CVE-2026-109841 in undici
Summary
by MITRE • 10/15/2026
undici is an HTTP client library for Node.js. In versions 7.15.0 through 7.30.0 and 8.0.0 through 8.12.1, its fetch() implementation verifies Subresource Integrity metadata by matching the algorithm token against a lowercase-only set, even though the SRI specification defines the token by its ASCII-lowercase form, so a valid but non-lowercase token such as an uppercase or mixed-case SHA256 is treated as unknown and skipped. When no recognized algorithm remains, undici treats the integrity list as empty and accepts the response, so a server returning a body that does not match the pinned digest passes the check. An attacker who controls only the response body can therefore deliver attacker-controlled bytes to an application that pinned a valid uppercase or mixed-case SRI token, with no integrity error, affecting response integrity only. The issue is fixed in undici 7.30.1 and 8.12.2, which normalize the algorithm token to ASCII lowercase before validation, and users should upgrade to one of those versions or later. As a workaround, use lowercase algorithm tokens such as sha256, sha384, or sha512 in integrity metadata.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified within undici, an HTTP client library for Node.js, stems from a flaw in the implementation of Subresource Integrity (SRI) verification logic during versions 7.15.0 through 7.30.0 and 8.0.0 through 8.12.1. SRI is a security feature that enables browsers and HTTP clients to verify that fetched resources have not been tampered with by comparing cryptographic hashes of the received content against expected values embedded in the application code or configuration. In this specific case, the fetch implementation incorrectly validates Subresource Integrity metadata by strictly matching the algorithm token against a lowercase-only set. This behavior contradicts the SRI specification, which defines the algorithm tokens based on their ASCII-lowercase form but does not mandate that incoming tokens must be pre-normalized to lowercase before comparison. Consequently, when an integrity attribute contains a valid hash with an uppercase or mixed-case algorithm identifier, such as SHA256 instead of sha256, the library fails to recognize it as a known algorithm.
This technical flaw leads to a critical operational impact where the integrity check is effectively bypassed for any resource utilizing non-lowercase algorithm tokens. When undici encounters an unrecognized algorithm token due to case sensitivity issues, it treats the entire integrity list as empty rather than raising an error or attempting alternative validation methods. An attacker who controls only the response body can exploit this behavior by delivering malicious content that does not match the pinned digest. Because the library skips verification for these malformed tokens, the application accepts the unverified and potentially compromised response without triggering any integrity errors. This results in a loss of response integrity, allowing potential code injection or data manipulation attacks to proceed undetected within applications relying on this specific version range of undici for network requests.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation and CWE-345 Insufficient Verification of Data Authenticity. The failure to correctly normalize input before validation represents a classic case where strict adherence to implementation details overrides compliance with established security standards like the W3C Subresource Integrity specification. Furthermore, in the context of the MITRE ATT&CK framework, this flaw facilitates techniques related to Supply Chain Compromise or Intercepted Communication if an attacker can manipulate network responses targeting applications using these vulnerable versions. The impact is strictly limited to response integrity verification failures and does not extend to other aspects of HTTP processing such as header injection or request smuggling.
The issue has been resolved in undici versions 7.30.1 and 8.12.2, where the developers implemented a fix that normalizes the algorithm token to ASCII lowercase before performing validation checks. This ensures compliance with the SRI specification regardless of how the client initially receives or formats the integrity metadata. Users are strongly advised to upgrade their dependencies to one of these patched versions immediately to restore proper security guarantees. For environments where an immediate upgrade is not feasible, a recommended workaround involves ensuring that all Subresource Integrity attributes in application code use strictly lowercase algorithm tokens such as sha256, sha384, or sha512. This manual normalization prevents the library from encountering unrecognized uppercase variants and ensures that integrity checks are performed correctly until the underlying dependency can be updated.