Attachment 15920 Details for Bug 14334 – Advisory v2 with release versions

Advisory v2 with release versions

CVE-2020-10704-ldap-stack-v2.txt (text/plain), 3.91 KB, created by Andrew Bartlett on 2020-04-16 20:32:17 UTC

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Creator: Andrew Bartlett

Created: 2020-04-16 20:32:17 UTC

Size: 3.91 KB

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obsolete

>===========================================================
>== Subject:     LDAP Denial of Service (stack overflow) in
>==              Samba AD DC
>==
>== CVE ID#:     CVE-2020-10704
>==
>== Versions:    All versions of Samba since Samba 4.0.0
>==
>== Summary:     A deeply nested filter in an un-authenticated
>==              LDAP search can exhaust the LDAP server's stack
>==              memory causing a SIGSEGV.
>===========================================================
>
>===========
>Description
>===========
>
>LDAP is encoded as ASN.1, and LDAP filters are defined recursively as
>   Filter ::= CHOICE {
>                and             [0] SET OF Filter,
>                or              [1] SET OF Filter,
>                not             [2] Filter,
>		
>This recursion is mirrored in Samba's recursive decent parser, which
>consumes around 600 bytes of stack per filter sent by the client.
>
>In Samba, LDAP packets are parsed pre-authentication.
>
>As an example on Linux x86_64, a LDAP search expression of (|(|(x=y)))
>will consume over 1k of stack (600 bytes or so per OR).  Therefore,
>even a fairly small, un-authenticated LDAP packet can cause the server
>to fault with SIGSEGV as the stack reaches the OS-imposed limit (8MB
>in this case).
>
>If the network architecture allows a CLDAP packet (to UDP port 389) of
>over 13,000 bytes (the maximum UDP packet size is 65,535) this
>would also fit enough ASN.1 to crash the CLDAP server.
>
>Samba 4.11 and later use the 'prefork' process model by default for
>LDAP, and all versions use single process for CLDAP.
>
>This shares one process between multiple network clients.  By crashing
>one worker, legitimate service is disrupted to other clients.  The
>process is restarted in all supported versions, but with a back-off.
>
>NOTE WELL: Unsupported Samba versions before Samba 4.7 use a single
>process for the (C)LDAP servers.  All versions of Samba before Samba
>4.10 do not restart that process.
>
>To address further concerns about resource use from LDAP packets, new
>pre-parse limits are placed on LDAP searches (250k packet size limit),
>all un-authenticated packets (250k packet size limit) other
>authenticated packets (16MB) and CLDAP packets (4k).
>
>* For authenticated connections the maximum packet size is controlled by
>  the smb.conf parameter "ldap max authenticated request size"
>
>* For anonymous connections the maximum packet size is controlled by
>  the smb.conf parameter "ldap max anonymous request size"
>
>* For searches, the maximum packet size is controlled by
>  the smb.conf parameter "ldap max search request size"
>
>==================
>Patch Availability
>==================
>
>Patches addressing both these issues have been posted to:
>
>    https://www.samba.org/samba/security/
>
>Additionally, Samba 4.10.15, 4.11.8 and 4.12.2 have been issued
>as security releases to correct the defect.  Samba administrators are
>advised to upgrade to these releases or apply the patch as soon
>as possible.
>
>==================
>CVSSv3 calculation
>==================
>
>CVSSv3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (7.5)
>
>================================
>Workaround and mitigating factors
>================================
>
>By default, Samba 4.10 is run using the "standard" process model,
>which would leave only the CLDAP server affected.  (Later versions use
>'prefork').
>
>This is controlled by the -M or --model parameter to the samba binary.
>
>All Samba versions are impacted if -M prefork or -M single is used. To
>mitigate this issue for LDAP only, select -M standard (however this
>will use more memory, may allow resource exhaustion, and will still
>leave the CLDAP server exposed).
>
>=======
>Credits
>=======
>
>Credit to oss-fuzz.
>
>Triage of the oss-fuzz report and advisory written by Andrew Bartlett
>of Catalyst and the Samba Team.
>
>Patches provided by Gary Lockyer of Catalyst and the Samba Team.
>
>==========================================================
>== Our Code, Our Bugs, Our Responsibility.
>== The Samba Team
>==========================================================
>

Flags:

dbagnall: review+
gary: review+
dbagnall: review+
abartlet: review+

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