookie() for security-sensitive operations. The confidentiality and integrity impacts are both rated Low, with no availability impact, as the attacker can read or manipulate session state but cannot directly crash the service (GitHub Advisory).

No public proof-of-concept exploit code or evidence of in-the-wild exploitation has been reported. The EPSS score is approximately 0.03–0.04% (9th percentile), indicating a low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires the attacker to have the ability to set cookies on the target domain, which raises the attack complexity to High (GitHub Advisory).

1. Identify a cookie injection vector: Determine a means to set cookies on the target domain — for example, via a man-in-the-middle position on a non-HTTPS subdomain, an XSS vulnerability, or a related-domain cookie injection.
2. Craft a malicious cookie: Create a cookie whose name is the target cookie name prefixed with a non-breaking space (U+00A0), e.g., \u00a0session-token=attacker-controlled-value.
3. Inject the cookie: Deliver the crafted cookie to the victim's browser through the identified injection vector, so the browser stores both \u00a0session-token and the legitimate session-token as distinct cookies.
4. Trigger normalization: When the victim's browser sends a request to the Hono application, both cookies are included in the Cookie header. Hono's vulnerable parse() normalizes \u00a0session-token to session-token, potentially overriding the legitimate cookie value.
5. Achieve objective: Depending on application logic, the attacker's value is returned by getCookie('session-token'), enabling session fixation, bypassing __Secure-/__Host- prefix checks, or overriding security-sensitive cookie values (GitHub Advisory).

• Network: HTTP requests containing Cookie headers with cookie names prefixed by a non-breaking space character (U+00A0, hex %C2%A0 in URL encoding) alongside a legitimate same-named cookie.
• Logs: Application or web server access logs showing Cookie header values with %C2%A0 or the raw UTF-8 byte sequence 0xC2 0xA0 preceding a cookie name.
• Application Behavior: Unexpected session behavior such as session fixation events, authentication bypasses, or __Secure-/__Host- prefixed cookie checks failing in Hono-based applications.

Upgrade the Hono npm package to version 4.12.12 or later, which replaces the vulnerable trim() call with a RFC-compliant trimCookieWhitespace() function that only strips SP and HTAB characters (Hono Release, Patch Commit). IBM customers using affected products (Cloud Pak for AIOps, App Connect Enterprise, DevOps Solution Workbench, watsonx Orchestrate, IBM Bob) should apply the relevant IBM security bulletins (IBM Advisory). As a temporary workaround where upgrading is not immediately possible, applications should avoid relying solely on getCookie() for security-critical cookie validation and consider server-side session validation mechanisms that do not depend on cookie name matching.

The advisory was published by Hono maintainer yusukebe on April 7, 2026, as part of a batch security release (v4.12.12) that addressed five separate vulnerabilities. The release received positive community engagement on GitHub, with 12 users reacting to the release notes (Hono Release). No significant independent researcher commentary or major media coverage has been identified beyond the GitHub advisory and downstream IBM bulletins.