CVE-2026-109841: undici vulnerable to Subresource Integrity bypass via non-lowercase algorithm tokens
Impact
undici's fetch() verifies Subresource Integrity (SRI) metadata by looking up the algorithm token in a lowercase-only set, but the SRI specification defines a valid token by its ASCII-lowercase form. A correct but non-lowercase token such as SHA256-... or Sha256-... is therefore treated as unknown and skipped. When no recognized algorithm remains, undici treats the integrity list as empty and accepts the response, so a server that returns a body different from the pinned digest passes the check.
An application that pins a response with a valid uppercase or mixed-case SRI token receives attacker-controlled bytes with no integrity error. The attacker needs only to control the response body, not the application's integrity metadata. The issue affects response integrity only. The regression was introduced in undici 7.15.0; releases 7.14.0 and earlier lowercase the token and are not affected.
Patches
Upgrade to 7.30.1 or 8.12.2. undici now normalizes the algorithm token to ASCII lowercase before validating it and before selecting the strongest algorithm, so case variants of SHA-256, SHA-384, and SHA-512 are verified correctly.
Workarounds
Use lowercase algorithm tokens (sha256, sha384, sha512) in integrity metadata.
Other sources
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.
— Ubuntu
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 8.12.2 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 7.30.1 - Upgrade
Upgrade
debian/node-undicito a version that resolves this vulnerability.Fixed in 5.15.0+dfsg1+~cs20.10.9.3-1+deb12u4Fixed in 5.15.0+dfsg1+~cs20.10.9.3-1+deb12u3Fixed in 7.3.0+dfsg1+~cs24.12.11-1Fixed in 8.12.2+dfsg+~cs3.2.3-2 - Upgrade
Upgrade
debian/node-undicito a version that resolves this vulnerability.Fixed in 8.12.2+dfsg+~cs3.2.3-1 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 7.30.1 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 8.12.2 - Configuration
Use lowercase algorithm tokens such as sha256, sha384, or sha512 in integrity metadata.
undici fetch() SRI integrity metadata algorithm tokens = lowercase sha256, sha384, or sha512
Event History
Frequently Asked Questions
Which applications are exposed to this bypass?
Applications using undici fetch() in versions 7.15.0 through 7.30.0 or 8.0.0 through 8.12.1 are exposed only when their integrity metadata contains a valid uppercase or mixed-case algorithm token. Lowercase tokens such as sha256, sha384, and sha512 are not affected by this parsing issue.
What does an attacker need to exploit it?
An attacker needs control of the HTTP response body for a request whose integrity metadata uses an affected non-lowercase algorithm token. No authentication or user interaction is required, but the attacker does not need to alter the integrity metadata itself.
What is the impact if exploitation succeeds?
The application can accept attacker-controlled response bytes even though they do not match the pinned digest, without an integrity error. The described impact is limited to response integrity; confidentiality and availability are not affected.
What can be done before upgrading?
Change all SRI algorithm tokens in integrity metadata to lowercase, such as sha256, sha384, or sha512. Upgrade undici to version 7.30.1 or later, or 8.12.2 or later, when possible.