JWT Algorithm Confusion (RS256 → HS256 / none)
jwt-alg-confusion is a critical severity check in the
web category, written in python.
Its source is templates/web/jwt/jwt-alg-confusion.py in
cert-x-gen-templates.
What it checks
Section titled “What it checks”Tests for JWT algorithm confusion attacks — forging tokens by switching RS256 to HS256 (using the public key as HMAC secret) and the none-algorithm bypass. Requires a valid bearer token from pipeline context.
Header
Section titled “Header”| Field | Value |
|---|---|
| Id | jwt-alg-confusion |
| Severity | critical |
| Language | python |
| Category | web |
| Author | BugB Security Team |
| Template version | 1.0.0 |
| Confidence | 85 |
| CVSS | 9.1 |
| Weakness | CWE-327 |
| Tags | jwt, algorithm-confusion, authentication, owasp-a02, token-forgery, none-alg |
| Target kind | not recorded |
| Oracle | not recorded |
The file declares @auto_probe, @batch_group, @context_vars, @hypothesis_tags, @vuln_class as well. Those are not tabled above; read the
source for what they carry.
References
Section titled “References”Declared in the header. cxg parses @references and then discards it, and
nothing at scan time reads it, so this is the only place the links a
template cites are surfaced.
- https://portswigger.net/web-security/jwt/algorithm-confusion
- https://cwe.mitre.org/data/definitions/327.html
- https://owasp.org/API-Security/editions/2023/en/0xa2-broken-authentication/
Run it
Section titled “Run it”To see what the copy on your machine says about itself, and to confirm it is installed at all:
cxg template info jwt-alg-confusionThe id it prints is the one to pass anywhere a template is selected. See
cxg template for the rest of
the subcommand, Scan a target for running
a scan, and
A match is not a finding for how
to read what comes back.
Related
Section titled “Related”- Template catalog is the whole corpus.
- web is the rest of this category.
- Templates by weakness class groups checks by the defect they look for.
- Contribute a template is the route from a check you needed and could not find to a merged one.