Daniel Berlin on Security Insight on SAP security, development stuff… and all the rest

25Apr 16

Protection of the Security Audit Log against deletion

Hi SAP security enthusiasts,How to prevent erasure of log data
the Security Audit Log [SAL] contains -obviously- information about important system events and should therefore better not get lost unintentionally!
Let's see, how the Audit Log can be erased and what we can do to prevent this and maximize its protection.

The Audit Log is stored in log-files that are located in the file system (see parameter DIR_AUDIT) and is either rotated daily or when the current file is full (see parameter rsau/max_diskspace/per_file).

Access to these files is possible via kernel functionality (from within the SAP system) or on operating system level (e.g. via an external OS command). We’ll focus on access via the SAP application server and won’t take a deeper look at manipulations directly from OS level (i.e. from the command line).

Ways to delete the Security Audit Log

From inside the SAP system, three variants of deleting the SAL files exist.
Below we'll check them and see, which measures exist to protect the logs.

Variant 1: SM18 (or function module RSAU_CLEAR_AUDIT_LOG or system function C_REMOVE)

SM18 is the SAP standard way of removing old SAL files.
The transaction is protected by an authorization check for S_ADMI_FCD with value AUDA (AUDit log Administration). The minimum age of files to be erased is 3 days − a nice feature, because an attacker cannot remove fresh logs and hide what he/she did moments ago.

Protection
SM18 is not really problematic… S_ADMI_FCD with value AUDA is a critical authorization, which nobody should be assigned to on production.

Diving a bit deeper…
A clever developer might try to copy the code that is responsible for the deletion to a customer report − fortunately this does not work... see below:

REPORT.
WRITE 'SM18 (C_REMOVE):'.
 
DATA: errno  TYPE rsautypes-errno
    , errmsg TYPE rsautypes-errmsg.
 
CALL 'C_REMOVE' ID 'DIR'    FIELD '/usr/sap/<SID>/DVEBMGS00/log'
                ID 'FILE'   FIELD 'audit_20160425'
                ID 'GROUP'  FIELD 'AUDIT'
                ID 'ERRNO'  FIELD errno
                ID 'ERRMSG' FIELD errmsg.
 
IF sy-subrc = 0. WRITE: 'successful'.
ELSE.            WRITE: 'failed;', errmsg. " caller not registered
ENDIF.

⇒ This report always runs into an error, because the SAP kernel checks the program name that calls the C_REMOVE function against an internal whitelist; we'll always get a "failed; caller not registered" message…

Variant 2: DELETE DATASET

A more sophisticated way of getting rid of audit log files is to simply delete them via the DELETE DATASET statement from within an ABAP report.

A simple example:

REPORT.
WRITE 'DELETE DATASET:'.
 
TRY.
    DELETE DATASET '/usr/sap/<SID>/DVEBMGS00/log/audit_20160425'.
 
    IF sy-subrc = 0. WRITE 'successful'.
    ELSE.            WRITE 'failed / not found'.
    ENDIF.
 
  CATCH cx_sy_file_authority.
    WRITE 'missing authorization'.
ENDTRY.

Basic protection
The kernel checks for S_DATASET authorizations each time a DELETE DATASET statement is executed (this cannot be prevented, since the check takes place in the kernel).
The field ACTVT is checked for value 06 (delete) and the FILENAME field for the path of the file to be deleted.

Better protection
A check for authorization object S_PATH can be activated to implement an additional protection for file system accesses. It is then evaluated after a successful S_DATASET check.

Alternatively, we can also prevent unauthorized log file deletion completely… even if a user has S_DATASET with all field values set to "*"!

For the additional check, the logic is as follows (see SAP Note 177702):

  • first, the system checks whether the SPTH table contains a PATH, which matches the start of the path name of the log file that we want to delete
  • if an authorization group is set in the FS_BRGRU field, the system then performs an authorization check on S_PATH with ACTVT = 02 (write or delete)

Additionally, the table SPTH has two flags, which can further limit file access and overwrite the authorization checks on S_PATH and S_DATASET:

  • if FS_NOREAD has the value "X", neither read nor write access is possible
  • and FS_NOWRITE = "X" prevents write access

SPTH Log File Protection

To effectively prevent Security Audit Log deletion, you can add the following entry to SPTH:

PATH = /usr/sap/<SID>/DVEBMGS00/log/audit_
FS_NOREAD = X
FS_NOWRITE = X

Afterwards, no user will be able to read or modify the SAL log files – disregarding their authorizations!
Anyway, SM20 will continue to work, as the access takes place in the kernel.

Variant 3: External operating system command

The third variant does not use the SAP kernel to delete the file, but rather an OS command (in the following example we’ll use the Unix/Linux rm command).

The following report creates a new external command, executes it to delete the same audit log file as above and finally deletes the command.

Note
The creation and deletion of the command is logged via table logging for table SXPGCOSTAB… but at least at a first glance, we can hide what we did by deleting the command after execution…

REPORT.
WRITE 'External command (SM69):'.
 
DATA command TYPE sxpgcolist.
command-NAME      = 'Z_DEL_AUDITLOG'.
command-opsystem  = sy-opsys.
command-opcommand = 'rm'.
command-addpar    = 'X'.
 
CALL FUNCTION 'SXPG_COMMAND_INSERT'         " --- Create command
  EXPORTING
    command = command
  EXCEPTIONS
    OTHERS  = 1.
 
IF sy-subrc = 0. WRITE 'creation successful;'.
ELSE.            WRITE 'creation failed.'.
                 LEAVE PROGRAM.
ENDIF.
 
DATA exitcode TYPE extcmdexex-exitcode.
 
CALL FUNCTION 'SXPG_COMMAND_EXECUTE'        " --- Execute command
  EXPORTING
    commandname           = command-NAME
    additional_parameters = '/usr/sap/&lt;SID&gt;/DVEBMGS00/log/audit_20160425'
  IMPORTING
    exitcode              = exitcode
  EXCEPTIONS
    OTHERS                = 1.
 
IF sy-subrc = 0 AND exitcode = 0. WRITE 'execution successful;'.
ELSE.                             WRITE 'execution failed;'.
ENDIF.
 
DATA: BEGIN OF command_del.
        INCLUDE STRUCTURE sxpgcostab.
DATA:   sapcommand   TYPE sxpgcolist-sapcommand
    ,   TYPE(8)      TYPE c
    , END   OF command_del
    , rc             TYPE i.
 
command_del-NAME     = command-NAME.
command_del-opsystem = command-opsystem.
 
PERFORM command_delete(saplsxpt)            " --- Delete command
        USING    SPACE
        CHANGING command_del rc.
 
IF sy-subrc = 0 AND rc = 0. WRITE 'deletion successful.'.
ELSE.                       WRITE 'deletion failed.'.
ENDIF.

Of course, you could also create, execute and delete the command via transaction SM69.

Protection
The above method works fine, if you have S_RZL_ADM (ACTVT = 01) for the creation of the command and S_LOG_COM authorizations for execution.
Unfortunately, the SPTH-protection from variant 2 does not work here.

If table logging is enabled (profile parameter rec/client), changes are recorded in table DBTABLOG and can be evaluated via transaction SCU3 (for table SXPGCOSTAB).

Bottom line

All three variants can be protected quite well via authorizations, so log deletion can be prevented by strictly controlling these authorizations.

Anyway high-privileged users – e.g. emergency users – still have access, so I suggest also implementing the SPTH-protection described in variant 2. 💡 It provides a good additional line of defense around the log files.

Please keep in mind that you should also take care of protecting table SPTH itself… 💡

Apart from the access perspective, organizational measures should be implemented to make sure that malicious code like to one above is checked (and refused) before a transport to production takes place!

So long!