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How would an attack on a git repo work? You create a repo with identical hashes but different content and next time the user clones from scratch they get your m

How would an attack on a git repo work? You create a repo with identical hashes but different content and next time the user clones from scratch they get your modified version?

flatiron 7y ago
yeah my thoughts about git are similar. look at the two messages they have an an example: Key is part of a collision! It's a trap!yE'NsbK#ދW]{1gKmCx's/vr| -pJO_,1$)uB1qXv#U)9ESU;p~0G:Y ݕbBIjFra눰3&t'lB_!h5M([,˴QMK#|o5pv|i,+yYpݍD7_Rf\'GUZ,ϵdvAYAugV=Lk8_E 2 +nolBtxXoQt&+?Y3LP:'Qt(,ۛuԪWJm:A"M6<|B4kVv̨ޠA=M+m%殺j5N|EMA\Ed- s&@u@:a?pq^Xf0U?R} and Practical SHA-1 chosen-prefix collision!'lka}vbI3,·W]Ǟ+gK}Cxs/v&r| }-hRJO_ rO̳;bzC ,1&uRP-MXrU3aO;pr0:sY'2 l&r7#(A{oNyCJ_W,8 əbحBYީpFr2a8#&t+n_15q(_,ˤQMW#hzYMgVV=L,kO0E*N +oc@BpXoᯖd&?+?[{3LвP&'U t ( WJÏm\:A"6>>|SB(k;Vv̨ޠ^A=Y ;om%j-|cUAAۜEТ&@o@:La3psH^eXf0QJm ݶd they have the same sha1sum, but in all practicality its nonsense since both messages are pure trash. you couldn't have malicious C code that would have the same hash as non malicious C code in this example
- saalweachter 7y ago
  Isn't that like incredibly simple? Dump your garbage string behind a // or inside an #if 0, restrict the garbage string character set to characters which will not disturb that, and your compiler will whistle while it works.
  - pathseeker 7y ago
    Depends on if the chosen prefix attack allows the content to appear arbitrarily in the middle of the byte stream like that.
    - Thorrez 7y ago
      That's exactly what a chosen prefix attack means. You choose the arbitrary prefixes. Then the garbage is inserted. Then (due to SHA1's Merkle–Damgård construction) you append a postfix that's mostly arbitrary (but the same in both files).
  - flatiron 7y ago
    anyone checking diffs would notice that, or working on the file, etc. it wouldn't survive long
    - saalweachter 7y ago
      Hmmm, does the garbage string actually have to survive long? If there's a followup CL to "delete a garbage string that accidentally made it into the repo", which doesn't actually fix whatever else was added, would that get you anywhere?
    - munk-a 7y ago
      I think active projects would detect this fine - but what if that commit was pushed to lpad and everyone ended up pulling it to local because it's a dependency of a dependency of a dependency in NPM? Or what if it's a really obscure library for parsing like... pyramidal jpeg2000s, are the library consumers going to be checking the source? Heck, most people already don't check download checksums unless their downloader does it automatically.
thenewnewguy 7y ago
You'd probably be most successful modifying the original repo - either by being the creator of the software or gaining their trust. However, it would have to be a rather powerful SHA1 attack for the commit to still be valid syntax, hard to detect, and make a meaningful malicious change.
munk-a 7y ago
If you could push up a commit that computed to the same hash of the last tagged release in a repo... I'm not certain, the tag might end up referencing the new object? Certain versions of git (i.e. maybe git for windows) may also react in different manners. In theory you might get people building software packages for distros to build your malicious version, you may also just temporarily shut down the ability for anyone to check out the version (basically denial of service for making?) but the time window would be weird.

How would an attack on a git repo work? You create a repo with identical hashes but different content and next time the user clones from scratch they get your modified version?

flatiron 7y ago

yeah my thoughts about git are similar. look at the two messages they have an an example: Key is part of a collision! It's a trap!yE'NsbK#ދW]{1gKmCx's/vr| -pJO_,1$)uB1qXv#U)9ESU;p~0G:Y ݕbBIjFra눰3&t'lB_!h5M([,˴QMK#|o5pv|i,+yYpݍD7_Rf\'GUZ,ϵdvAYAugV=Lk8_E 2 +nolBtxXoQt&+?Y3LP:'Qt(,ۛuԪWJm:A"M6<|B4kVv̨ޠA=M+m%殺j5N|EMA\Ed- s&@u@:a?pq^Xf0U?R} and Practical SHA-1 chosen-prefix collision!'lka}vbI3,·W]Ǟ+gK}Cxs/v&r| }-hRJO_ rO̳;bzC ,1&uRP-MXrU3aO;pr0:sY'2 l&r7#(A{oNyCJ_W,8 əbحBYީpFr2a8#&t+n_15q(_,ˤQMW#hzYMgVV=L,kO0E*N +oc@BpXoᯖd&?+?[{3LвP&'U t ( WJÏm\:A"6>>|SB(k;Vv̨ޠ^A=Y ;om%j-|cUAAۜEТ&@o@:La3psH^eXf0QJm ݶd they have the same sha1sum, but in all practicality its nonsense since both messages are pure trash. you couldn't have malicious C code that would have the same hash as non malicious C code in this example

saalweachter 7y ago
Isn't that like incredibly simple? Dump your garbage string behind a // or inside an #if 0, restrict the garbage string character set to characters which will not disturb that, and your compiler will whistle while it works.
- pathseeker 7y ago
  Depends on if the chosen prefix attack allows the content to appear arbitrarily in the middle of the byte stream like that.
  - Thorrez 7y ago
    That's exactly what a chosen prefix attack means. You choose the arbitrary prefixes. Then the garbage is inserted. Then (due to SHA1's Merkle–Damgård construction) you append a postfix that's mostly arbitrary (but the same in both files).
- flatiron 7y ago
  anyone checking diffs would notice that, or working on the file, etc. it wouldn't survive long
  - saalweachter 7y ago
    Hmmm, does the garbage string actually have to survive long? If there's a followup CL to "delete a garbage string that accidentally made it into the repo", which doesn't actually fix whatever else was added, would that get you anywhere?
  - munk-a 7y ago
    I think active projects would detect this fine - but what if that commit was pushed to lpad and everyone ended up pulling it to local because it's a dependency of a dependency of a dependency in NPM? Or what if it's a really obscure library for parsing like... pyramidal jpeg2000s, are the library consumers going to be checking the source? Heck, most people already don't check download checksums unless their downloader does it automatically.

thenewnewguy 7y ago

You'd probably be most successful modifying the original repo - either by being the creator of the software or gaining their trust. However, it would have to be a rather powerful SHA1 attack for the commit to still be valid syntax, hard to detect, and make a meaningful malicious change.

munk-a 7y ago

If you could push up a commit that computed to the same hash of the last tagged release in a repo... I'm not certain, the tag might end up referencing the new object? Certain versions of git (i.e. maybe git for windows) may also react in different manners. In theory you might get people building software packages for distros to build your malicious version, you may also just temporarily shut down the ability for anyone to check out the version (basically denial of service for making?) but the time window would be weird.