EUVD-2026-37521Severity by source
AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Adjacent PPP-link reach (AV:A) and no auth (PR:N); a zero-length field is trivial to craft so AC:L versus NVD's AC:H; auth bypass yields authorized network access, giving scope change and high CIA.
Primary rating from Vendor (mitre).
CVSS VectorNVD
Lifecycle Timeline
9DescriptionNVD
sppp_pap_input in sys/net/if_spppsubr.c in OpenBSD before 076e2b1 allows authentication bypass via certain zero values for lengths.
AnalysisAI
Authentication bypass in OpenBSD's sppp PAP handler (sppp_pap_input in sys/net/if_spppsubr.c) lets an adjacent attacker on a PPP link authenticate without valid credentials by supplying zero-length name/password fields, which the kernel only bounds-checked against an upper limit (AUTHMAXLEN) instead of the exact expected credential length. All OpenBSD builds before commit 076e2b1 that terminate PPP sessions with PAP authentication are affected. Publicly available exploit code exists (an Argus Systems PoC script) and a vendor fix is available, though EPSS is low (0.21%, 12th percentile) and the flaw is not in CISA KEV, so this is proof-of-concept rather than confirmed active exploitation.
Technical ContextAI
The bug lives in OpenBSD's synchronous/asynchronous PPP subsystem (sppp), which implements PAP (Password Authentication Protocol, RFC 1334) inside the kernel network stack. When a peer sends a PAP Authenticate-Request, sppp_pap_input parses a name field and a password field, each preceded by a length byte. The vulnerable code validated only that name_len and passwd_len did not exceed AUTHMAXLEN, never that they matched the length of the locally configured expected credential (sp->hisauth.name / sp->hisauth.secret). This is a classic CWE-1284 (Improper Validation of Specified Quantity in Input): with a length of zero, the subsequent comparison against the configured secret degenerates and the credential check passes, granting a successful PAP authentication. The fix (commit 076e2b1c1fc4ac0883a72d3544131ad5cee7adf8, revision 1.202) replaces the '> AUTHMAXLEN' bounds checks with strict equality checks 'name_len != strlen(sp->hisauth.name)' and 'passwd_len != strlen(sp->hisauth.secret)', rejecting any request whose field lengths do not exactly match the expected credential. The affected CPE is cpe:2.3:a:openbsd:openbsd, and reporting notes the logic error persisted for roughly 27 years.
RemediationAI
Apply the vendor fix by updating sys/net/if_spppsubr.c to revision 1.202 or later (commit 076e2b1c1fc4ac0883a72d3544131ad5cee7adf8) and rebuilding the kernel, or install an OpenBSD errata/syspatch or release build that incorporates it - this is the primary and complete remediation, since the patch enforces exact-length credential validation (see https://github.com/openbsd/src/commit/076e2b1c1fc4ac0883a72d3544131ad5cee7adf8 and the disclosure at https://www.openwall.com/lists/oss-security/2026/06/16/9). Because the released tagged patched build should be confirmed against OpenBSD errata, treat the commit as the authoritative fix reference. If you cannot patch immediately, the most effective compensating control is to avoid PAP: switch affected PPP links to CHAP authentication, which does not traverse the vulnerable sppp_pap_input path (trade-off: requires peer support and reconfiguration of both ends). Where PAP cannot be removed, restrict the PPP link to trusted physical/adjacent peers and place the concentrator behind link-layer access controls so untrusted parties cannot present PAP requests (trade-off: does not help against a malicious or spoofed peer already on the link). Disabling or removing sppp/PPP interfaces entirely eliminates exposure but breaks any dependent connectivity.
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Same technique Authentication Bypass
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External POC / Exploit Code
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