Apache patch proves patchy – now you need to patch the patch


Software patches are sometimes a bit like buses.

You don’t get one for a while, and then three come at once.

For buses on busy urban routes, at least, the explanation of the phenomenon goes something like this.

If three buses start out travelling the same route together in a nicely spaced sequence, then the first one is most likely to be the slowest, because it will be stopping to scoop up most of the waiting passengers, while the ones behind will tend to travel faster because they need to stop less often or for shorter periods.

So buses naturally tend to scrunch up and arrive in bursts.

Burst-mode software patches

When it comes to software patches, however, the problem often works the other way around.

If the first patch arrives too quickly, then it may not have been reviewed or tested quite as much as you might like.

So it’s not so much that the next patch in the queue catches up because the first one is too slow, but that the next one has to be completed in a rush to keep up…

…and, if you aren’t careful, then that second patch might itself beget a third patch, needed to patch the patch that patched the first patch.

Three Apache buses

And thus with Apache: just two days ago, we reported a path validation bug dubbed CVE-2021-41773 that was introduced in Apache 2.4.49:

We advised you to update to 2.4.50, which would indeed have protected you against at least some of the known exploits already circulating on Twitter.

But the 2.4.50 update itself was incomplete, having been put together in something of a hurry, so that although it blocked some ways of exploiting the bug, it didn’t reliably block all of them.

The bottom line is that if you have Apache 2.4.49 (released 2021-09-15) or Apache 2.4.50 (released 2021-10-04) then you now need to update to Apache 2.4.51 (released 2021-10-07).

What went wrong?

We haven’t investigated this in detail, but a quick look at the three versions listed above suggests that the vulnerability progressed like this:

  • Apache 2.4.49 made it possible to embed the string ../ (dot-dot-slash) in a URL by using a sneaky text encoding that disguised the second dot. Filenames with dot-dot-slash sequences are dangerous because they tell the operating system to go up one directly level, thus “cancelling out” the previous subdirectory specified in the path. Multiple dot-dot-slash sequences could allow an attacker to ascend far enough up the directory tree to “escape” from the web server’s official sub-tree of official files, and from there to descend again into forbidden parts of the operating system, such as /etc on Unix or C:WindowsSystem32 in Windows.
  • Apache 2.4.50 watched out for suspiciously encoded dots in any dot-dot-slash sequence. But the patch wasn’t always able to block suspiciously encoded slashes in the pathname, so that by shifting the trick from disguising a dot to disguising a slash, an attack was still theoretically possible.
  • Apache 2.4.51 now watches out for a wider range of suspcious encodings. The code now explicitly includes an internal flag named AP_UNESCAPE_URL_FORBID_SLASHES, and is stricter about reporting errors for inappropriate and therefore exceptionable URL encodings, even if they might inadvertently appear in non-malicious URLs.

What next?

Will the sudden arrival of the third bus in this burst of patches mean that we’ll soon have 2.4.52 to follow?

We don’t know.

But at least the patch-for-the-patch came out within two days of the 2.4.50 update, which, though imperfect, would have stopped a range of already-known attacks that were widely circulated in the wild.

So we don’t regret having urged you to update to Apache 2.4.50 earlier this week, even though it now means updating once again.

(Our Linux distro took care of both of these updates for us quickly and automatically, but if you’re using an Apache version you built yourself, don’t forget that you need to recompile it.)

Our earlier article about this CVE-2021-41773 bug includes an explanation of jargon terms path validation and path traversal, and provides some suggestions on how to find Apache servers that you might not realise are in use on your network:

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