Document security expectations (AcademySoftwareFoundation#1623)

* Document security expectations

Signed-off-by: Cary Phillips <cary@ilm.com>

* Menion Imath as a dependency

Signed-off-by: Cary Phillips <cary@ilm.com>

* Update SECURITY.md

Co-authored-by: Nick Porcino <meshula@hotmail.com>
Signed-off-by: Cary Phillips <cary@ilm.com>

* change 'Threat Model' to 'Potential Vulnerabilties'

Signed-off-by: Cary Phillips <cary@ilm.com>

* Mention GitHub issue as fallback security contact

Signed-off-by: Cary Phillips <cary@ilm.com>

* github security advisory

Signed-off-by: Cary Phillips <cary@ilm.com>

* mention exrcheck

Signed-off-by: Cary Phillips <cary@ilm.com>

---------

Signed-off-by: Cary Phillips <cary@ilm.com>
Co-authored-by: Nick Porcino <meshula@hotmail.com>

SECURITY.md

@@ -6,17 +6,15 @@
 ## Reporting a Vulnerability
 
 If you think you've found a potential vulnerability in OpenEXR, please
-report it by emailing security@openexr.com. Only Technical Steering
-Committee members and Academy Software Foundation project management
-have access to these messages. Include detailed steps to reproduce the
-issue, and any other information that could aid an investigation. Our
-policy is to respond to vulnerability reports within 14 days.
-
-Our policy is to address critical security vulnerabilities rapidly and
-post patches as quickly as possible. If you do not get a response to a
-message sent to security@openexr.com within 48 hours, contact the
-project maintainers via a GitHub
-[Issue](https://github.com/AcademySoftwareFoundation/openexr/issues).
+report it by filing a GitHub [security
+advisory](https://github.com/AcademySoftwareFoundation/openexr/security/advisories/new). Alternatively,
+email security@openexr.com and provide your contact info for further
+private/secure discussion.  If your email does not receive a prompt
+acknowledgement, your address may be blocked.
+
+Our policy is to acknowledge the receipt of vulnerability reports
+within 48 hours. Our policy is to address critical security vulnerabilities
+rapidly and post patches within 14 days if possible.
 
 ## Known Vulnerabilities
 
@@ -61,3 +59,108 @@ These vulnerabilities are present in the given versions:
 
 See the [release notes](CHANGES.md) for more information.
 
+## Supported Versions
+
+This gives guidance about which branches are supported with patches to
+security vulnerabilities.
+
+| Version / branch  | Supported                                            |
+| --------- | ---------------------------------------------------- |
+| main      | :white_check_mark: :construction: ALL fixes immediately, but this is a branch under development with a frequently unstable ABI and occasionally unstable API. |
+| 3.2.x    | :white_check_mark: All fixes that can be backported without breaking ABI compatibility. |
+| 3.1.x    | :warning: Only the most critical fixes, only if they can be easily backported. |
+| 3.0.x    | :warning: Only the most critical fixes, only if they can be easily backported. |
+| 2.5.x    | :warning: Only the most critical fixes, only if they can be easily backported. |
+| <= 1.x   | :x: No longer receiving patches of any kind. |
+
+## Security Expectations
+
+### Software Features
+
+- The OpenEXR project implements the EXR image file format, used
+  throughout the motion picture industry and beyond, on Linux, macOS,
+  and Windows.
+
+- The project consists of a software run-time library, implemented in
+  C/C++ and built via cmake, that reads and writes image data
+  files. The project also distributes python wrappings for the C/C++
+  I/O API.
+
+- The library reads and writes binary image data and text-based
+  metadata, treated as blind data, none of which is executable code.
+
+- Other than the website and online technical documentation, the
+  project implements no web/online services or network communication
+  protocols.  The library never requests any security or
+  authentication credentials or login information from users.
+
+  The website implements no interactive features and requires no login
+  credentials.
+
+- The library reads and writes only to file paths specificly requested
+  via the C/C++ API. The runtime library uses no system configuration
+  files or sidecar data files. Access to data files uses only standard
+  file I/O system calls.
+
+- The library compresses/decompresses data via standard compression
+  algorithms but uses no cryptographic or confidentiality protocols.
+
+### Software Dependencies
+
+OpenEXR depends on
+[Imath](https://github.com/AcademySoftwareFoundation/Imath), a library
+of basic math operations also maintained and distributed by the
+OpenEXR project. Imath follows the same security conventions
+documented here for OpenEXR itself. The core Imath library has no
+external dependencies. The Imath python bindings depend on python and
+boost.
+
+The only
+external library dependency of OpenEXR is
+[libdeflate](https://github.com/ebiggers/libdeflate), which implements
+standard deflate/zlib/gzip compression and decompression.
+
+The project uses
+[Snyk](https://github.com/AcademySoftwareFoundation/openexr/blob/main/.github/workflows/snyk-scan-pr.yml)
+to scan for dependency vulnerability.
+
+### Potential Vulnerabilities
+
+Potential entry points are images being loaded using the
+library. Malformed images could caused issues such as heap buffer
+overflows, out-of-memory faults, or segmentation faults that could be
+exploitable as denial-of-service attacks. 
+
+### Hardening
+
+#### Testing
+
+The OpenEXR project implements a comprehensive suite of validation
+tests, including fuzz testing to harden against malicious input
+data. Note that fuzz testing hardens only against *small* input data
+files and is not a comprehensive test against all potential input.
+
+Note that the
+[exrcheck](https://github.com/AcademySoftwareFoundation/openexr/tree/main/src/bin/exrcheck)
+utility is intended to be used by testers to demonstrate a particular
+proof-of-concept input file exposes a vulnerability, and it is very
+helpful to let us know if a vulnerability can be reproduced using that
+tool.
+
+The project also uses the [OSS
+Fuzz](https://bugs.chromium.org/p/oss-fuzz) service for continuous
+fuzz testing.
+
+#### Development Cycle and Distribution
+
+OpenEXR is downloadable and buildable by C/C++ source via GitHub. Only
+members of the project's Technical Steering Committee, all veteran
+software engineers at major motion picture studios or vendors, have
+write permissions on the source code repository. All critical software
+changes are reviewed by multiple TSC members.
+
+The library is distributed in binary form via many common package
+managers across all platforms.
+
+
+