Attachment Scrubbing

PII can be included in attachments which are included in events. Minidumps, a common attachment type for native crash reports, may contain PII in various sections of the file.

Data Scrubbing Methods in Attachments

Scrubbing attachments, and especially minidumps, should be done carefully and without destroying the original file format. Sentry provides some built-in knowledge of some file formats and provides custom source-selectors to help you write scrubbing rules that won't damage the file when used carefully.

When modifying data in the file, Sentry cannot modify the length of the file. This has implications for how the Methods behave:

  • Any modification that results in replacement text longer than the original will be truncated.
  • Any modification that results in replacement text shorter than the original will be padded using x as the padding character.

Here is how this works specifically for each method:

  • Remove: As the entire field is removed, it is essentially replaced with the padding character.
  • Mask: This behaves as normal, all data is replaced with * masking character.
  • Hash: Hashing happens as usual, however the binary representation of the match is hashed, regardless of the encoding in which the match was made. The resulting hash is padded or truncated as described above.
  • Replace: This behaves like normal, with the replacement being padded or truncated as described above.

Attachment Source Selectors

All attachments can be selected using the $attachments Value Type selector at the root of the selector, followed by the specific filename. The filename must be in quotes to write a specific filename, but wildcards can also be used.

For example, for the filename minidump.dmp, you can select all fields to be scrubbed using $attachments.'minidump.dmp'.**. To write the same using a wildcard to select all attachments, you would write $attachments.*.** or even simply $attachments.**.

Minidump Selectors

A minidump is always an attachment, but it also has its own value type of $minidump, which allows you to select all fields that can be scrubbed in a minidump using $minidump.**. This really is a shorthand for selecting the fields of all attachments that are minidumps: $attachments.$minidump.**.

Our examples thus far have repeatedly referred to all fields in a minidump that can be scrubbed. For the purpose of PII scrubbing, we currently represent the minidump as a flat structure with the following fields:

  • The stack memory: These are memory regions in the minidump that are used as stack memory by a thread. You can use the stack_memory path item to select these regions. Generally they will contain binary data, so be careful with the data types used to scrub these regions.

    We do not recommend scrubbing stack memory since it is required by Sentry to build a readable stack trace for the event. Inadvertently modifying the stack in an undesired way will make this process impossible and the event far less useful. Because of this, you cannot select the stack memory by a more generic selector like a value type. You must select it explicitly, such as by stack_memory or $minidump.stack_memory, for it to be scrubbed.

    In general, all data types result in a textual regular expression match on some data. For the stack, this regular expression is matched as a binary UTF-8 regular expression so that any UTF-8 embedded in the binary data will be matched. Additionally the regular expression is applied to any valid UTF-16LE strings found within the binary data.

  • The heap memory: This is considered to be any memory region which is not used as stack memory. This means these memory regions could include other non-stack regions, such as files that are mapped to the process' memory using mmap. All these memory regions can be selected using the heap_memory path item selector. Additionally these regions also have $binary as a value type selector, allowing you to write more generic rules.

    Since Sentry does not require heap memory for extracting stack traces, it is much safer to modify these memory regions.

    Similar to stack memory, all data types are matched as a binary UTF-8 regular expression as well as in any valid UTF-16LE strings found within the binary memory region.

  • A minidump also contains a list of code modules, which are loaded at the time the minidump is created. This module list contains information about each module, including two fields that may contain file paths: the path of the code file and the path of the associated debug file.

    Since these fields are pathnames, they have the potential to contain a user's home directory. As a result, it makes sense to apply the Usernames in filepaths data type to these fields. The fields can be selected individually by their respective field names:

    • code_file
    • debug_file

    Both fields have a value type of $string. Any modification will exclude the basename of the pathnames to ensure the event processing pipeline can still operate correctly on the minidump.

    This example illustrates how a rule to scrub these fields displays in sentry.io:

    Copied
    [Remove] [Usernames in filepaths] from [$minidump.code_file || $minidump.debug_file]

    As this rule is more generic, it is also possible to apply it more generically using the value type:

    Copied
    [Remove] [Usernames in filepaths] from [$string]
  • Linux minidumps can contain both the command line and environment variables, which are included as copies of the files in /proc/$pid/cmdline and /proc/$pid/environ. Since these are direct copies of these files, they have this file format: NULL-separated records of argv arguments or VAR=val pairs respectively.

    To select these, you can currently use only their value types: $binary, which will be successful because these are currently the only fields in the minidump with a binary value type.

    This example is how the UI displays a rule to scrub the home directory from the $HOME env var:

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    [Remove] [HOME=[^\u0000+]\u0000] from [$minidump.$binary]

    Note the syntax to denote a string delimited by NULL in the regular expression: any sequence of characters not containing NULL, but ending in NULL. Here, NULL itself is represented by its unicode codepoint: \u0000.

Malformed Minidumps

If a minidump fails to be parsed because it is malformed it could still contain PII. For this reason the minidump attachment is also scrubbed as one binary field if it can not be parsed.

In this case the entire attachment can be selected as normal using a filename selector or a wildcard: $attachments.'minidump.dmp' or $attachments.*. Additionally since the entire attachment is now a single field it can be selected using the $binary value type.

Example

A process usually contains a struct with it's environment variables early in the stack memory, in C semantics this can often be consumed by the process using the char *envp[] parameter to main(). The environment often contains $HOME which contains the username which could be PII. This block can occur in a number of places in a minidump:

  • Early on the stack, as passed via **envp.
  • In the /proc/$pid/environ block, which is an exact copy of the data on the stack.
  • As a raw attachment, if the minidump failed to parse.

A rule could be written to scrub the $HOME environment variable in all these locations as:

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[Remove] [HOME=[^\u0000+]\u0000] from [stack_memory || $binary]

Note that stack_memory needs to be explicitly used as it can not be addressed by a more general rule because scrubbing this field can easily break minidump processing. However the last two cases are covered together with $binary which will match both $attachment.$minidump.$binary (the environ file) and $attachment.'minidump.dmp'.

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