CVE-2026-88837 in BusyBox
Summary
by MITRE • 09/23/2026
BusyBox httpd treats yescrypt ($y$) password hashes as plaintext during Basic Authentication, inverting the authentication check.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 09/23/2026
The vulnerability identified in BusyBox HTTPD represents a critical failure in the implementation of HTTP Basic Authentication mechanisms when handling modern cryptographic hash formats. Specifically, the web server component fails to properly parse or validate yescrypt password hashes, which are denoted by the prefix $y$. Instead of processing these hashed values through the appropriate verification algorithm, the system treats the entire string representing the hash as a literal plaintext password candidate for comparison against user input. This fundamental logic error inverts the authentication check, creating a severe security flaw that undermines the integrity of access controls provided by this widely used embedded systems utility.
From a technical perspective, HTTP Basic Authentication relies on comparing credentials submitted by the client with stored credential data. In secure implementations, stored passwords are never kept as plaintext but rather as cryptographic hashes generated using algorithms such as bcrypt, scrypt, or yescrypt. When an authentication request arrives, the server extracts the hash identifier and applies the corresponding algorithm to verify if the provided password matches the stored value. However, in this vulnerable scenario, BusyBox HTTPD does not recognize the $y$ prefix as a signal to invoke the yescrypt verification routine. Consequently, it proceeds with a direct string comparison between the user-supplied plaintext password and the literal text of the hash itself. This means that authentication succeeds only if an attacker knows or can guess the exact string representation of the stored hash, effectively bypassing all cryptographic protection intended by the use of hashing in the first place.
The operational impact of this vulnerability is significant, particularly given BusyBox's prevalence in resource-constrained environments such as IoT devices, routers, and embedded appliances where security configurations are often minimal or default. If an administrator has configured authentication using yescrypt hashes for any protected directory within the HTTP server, attackers can potentially gain unauthorized access by supplying specific crafted inputs that match the hash string rather than the actual password. While this does not allow arbitrary code execution directly, it constitutes a complete bypass of authentication controls. This flaw aligns with CWE-287 Improper Authentication and CWE-521 Weak Password Requirements if combined with weak initial setup practices, but more critically reflects CWE-345 Insufficient Verification of Data Authenticity or logic flaws akin to CWE-602 Client-Side Enforcement of Server-Side Security. The inversion of the check means that valid credentials are rejected while specific invalid inputs may be accepted, leading to unpredictable and insecure access patterns.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. Administrators should immediately upgrade BusyBox to a version where this parsing logic has been corrected to properly handle yescrypt hashes or switch to using plaintext passwords for testing purposes only if the environment is isolated and non-production, though relying on plaintext in production is never recommended. Alternatively, administrators can migrate away from yescrypt hashing within BusyBox HTTPD until support is fully stabilized, opting instead for other supported hash formats that are correctly implemented by the current version of the software. For environments where upgrading is not immediately feasible, placing a reverse proxy or Web Application Firewall (WAF) in front of the BusyBox instance can provide an additional layer of authentication and validation, effectively shielding the vulnerable backend from direct exploitation attempts. This vulnerability highlights the risks associated with using lightweight utilities for security-sensitive tasks without rigorous verification of their cryptographic implementations against modern standards such as those outlined by NIST SP 800-132 regarding password hashing techniques.