3 ms·

CSP is optional and designed to be one part of a defense-in-depth strategy (to extent that it was thoughtfully designed at all—it's an awful standard that shoul

by cxr 4mo ago

CSP is optional and designed to be one part of a defense-in-depth strategy (to extent that it was thoughtfully designed at all—it's an awful standard that should not have made it past proposal stage). It's not a solution for sandboxing untrusted content and should not be relied upon that way. Treating it like one is a great demonstration of how some uses of CSP make people more vulnerable.

simonw 4mo ago
Right, which is why I'm combining it with <iframe sandbox=""> - which really is designed to be used as a sandbox (if you can figure out the right way to implement it.)
- cxr 4mo ago
  > <iframe sandbox=""> - which really is designed to be used as a sandbox Not for untrusted content living on the same origin to prevent it from exercising any of the powers that it would ordinarily have to be able to access sensitive data. It's a misleading name and shouldn't have been chosen. There is no combination of CSP or the iframe sandbox attribute that can be relied upon for that purpose. This is a fundamental limitation of the way the specs were written. (There needs to be a big warning about this on MDN, but moving from the old wiki to a wiki with GitHub for login to the GitHub-based pull request process really didn't help the there's-a-problem-on-this-page-but-limited-resources-to-make-things-better problem.)
  - simonw 4mo ago
    That's why I'm careful not to include allow-same-origin in the sandbox attribute - without that the iframe content is treated as a separate origin from the parent. And I serve the content in srcdoc= to ensure there's no URL a user can visit which would directly execute the content outside of that iframe sandbox.
    - cxr 4mo ago
      > That's why I'm careful not to include allow-same-origin in the sandbox attribute It doesn't matter. I just said there is no combination of CSP or the iframe sandbox attribute that can be relied upon here.
      - simonw 4mo ago
        If that's true then my project is fatally flawed and I need to stop distributing it. I'm not convinced it's true - I've been thinking about this for months, and building experimental prototypes to help me get to the combination that I think makes sense. Can you describe an exploit that the combination I'm using of iframe sandbox= srcdoc= with an injected meta CSP tag doesn't handle? Would moving the untrusted content to be served from a separate domain entirely close the hole? (In case it's not clear the iframe sandbox= is the bit that's doing most of the work here - the CSP stuff is there mainly to protect against malicious apps that deliberately exfiltrate stolen private data.)
        cxr 4mo ago
        > Would moving the untrusted content to be served from a separate domain entirely close the hole? Yes.

by cxr 4mo ago

simonw 4mo ago

Right, which is why I'm combining it with <iframe sandbox=""> - which really is designed to be used as a sandbox (if you can figure out the right way to implement it.)

cxr 4mo ago
> <iframe sandbox=""> - which really is designed to be used as a sandbox Not for untrusted content living on the same origin to prevent it from exercising any of the powers that it would ordinarily have to be able to access sensitive data. It's a misleading name and shouldn't have been chosen. There is no combination of CSP or the iframe sandbox attribute that can be relied upon for that purpose. This is a fundamental limitation of the way the specs were written. (There needs to be a big warning about this on MDN, but moving from the old wiki to a wiki with GitHub for login to the GitHub-based pull request process really didn't help the there's-a-problem-on-this-page-but-limited-resources-to-make-things-better problem.)
- simonw 4mo ago
  That's why I'm careful not to include allow-same-origin in the sandbox attribute - without that the iframe content is treated as a separate origin from the parent. And I serve the content in srcdoc= to ensure there's no URL a user can visit which would directly execute the content outside of that iframe sandbox.
  - cxr 4mo ago
    > That's why I'm careful not to include allow-same-origin in the sandbox attribute It doesn't matter. I just said there is no combination of CSP or the iframe sandbox attribute that can be relied upon here.
    - simonw 4mo ago
      If that's true then my project is fatally flawed and I need to stop distributing it. I'm not convinced it's true - I've been thinking about this for months, and building experimental prototypes to help me get to the combination that I think makes sense. Can you describe an exploit that the combination I'm using of iframe sandbox= srcdoc= with an injected meta CSP tag doesn't handle? Would moving the untrusted content to be served from a separate domain entirely close the hole? (In case it's not clear the iframe sandbox= is the bit that's doing most of the work here - the CSP stuff is there mainly to protect against malicious apps that deliberately exfiltrate stolen private data.)
      - cxr 4mo ago
        > Would moving the untrusted content to be served from a separate domain entirely close the hole? Yes.