https://github.com/angea/pocorgtfo#0x14
And yes, documents are not normally supposed to be able to display their own MD5 hash.
Also, a more recent innovation in MD5 collisions is textcoll, which creates colliding blocks that are completely plaintext. This would allow for colliding PHP source files like in OP but without any obvious binary artefacts (although this requires identical prefixes).
https://github.com/cr-marcstevens/hashclash?tab=readme-ov-fi...
[1]: https://lemire.me/blog/2025/01/11/javascript-hashing-speed-c...
Try this on your own system:
$ head -c 1000000000 /dev/urandom > random-1gb
$ time md5sum random-1gb
ef72a3616aad5117ddf40a7d5f5d0162 random-1gb
real 0m2.428s
user 0m2.192s
sys 0m0.202s
$ time sha256sum random-1gb
ec7d7f31c4489acae8328fddbe54157f1cb9e97b220ef502a07e1f9230969310 random-1gb
real 0m3.894s
user 0m3.697s
sys 0m0.181s
$ time b3sum random-1gb
11fe11cc5721faf65369d18893d7b7631f6178b4692bc0bb03b1b180273cd384 random-1gb
real 0m0.282s !!!
user 0m0.876s
sys 0m0.124s
$ time b3sum --num-threads=1 random-1gb
11fe11cc5721faf65369d18893d7b7631f6178b4692bc0bb03b1b180273cd384 random-1gb
real 0m0.597s
user 0m0.488s
sys 0m0.107s
The only CPUs still likely to be in use and without SHA support are the Intel Core CPUs until and including the Skylake derivatives (i.e. up to Comet Lake, i.e. up to 6 years ago).
The Intel Atoms have received SHA support many years before Intel Core, because they competed with ARM, which already had such support.
The support in Intel Core has been added due to AMD Zen, but the products with it have been delayed by the failure of Intel to achieve acceptable fabrication yields in their 10-nm CMOS process, before 2019/2020.
1. You can upload scripts that get scanned for malicious code 2. These scripts can be executed once deemed "safe" 3. The server is using MD5 hashes to determine if you uploaded the same file or if it should re-scan it
3. Is where the issue is. It should probably always re-scan it and it definitely should not be using MD5.
Wouldn't the sensible thing for a server that gets an upload matching an existing file's hash be to just treat it as an idempotent no-op? What reason would it have to replace the old version with a presumably identical copy? What am I missing?
Most I've seen (sec scans, backup validation/dedup etc) pushed to phase out md5 very long time ago.
if normal.php had actual php code in there, being really 'normal' as the name implies, this would be much more severe / interesting because it might be more easy to convince modern security products it's actually a benign file.
Currently if it would be analysed, it would be flagged as suspicious simply because its not a valid file. and really, it dont need to be php, it could be any valid file format as long as it's an actually file that has benign behavior or contents.
plaintext might be easier to generate, but you'd need it to be 'executable' format or something interpretable like a script to have it actually stored in databases marking files as malicious or benign. matching filetype with the malicious file, in a valid form that does actual benign behavior would be 'best'.
don't take me wrong tho. still fun to see these things and honestly props, if it bypasses anything that's always a 'nice result' :)
its funny often web basted languages have this property tho , i mean, how else you gonna poison logs and execute them :')... js and php are just adorable for providing opportunities :D
But there are two applications: the first is breaking in to a system under some very obscure set of circumstances that you are very unlikely to encounter in the real world. The second is to bump up your karma on HN.
If you do know, then you also know md5 being broken is really really old news.
Seriously. Cryptographers have been warning that md5 seems weak since 1996. There are probably people reading this thread who weren't even alive yet. (It got totally broken in 2004 but the warning signs were way earlier).
Is there any way to use HN karma? Like, can I sell my account on some shady exchange like people sell big twitter accounts? And if I can, what's the going rate for internet points these days? Asking for an unscrupulous friend.
