Skip to content

(CVE-2020-25219) pac server can trigger unbounded recursion in url.cpp recvline() #134

Closed
@mcatanzaro

Description

@mcatanzaro

I found this in url.cpp:

static inline string recvline(int fd) {
	// Read a character.
	// If we don't get a character, return empty string.
	// If we are at the end of the line, return empty string.
	char c = '\0';
	
	if (recv(fd, &c, 1, 0) != 1 || c == '\n')
		return "";

	return string(1, c) + recvline(fd);
}

Looks like the server that hosts the proxy authconfig file can cause libproxy to overflow the stack by sending an unending stream of characters without a newline. The PAC server should be trusted to not do that, but it's still not good. Normal use with a non-malicious server looks like this:

(gdb) bt
#0  recvline (fd=4) at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:389
#1  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#2  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#3  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#4  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#5  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#6  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#7  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#8  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#9  0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#10 0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#11 0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#12 0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#13 0x00007ffff7f987ac in recvline (fd=<optimized out>)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:398
#14 0x00007ffff7f99749 in libproxy::url::get_pac (this=this@entry=0x7fffffffdd30)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/url.cpp:464
#15 0x00007ffff7f8b7f9 in libproxy::proxy_factory::expand_pac (this=0x416eb0, confurl=...)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/proxy.cpp:393
#16 0x00007ffff7f9120a in libproxy::proxy_factory::get_proxies (this=0x416eb0, realurl=...)
    at /usr/src/debug/libproxy-0.4.15-17.fc32.x86_64/libproxy/proxy.cpp:215
#17 0x00007ffff7f916dc in px_proxy_factory_get_proxies (self=0x416eb0, url=0x402010 "https://lwn.net")
    at /usr/include/c++/10/bits/char_traits.h:300
#18 0x0000000000401188 in main ()

Metadata

Metadata

Assignees

No one assigned

    Labels

    No labels
    No labels

    Type

    No type

    Projects

    No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions