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author | Joey Hess <joeyh@joeyh.name> | 2017-02-24 00:17:25 -0400 |
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committer | Joey Hess <joeyh@joeyh.name> | 2017-02-24 00:17:25 -0400 |
commit | 24115d7fe885e3c15603daca9c2bd5e25c7c5a14 (patch) | |
tree | fb3d90f72b86a78ab46163dd3c3f1ab44a4ee1d6 /doc | |
parent | 55983c0a8e4ad4908b57b69f64256fbb7aa24397 (diff) |
Tighten key parser to not accept keys containing a non-numeric fields, which could be used to embed data useful for a SHA1 attack against git.
Also todo about why this is important, and with some further hardening to
add.
This commit was sponsored by Ignacio on Patreon.
Diffstat (limited to 'doc')
-rw-r--r-- | doc/todo/sha1_collision_embedding_in_git-annex_keys.mdwn | 36 |
1 files changed, 36 insertions, 0 deletions
diff --git a/doc/todo/sha1_collision_embedding_in_git-annex_keys.mdwn b/doc/todo/sha1_collision_embedding_in_git-annex_keys.mdwn new file mode 100644 index 000000000..069fef85b --- /dev/null +++ b/doc/todo/sha1_collision_embedding_in_git-annex_keys.mdwn @@ -0,0 +1,36 @@ +Some git-annex backends allow embedding enough data in the names of keys +that it could be used for a SHA1 collision attack. So, a signed git commit +could point to a tree with such a key in it, and the blob for the key could +have two versions with the same SHA1. + +Users who want to use git-annex with signed commits to mitigate git's own +SHA1 insecurities would like at least a way to disable the insecure +git-annex backends: + +* WORM can contain fairly arbitrary data in a key name +* URL too (also, of course, URLs download arbitrary data from the web, + so a signed git commit pointing at URL keys doesn't have any security + even w/o SHA1 collisions) +* SHA1 and MD5 backends are insecure because there can be colliding + versions of the data they point to. + +A config setting to prevent git-annex from using insecure backends would be +useful. + +(git-annex might suggest enabling that configuration if commit.gpgSign +is enabled) + +A few other potential problems: + +* `*E` backends could embed sha1 collision data in a long filename + extension. That this is much harder to exploit because git-annex + checks the hash of the data when it enters the repository, and git-annex + fsck also verifies it. It still might be worth limiting the length + of an extension in such a key to the longest such extension git-annex has + ever supported (probably < 20 bytes or so), which would be less than the + size of the data needed for current SHA1 collision attacks. +* It might be possible to embed colliding data in a specially constructed + key name with an extra field in it, eg "SHA256-cXXXXXXXXXXXXXXX-...". + Need to review the code and see if such extra fields are allowed. + Update: All fields are numeric, but could contain arbitrary data + after the number. (fixed now) |