The thing that makes this blog post not realistic is:
* Such tricks would make much more sense with normal programs, where you're trying to trick an user to download and execute it. Webshells are downloaded by the attacker knowingly.
* Md5 is not used anymore (although I know security vendors who used it for embarrassingly long time). If this was SHA256, that attack would be devastating for many more severe reasons.
But it's still a fun PoC.
That's only true if you ignore all the details.
As usual, you cannot make a coherent understanding on just about any subject by reading headlines alone. Life would have taught you by now that the devil is in the details.
WP uses salt and multiple rounds of hashing, fully mitigating the md5 collisions being topic of discussion here.
So no, wp doesn't "use md5" in the sense that they would be vulnerable to this type of attack.
Source: https://developer.wordpress.org/reference/functions/wp_hash_...
> Can use it bypass some cached webshell detections.
Nothing other than vanity AFAIK.
It's actually a bit of a scam because karma accumulates and never expires. I've been on the leaderboard for a long time, not because I'm making particularly valuable contributions (I only post a few times a week) but just because I've been on HN since it launched.
Such security! Much wow!
As the OP article/PoC is about hashing uploaded files, not passwords btw, I think you should read it again.
Because as I pointed out, wp_hash() is used to check against uploaded files.
Oh, and source: https://developer.wordpress.org/reference/functions/wp_hash/
And as I cannot resist quoting you for trying to smartass while literally not having read the source code the PoC was about:
> As usual, you cannot make a coherent understanding on just about any subject by reading headlines alone. Life would have taught you by now that the devil is in the details.
>Can use it bypass some cached webshell detections.
The amount of sweet, sweet irony displayed here will make me diabetic. Did you read the article at all? Salting? What are you on about?
Honestly, it feels that some HN commenters are LLMs instructed to defend a given entity.
Is this not an oxymoron? E.g. b3 then ought to be an order of magnitude easier to brute force.
For a 256-bit cryptographic hash function, it should take an expected 2^256 attempts to find a message with a given hash (preimage attack) and around 2^128 attempts to find any collision (due to the birthday paradox), and a few other properties like that. This holds for both SHA-256 and Blake3 (as far as we know—neither algorithm has proven security*) but not for MD5.
MD5 is insecure not just because its output size of 128 bit is too short (though that's a problem too), but also because it has weaknesses that allow constructing collisions with much less than the 2^64 attempts than you would expect on the basis of its output size. That's why MD5 is considered insecure even for its size.
Generally speaking, you want your hashing primitives to be as fast as possible. The practical security then comes from the output size. If someone discovered a secure 320-bit cryptographic hash that is a trillion times faster than even Blake3 (10^12 or about 2^40), everyone should adopt it, because it would be much faster and even more secure against brute force attacks than SHA-256/Blake3 are (since 320 > 256 + 40).
While there are use cases for deliberately slow hash functions too (notably password hashing) those can be constructed using fast hash functions as primitives. For example, one of the strongest password hashing schemes (Argon2) is based on one of the fastest hashing primitives (Blake2), not a slow one as you might have expected.
While BLAKE3 can be many times faster than SHA-256, by consuming many times more power, the amount of work for computing a hash differs much less between the 2 hashes than the execution time on a multi-core CPU.
The speed difference quoted by you for a single thread is caused by your Skylake-based CPU, which does not have the SHA hardware instructions.
Moreover, even the programs that claim to use the SHA hardware instructions may have a speed several times lower than allowed by the hardware, because the more recent CPUs, e.g. from the last 4 years, have wider SHA instructions than the older CPUs, but the programs must have been compiled to support such CPUs, e.g. Zen 3 and newer or Alder Lake and newer.
Such an algorithm has been first published by Ralph Merkle, in 1979, but it has been improved later:
https://en.wikipedia.org/wiki/Merkle_tree
For security, it is necessary to use different hash functions at different levels in the hash tree, but this is trivially achieved by using the same hash function, but also hashing some extra distinguishing data besides the hashes from the previous level.