CVE-2026-76504, Cisco Catalyst SD-WAN Manager: one percent-encoded character in the login path skips the password check and mints an admin API session, exploited in the wild (CVSS 9.8)
Cisco confirms exploitation of an unauthenticated admin bypass in the SD-WAN controller; no workaround, fixes per train
Defender actions
- Upgrade every Catalyst SD-WAN Manager to the first fixed release of its train (20.9.10.1, 20.12.8.2, 20.15.6.1, 20.18.4.1, 26.1.2.1 or 26.2.1; anything earlier than 20.9 must migrate) and, until the upgrade is done, restrict the Manager's web and API access to known trusted hosts behind a firewall; Cisco's Live Protect shield for this CVE adds only temporary, partial protection and may block legitimate logins that use URI encoding.
- On every Manager that was internet-reachable before the upgrade, search serviceproxy-access.log for POSTs to the login path with a percent-encoded character from unknown addresses and vmanage-server.log for login-path requests tied to viptela-reserved- user names, and open a Severity 3 Cisco TAC case with the CVE id in the title if anything matches.
Analysis
Cisco published an advisory on 2026-09-30 for CVE-2026-76504, an authentication bypass in the API session management of Cisco Catalyst SD-WAN Manager that lets an unauthenticated remote attacker reach the API with the privileges of the admin user (Cisco PSIRT, 2026-09-30). Cisco states its PSIRT became aware of active exploitation in September 2026 (Cisco PSIRT, 2026-09-30), CISA added the CVE to its Known Exploited Vulnerabilities catalog the same day (CISA, 2026-09-30), and no source names an actor. The flaw affects the Manager regardless of configuration and Cisco lists no workaround (Cisco PSIRT, 2026-09-30); Cisco's revision of 2026-10-02 adds a Live Protect shield that it describes as temporary, partial protection, and says the only way to remediate is to upgrade to the first fixed release (Cisco PSIRT, 2026-09-30).
VulnCheck's reproduction explains the mechanism. The application server decodes the request path when it matches its login security constraint, but Cisco's login module tests the raw, undecoded path for the string j_security_check before it decides whether to run the real password check, so a request to /%6a_security_check skips the password check (VulnCheck, 2026-10-01). The request then falls into a branch written for continuing an already trusted session, whose only gate is a substring match against four internal viptela-reserved- account names, and supplying any of them builds a fresh login session (VulnCheck, 2026-10-01). Two of the four accounts hit role checks on admin-only endpoints and two are expected to carry the needed permissions (VulnCheck, 2026-10-01). Cisco stresses that any single encoded character works, not only %6a (Cisco PSIRT, 2026-09-30). VulnCheck counts about 1,500 internet-exposed Managers and found no legitimate public exploit as of 2026-10-01; it rejected a fake proof-of-concept repository (VulnCheck, 2026-10-01).
Cisco's first fixed releases are 20.9.10.1, 20.12.8.2, 20.15.6.1, 20.18.4.1, 26.1.2.1 and 26.2.1; releases earlier than 20.9 must migrate to a fixed train, and the Cisco-managed cloud service (release 20.15.605) needed no customer action (Cisco PSIRT, 2026-09-30). NCSC Switzerland posted its own advisory on 2026-09-30 and lists the flaw as actively exploited (NCSC Switzerland, 2026-09-30).
Triage: Cisco warns that these log entries can also occur during standard operation and must be assessed against normal network posture; a viptela-reserved- name is an internal service account, so the signal is a login-path request with an encoded character from an address that is not part of the fabric (Cisco PSIRT, 2026-09-30).
Cited evidence
In September 2026, the Cisco PSIRT became aware of active exploitation of this vulnerability.
This vulnerability affects Cisco Catalyst SD-WAN Manager, regardless of system configuration.
there are no legitimate public exploits for CVE-2026-76504 as of October 1, 2026
This shield offers only temporary partial protection.
A legitimate user with URI encoding might not be able to login to SDWAN Manager.
Updates1
Cisco revised its advisory to version 1.1 on 2026-10-02 ("Added information about Live Protect shield availability") and now states it has released a Live Protect shield for CVE-2026-76504 "to provide temporary security coverage" while upgrades are planned (Cisco PSIRT, 2026-09-30). Cisco says the shield offers only temporary partial protection and that, once applied, a legitimate user with URI encoding might not be able to log in to the Manager (Cisco PSIRT, 2026-09-30). Cisco's Live Protect page lists the feature from Catalyst SD-WAN Control Components release 20.18.3, describes a monitoring mode that shows exploit attempts without enforcing and an enforce mode that applies the mitigation, and says Cisco creates shields for the release current when a shield is published and for the two immediately preceding releases in each supported release train that includes Live Protect, so older releases may have no shield (Cisco, 2026-07-01). The fixed releases, the exploitation statement and the KEV listing are unchanged, and Cisco keeps its position that the only remediation is the upgrade.
Sources5
Revision history
- Published 2026-10-02T0404Z-intel
- Update 2026-10-03T0404Z-intel
Cisco revised the advisory on 2026-10-02 (version 1.1) to add a Live Protect shield for this CVE, which it describes as temporary, partial protection that may block legitimate logins using URI encoding. The first fixed releases are unchanged and the upgrade remains Cisco's only remediation.
Changed: summary immediate_action actions body sources evidence
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