Knowledge Search


2012-07 Security Bulletin: Junos: Incorrect behavior of SYN Cookie protection

  [JSA10517] Show Article Properties

Legacy Advisory Id:
Product Affected:
This issue can affect all SRX Series services gateways running Junos OS 11.2 or later with SYN flood protection enabled.

If SYN flood protection is enabled and the SYN Cookie protection threshold is exceeded, then the follow-on special additional handling for the handshake fails. The result is that the server sees an ACK for an unrecognized connection, and responds with a RST, causing the far end to shut down the connection. Note that SYN Cookie protection is enabled by default.

This issue only affect Junos 11.2 and later releases. Versions prior to 11.2 are unaffected by this vulnerability.

Juniper SIRT is not aware of any malicious exploitation of this vulnerability.

No other Juniper Networks products or platforms are affected by this issue.

Prior to the fix, the first "cookied" ACK from the client was not delivered to the SYN proxy plugin. The ACK is now delivered so that the first SYN can be sent to server.

All Junos OS software releases built on or after 2012-06-21 have fixed this specific issue. Releases containing the fix specifically include: 11.4R4, 12.1R2-S1, and all subsequent releases (i.e. all releases built after 12.1R2-S1).

This issue is being tracked as PR 755727 and is visible on the Customer Support website.

KB16765 - "In which releases are vulnerabilities fixed?" describes which release vulnerabilities are fixed as per our End of Engineering and End of Life support policies.

Disable SYN Cookie protection by switching to SYN proxy mode. For example:
user@host# set security flow syn-flood-protection-mode syn-proxy

How to obtain fixed software:

Security vulnerabilities in Junos are fixed in the next available Maintenance Release of each supported Junos version. In some cases, a Maintenance Release is not planned to be available in an appropriate time-frame. For these cases, Service Releases are made available in order to be more timely. Security Advisory and Security Notices will indicate which Maintenance and Service Releases contain fixes for the issues described. Upon request to JTAC, customers will be provided download instructions for a Service Release. Although Juniper does not provide formal Release Note documentation for a Service Release, a list of "PRs fixed" can be provided on request.
Related Links:
CVSS Score:
7.8 (AV:N/AC:L/Au:N/C:N/I:N/A:C)
Severity Level:
Severity Assessment:
Information for how Juniper Networks uses CVSS can be found at KB 16446 "Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories."