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OpenBSD sppp_pap_input: PAP authentication bypass
From: shj
Date: Tue, 16 Jun 2026 21:27:44 +0200
------------------------------------------------------------------------ OpenBSD sppp_pap_input: PAP Authentication Bypass via Zero-Length bcmp ------------------------------------------------------------------------ Affected: OpenBSD all versions through 7.6 (fixed in -current) Vendor: OpenBSD Severity: High Reporter: Argus Date: 2026-06-16 1. SUMMARY ========== The sppp_pap_input() function in sys/net/if_spppsubr.c uses the attacker-controlled name_len and passwd_len fields from the incoming PAP frame directly as the comparison length for bcmp() against configured credentials. When both fields are set to zero, bcmp() returns 0 unconditionally (bcmp with length 0 always succeeds). The existing upper-bound guard (> AUTHMAXLEN) allows zero through. As a result, a PAP Auth-Request with name_len=0 and passwd_len=0 passes credential validation and triggers a PAP_ACK, authenticating the peer without any knowledge of the configured username or password. A secondary kernel heap over-read exists via the same root cause: supplying a name_len larger than the allocation of the stored credential causes bcmp to read past the heap object. 2. AFFECTED VERSIONS ==================== The bcmp comparison pattern was introduced with the original sppp code import on 1999-07-01 (commit bda3414e, "lmc driver; ported by chris () dqc org"). The zero-length bypass has been exploitable since that date. In February 2009 (commit 9c2f3d605fc), auth credential fields were changed from fixed-size struct arrays to dynamically allocated malloc(strlen()+1), and the bounds check was changed to > AUTHMAXLEN (256). This decoupled the allocation size from the comparison bound, enabling the heap over-read. Confirmed against OpenBSD 7.6 (amd64) in QEMU/KVM. 3. DETAILS ========== Vulnerable code (sys/net/if_spppsubr.c, sppp_pap_input): if (name_len > AUTHMAXLEN || passwd_len > AUTHMAXLEN || bcmp(name, sp->hisauth.name, name_len) != 0 || bcmp(passwd, sp->hisauth.secret, passwd_len) != 0) { /* authentication failed */ name_len and passwd_len are parsed directly from the PAP frame payload. bcmp(a, b, 0) always returns 0. The > AUTHMAXLEN guard rejects values above 255 but permits zero. The CHAP handler in the same file already had the correct pattern with an exact-length pre-check: if (name_len != strlen(sp->hisauth.name) || bcmp(name, sp->hisauth.name, name_len) != 0) { The PAP handler never received the same treatment. 4. REACHABILITY =============== Both bugs are reachable via the PPPoE data path: pppoe_data_input -> pppoeintr -> sppp_input -> sppp_pap_input Precondition: the target system must be configured as a PAP authenticator (e.g. ifconfig pppoe0 peerproto pap peername peerkey ). The attacker does not need to know any credentials. 5. IMPACT ========= An attacker on the same network segment can authenticate to a PPPoE interface without credentials, establishing a full network-layer link (LCP -> PAP -> IPCP -> IP). When OpenBSD acts as a PPPoE client with mutual authentication, a rogue server in the same broadcast domain can exploit the bypass to impersonate a legitimate server, causing OpenBSD to route traffic through the attacker's endpoint. 6. PROOF OF CONCEPT =================== A Python PoC acts as a PPPoE server, completes discovery and LCP negotiation, then sends a PAP Auth-Request with name_len=0 and passwd_len=0. Result: PAP_ACK received with empty credentials VM accepted name_len=0, passwd_len=0 as valid auth. IPCP Config-Ack received - link is UP ICMP echo reply from 10.0.0.1 FULL LINK ESTABLISHED PoC and full technical report: https://blog.argus-systems.ai/blog/openbsd-pap-27-year-auth-bypass.html 7. FIX ====== Fixed in -current by mvs on 2026-06-14. The fix mirrors the CHAP handler's exact-length pre-check: if (name_len != strlen(sp->hisauth.name) || passwd_len != strlen(sp->hisauth.secret) || bcmp(name, sp->hisauth.name, name_len) != 0 || bcmp(passwd, sp->hisauth.secret, passwd_len) != 0) { Fix commit: https://github.com/openbsd/src/commit/076e2b1c1fc4ac0883a72d3544131ad5cee7adf8 8. TIMELINE =========== 2026-06-12 Reported to security () openbsd org with PoC 2026-06-14 Fix committed to -current 9. CREDIT ========= Discovered and reported by Argus (https://byteray.co.uk/ 10. REFERENCES ============== Advisory: https://pop.argus-systems.ai/advisory/adv-038.html Blog post: https://blog.argus-systems.ai/blog/openbsd-pap-27-year-auth-bypass.html Proof of concept: https://pop.argus-systems.ai/attachments/poc-001-pap-bypass.py _______________________________________________ Sent through the Full Disclosure mailing list https://nmap.org/mailman/listinfo/fulldisclosure Web Archives & RSS: https://seclists.org/fulldisclosure/
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- OpenBSD sppp_pap_input: PAP authentication bypass shj (Jun 20)