Subject emailAddress isn't checked against rfc822Name constraints when the leaf has no subjectAltName

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
22/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
rust
Domain
security

Research direction

Start with check_name_constraints in src/subject_name/mod.rs, which checks SAN entries and the subject directory name but never extracts the subject emailAddress when there is no subjectAltName. Run the linked gist repro (cargo run) to see forbidden_no_san accepted while forbidden_email_san is rejected, and compare with the openssl verify control. Done means the no-SAN case is rejected under a permitted rfc822Name constraint, with the RFC 5280 section 4.2.1.10 behaviour and related issues #445 and #70 taken into account.

Written by the indexing model from the issue text.

Description

Re-filing from GHSA-hc65-xjvf-77jj as asked.

rustls-webpki 0.103.16 (and main) accepts a certificate whose subject emailAddress is outside its issuer's permitted rfc822Name subtree when the leaf has no subjectAltName. If the same email is in a SAN, the constraint fails closed as expected.

As @ctz pointed out on the advisory, webpki has no API for asserting email identities and ignores names in the subject. So the impact is narrow: webpki accepts a constrained chain that isn't valid under its constraints. It only matters if something else extracts the subject email from a chain webpki has validated.

Details

RFC 5280 section 4.2.1.10 applies rfc822Name constraints to an email address in the subject DN when there's no subjectAltName. check_name_constraints checks the SAN entries plus the subject as a directory name. A directory name doesn't match an rfc822Name subtree, so it's skipped, and the subject emailAddress is never pulled out.

An email SAN under an email constraint hits the unsupported-name path and is rejected. The no-SAN subject email never reaches that path.

Related: #445 (email constraint support) and #70 (RFC 5280 deviations).

Repro

https://gist.github.com/DicksonWu654/7cb644c9485e48118a11899867fee090

cargo run builds against rustls-webpki = "=0.103.16" with the certs embedded. A root signs an intermediate with a critical permitted rfc822Name subtree for allowed.test, which signs three client-auth leaves:

allowed_no_san: accepted
forbidden_no_san: accepted
forbidden_email_san: rejected (NameConstraintViolation)

forbidden_no_san has emailAddress = [email protected] in the subject and no SAN. openssl verify on the same chain fails with error 47 (permitted subtree violation). The README has a one-liner for that.

Possible fix

When the leaf has no subjectAltName, treat subject emailAddress attributes like SAN emails for constraint purposes. With the current fail-closed handling, that means rejecting the chain when an email constraint is present.

I used AI tools (OpenAI Codex and Anthropic Claude) to find this, build the test and draft the report. The test and the OpenSSL control were re-run against 0.103.16 before the original report, and the cited code was checked against source.

Dominant language
Rust
Stars
167
Forks
112
Avg merge
4h 28m
Merged PRs (30d)
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