Rewriting programs in Rust has become a bit of a meme and one program that has been discussed a lot is cURL.
The first time people suggested rewriting cURL in Rust, the main author Daniel Stenberg wrote an article about why cURL is written in C and wouldn’t be rewriten in Rust. It includes this section:
C is not the primary reason for our past vulnerabilities
There. The simple fact is that most of our past vulnerabilities happened because of logical mistakes in the code. Logical mistakes that aren’t really language bound and they would not be fixed simply by changing language.
Of course that leaves a share of problems that could’ve been avoided if we used another language. Buffer overflows, double frees and out of boundary reads etc, but the bulk of our security problems has not happened due to curl being written in C.
Three years later news arrived that some Rust code would be used in cURL, though only as an optional HTTP backend - it isn’t a full rewrite. This news reignited the discussion (Reddit, Hacker News). It seems that some people are still under the impression that it is possible to write memory-safe C, and based on the above quote that cURL is memory safe C!
Is this true? Are the majority of cURL’s security vulnerabilities logic mistakes?
It’s easy to find out. The cURL authors have a great list of (known) cURL security vulnerabilities. If you skim it it becomes immediately obvious that no, cURL has plenty of memory safety bugs. Since there’s a nice list with great descriptions of each bug it seems like a nice opportunity to measure how many bugs Rust would have prevented.
I went through the entire list of cURL security issues, and categorised all of the bugs, together with whether or not I think Rust would have prevented them. I did not look at the code for all of them (e.g. if it says “buffer overflow” then it’s pretty clear Rust would prevent it), so take these results with a small pinch of salt. Corrections welcome!
There are 95 bugs. By my count Rust would have prevented 53 of these.
fgets(). Rust does not stop you making difficult to use APIs, but it definitely reduces the chance, e.g. by warning you if you don’t use a
Result. It’s hard to imagine this bug happening with Rust.
The remaining bugs are logic errors of some kind or another. There are definitely several of the sort “we should
have checked thing, but didn’t” that Rust couldn’t help with. But there are also a decent number of other bugs that come from cURL doing ad-hoc inline
character-by-character parsing of just about everything, whereas in Rust you would probably use a library to fully parse things. I’ve generously counted these as
No in my tally but I suspect they would be less likely with Rust.
It is safe to say that nobody can write memory-safe C, not even famous programmers that use all the tools. Here’s Daniel in 2017:
We keep scanning the curl code regularly with static code analyzers (we maintain a zero Coverity problems policy) and we run the test suite with valgrind and address sanitizers.
12 out of 15 of cURL’s security issues since that statement have been memory errors (or integer overflows leading to memory errors).
Rust proponents may seem overly zealous and I think this has led to a minor backlash of people thinking “Rust can’t be that great surely; these people must be confused zealots, like Trump supporters or Christians”. But it’s difficult to argue with numbers like these.
Some random things I noticed when reading the list.
These are how I classified the bugs. If I’ve got something drastically wrong let me know.
|#||Vulnerability||Classification||Rust prevention (0=no, 4=yes)|
|95||wrong connect-only connection||Logic, pointers||2|
|94||curl overwrite local file with -J||Logic||1|
|93||Partial password leak over DNS on HTTP redirect||Logic, quoting||1|
|91||TFTP small blocksize heap buffer overflow||Memory||4|
|90||Windows OpenSSL engine code injection||Logic||0|
|89||TFTP receive buffer overflow||Memory||4|
|88||Integer overflows in curl_url_set||Integer overflow leading to memory||3|
|87||NTLM type-2 out-of-bounds buffer read||Memory||4|
|86||NTLMv2 type-3 header stack buffer overflow||Memory||4|
|85||SMTP end-of-response out-of-bounds read||Memory||4|
|84||warning message out-of-buffer read||Memory||4|
|83||use-after-free in handle close||Memory||4|
|82||SASL password overflow via integer overflow||Integer overflow leading to memory||3|
|81||NTLM password overflow via integer overflow||Integer overflow leading to memory||3|
