CVE-2026-48758

@sigstore/core has DSSE payloadType type-binding failure

Reproduction

const { preAuthEncoding } = require('@sigstore/core/dist/dsse.js');
const payload = Buffer.from('hello world');

const original = preAuthEncoding('text/plain', payload);
// U+01xx chars whose low bytes match the original ASCII chars
const mutant = preAuthEncoding('\u0174\u0165\u0178\u0174/\u0170\u016c\u0161\u0169\u016e', payload);

console.log('PAE bytes equal:', original.equals(mutant)); // true, should be false

The preAuthEncoding function in @sigstore/core uses Node.js 'ascii' encoding when converting the PAE (Pre-Authentication Encoding) string to bytes. This allows payloadType to be mutated after signing without invalidating the signature, breaking the type-binding guarantee that DSSE is designed to provide.

In packages/core/src/dsse.ts, the PAE function builds a string containing payloadType and then encodes it with Buffer.from(prefix, 'ascii').

In Node.js, 'ascii' encoding for string-to-Buffer is equivalent to 'latin1', which truncates characters above U+00FF to their low byte. This means for any ASCII character, there exist Unicode characters (at U+01xx, U+02xx, etc.) that produce the identical encoded byte:

An attacker can substitute every character in payloadType with a Unicode variant whose low byte matches, producing identical PAE bytes and a passing signature verification.

Additionally, payloadType.length returns the JavaScript string length (UTF-16 code units) rather than the UTF-8 byte length required by the DSSE spec, though this is only a contributing factor for non-ASCII types.

CVE-2026-48758 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.2.1); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2026-48758? CVE-2026-48758 is a medium-severity security vulnerability in @sigstore/core (npm), affecting versions <= 3.2.0. It is fixed in 3.2.1.
2. How severe is CVE-2026-48758? CVE-2026-48758 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of @sigstore/core are affected by CVE-2026-48758? @sigstore/core (npm) versions <= 3.2.0 is affected.
4. Is there a fix for CVE-2026-48758? Yes. CVE-2026-48758 is fixed in 3.2.1. Upgrade to this version or later.
5. Is CVE-2026-48758 exploitable, and should I be worried? Whether CVE-2026-48758 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-48758 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-48758? Upgrade @sigstore/core to 3.2.1 or later.