Would Rust secure cURL?

16th Jan 2021

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 think “how many historical bugs would this have prevented” is a really good way of judging a programming language or feature. For example this great study shows that using Typescript would have prevented approximately 15% of all bugs that you find in typical Javascript code. It’s hard to argue against static types with evidence like that.

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.

  1. 47 are standard memory errors (overflows, use-after free, etc.). Rust would definitely prevent these. For comparison, Google found that 70% of Chrome’s high severity security bugs are memory errors.
  2. 5 are integer overflows, which Rust does not prevent by default in release mode (though it can via an optional flag), but they lead to memory errors which it does prevent.
  3. 1 was through misuse of 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.
image/svg+xml No Unlikely Possibly Likely Yes 0 25 50 75 100 Bug count Would Rust have prevented the bug?

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 other observations

Some random things I noticed when reading the list.

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
92 FTP-KRB double-free Memory 4
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
61 SSL_VERIFYSTATUS ignored Logic 0
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