|80||SMTP send heap buffer overflow||Memory||4|
|79||FTP shutdown response buffer overflow||Memory||4|
|78||RTSP bad headers buffer over-read||Memory||4|
|77||RTSP RTP buffer over-read||Memory||4|
|76||LDAP NULL pointer dereference||Memory||4|
|75||FTP path trickery leads to NIL byte out of bounds write||Memory||4|
|74||HTTP authentication leak in redirects||Logic||0|
|73||HTTP/2 trailer out-of-bounds read||Memory||4|
|72||SSL out of buffer access||Memory||4|
|71||FTP wildcard out of bounds read||Memory||4|
|70||NTLM buffer overflow via integer overflow||Integer overflow leading to memory||3|
|69||IMAP FETCH response out of bounds read||Memory||4|
|68||FTP PWD response parser out of bounds read||Memory||4|
|67||URL globbing out of bounds read||Memory||4|
|66||TFTP sends more than buffer size||Memory||4|
|65||FILE buffer read out of bounds||Memory||4|
|64||URL file scheme drive letter buffer overflow||Memory||4|
|63||TLS session resumption client cert bypass (again)||Logic, reuse||0|
|62||--write-out out of buffer read||Memory||4|
|60||uninitialized random||Type error||4|
|59||printf floating point buffer overflow||Memory||4|
|58||Win CE schannel cert wildcard matches too much||Logic||0|
|57||Win CE schannel cert name out of buffer read||Memory||4|
|56||cookie injection for other servers||Logic, difficult to use API||3|
|55||case insensitive password comparison||Logic, terrible function name||0|
|54||OOB write via unchecked multiplication||Memory||4|
|53||double-free in curl_maprintf||Memory||4|
|52||double-free in krb5 code||Memory||4|
|51||glob parser write/read out of bounds||Memory||4|
|50||curl_getdate read out of bounds||Memory||4|
|49||URL unescape heap overflow via integer truncation||Memory||4|
|48||Use-after-free via shared cookies||Memory||4|
|47||invalid URL parsing with '#'||Logic, used regex, now 2 problems||0|
|46||IDNA 2003 makes curl use wrong host||Logic, unicode insanity||1|
|45||curl escape and unescape integer overflows||Integer overflow leading to memory||3|
|44||Incorrect reuse of client certificates||Logic, reuse||0|
|43||TLS session resumption client cert bypass||Logic, reuse||0|
|42||Re-using connections with wrong client cert||Logic, reuse||0|
|41||use of connection struct after free||Memory||4|
|40||Windows DLL hijacking||Windows API nonsense||0|
|39||TLS certificate check bypass with mbedTLS/PolarSSL||Logic||0|
|38||remote file name path traversal in curl tool for Windows||Logic, quoting||0|
|37||NTLM credentials not-checked for proxy connection re-use||Logic, reuse||0|
|36||SMB send off unrelated memory contents||Memory, but I think this is still reading from valid allocated memory, heartbleed style||0|
|35||lingering HTTP credentials in connection re-use||Logic, reuse||0|
|34||sensitive HTTP server headers also sent to proxies||Logic||0|
|33||host name out of boundary memory access||Memory||4|
|32||cookie parser out of boundary memory access||Memory||4|
|31||Negotiate not treated as connection-oriented||Logic||0|
|30||Re-using authenticated connection when unauthenticated||Logic, reuse||0|
|29||darwinssl certificate check bypass||Logic, reuse||0|
|28||URL request injection||Logic||0|
|27||duphandle read out of bounds||Memory||4|
|26||cookie leak for TLDs||Logic, parsing||0|
|25||cookie leak with IP address as domain||Logic||0|
|24||not verifying certs for TLS to IP address / Winssl||Logic||0|
|23||not verifying certs for TLS to IP address / Darwinssl||Logic||0|
|22||IP address wildcard certificate validation||Logic||0|
|21||wrong re-use of connections||Logic||0|
|20||re-use of wrong HTTP NTLM connection||Logic, reuse||2|
|19||cert name check ignore GnuTLS||Logic||0|
|18||cert name check ignore OpenSSL||Logic||0|
|17||URL decode buffer boundary flaw||Memory||4|
|16||cookie domain tailmatch||Logic||0|
|15||SASL buffer overflow||Memory||4|
|14||SSL CBC IV vulnerability||Logic||0|
|13||URL sanitization vulnerability||Logic, parsing||0|
|12||inappropriate GSSAPI delegation||Logic||0|
|11||local file overwrite||Logic, parsing||0|
|10||data callback excessive length||Memory||4|
|9||embedded zero in cert name||Logic, null-terminated strings||4|
|8||Arbitrary File Access||Logic||0|
|7||GnuTLS insufficient cert verification||Logic||0|
|6||TFTP Packet Buffer Overflow||Memory||4|
|5||URL Buffer Overflow||Memory||4|
|4||NTLM Buffer Overflow||Memory||4|
|3||Authentication Buffer Overflows||Memory||4|
|2||Proxy Authentication Header Information Leakage||Logic||0|
|1||FTP Server Response Buffer Overflow||Memory||4|