Got an OpenBSD Web server? Better patch it

DoS-able bugs splatted

OpenBSD and two of its SSL libraries need patches against a pair of denial-of-service bugs that can crash Web-facing servers.

The first is in the operating system's SSL implementation, specifically in the HTTP daemon. An advisory says that daemon can be crashed with repeated SSL renegotiation.

A single renegotiation thread, the post claims, can soak up 70 per cent of CPU cycles, meaning if the attacker fires multiple renegotiation threads at the target, the daemon will crash, and “there is no trace of such attacks in the httpd logs.

The second, which has been given the common vulnerabilities and exposures number CVE-2017-5850, is a memory exhaustion bug, again in the HTTP daemon.

“Requesting file using a file-range will result in having a httpd process doing a full malloc() of the requested file,” the report states. “It appears the entry is not correctly free()'d.”

(Malloc() and free() are memory management calls in the standard C library.)

“Hence, it's possible to DoS the remote server by requesting a file over and over by specifying a custom file range”.

The other requirement for this attack is for the attacker to identify a file larger than around 10 MB served by the victim's machine.

OpenBSD has responded to the two issues. The memory exhaustion bug is dealt with in bug fixes outlined here for version 6.0, or here for version 5.9.

The SSL renegotiation bug is in the LibreSSL implementation used by OpenBSD. The fix is in patches in the SSL and TLS libraries so sysadmins can block client-initiated renegotiation. ®


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