Security: OOB write/read in protocol/dns.c dns_name_decode/encode and compression path

[LL-V]

Summary

protocol/dns.c has multiple memory-safety bugs in DNS name encode/decode used by dns_rr_unpack / dns_unpack / nslookup / dns_query.

I am reporting these as original findings against current libhv master (lab verification 2026-07-22). I did not find a prior issue/CVE specifically for dns_name_decode / dns_name_encode bounds.

Bug 1 — dns_name_decode unbounded write (CWE-120 / CWE-787)

// protocol/dns.c
int dns_name_decode(const char* buf, char* domain) {
    const char* p = buf;
    int len = *p++;
    int buflen = 1;
    while (*p != '\0') {
        if (len-- == 0) {
            len = *p;
            *domain = '.';
        } else {
            *domain = *p;
        }
        ++p;
        ++domain;   // never checks remaining space in domain[]
        ++buflen;
    }
    *domain = '\0';
    ...
}

Callers pass rr->name which is only DNS_NAME_MAXLEN (256) bytes (dns_rr_t.name[256]), but the decoder never enforces that limit. A crafted wire name that expands to >255 bytes overwrites past name[].

There is also no remaining-input length parameter: a missing terminating 0x00 causes unbounded read of buf.

Local confirmation: canary after a 256-byte domain buffer is smashed (buflen returned 513).

Bug 2 — compression-pointer path leaves name[] uninitialized (CWE-457 / CWE-125)

int dns_rr_unpack(...) {
    if (*(uint8_t*)p >= 192) {
        namelen = 2;
        // name is never written / NUL-terminated
    } else {
        namelen = dns_name_decode(buf, rr->name);
    }
    ...
}

Any RR using a compression pointer leaves rr->name untouched. Later strlen / %s use is an OOB read.

Bug 3 — dns_name_encode unbounded write (CWE-787)

int dns_name_encode(const char* domain, char* buf) {
    ...
    while (*p != '\0') {
        ...
        ++buf;  // no capacity check
        ++buflen;
    }

dns_rr_pack uses a fixed char encoded_name[256] and calls encode without checking domain length. A long rr->name overruns encoded_name.

Local confirmation: encode of a long name returned length 502 past a 256-byte buffer canary.

Attack surface

Any application using libhv DNS helpers (dns_unpack, dns_query, nslookup, examples under protocol DNS) that processes untrusted DNS responses (or packs attacker-influenced names) can hit these paths.

Not the default HTTP-only build path, but enabled with protocol/DNS (WITH_PROTOCOL / equivalent).

Suggested fix

1. Change APIs to pass capacity / remaining wire length, e.g.
   dns_name_decode(const char* buf, size_t buflen, char* domain, size_t domaincap)
2. Reject expanded names >= 256 and individual labels > 63.
3. On compression pointer, either fully resolve into name[] with bounds, or set name[0] = '\0' / explicit flag and never treat as C-string until resolved.
4. Cap dns_name_encode output to provided buffer size; return error if domain too long.
5. Add unit tests + ASan fuzz of dns_unpack.

Reproduction (lab)

Standalone canary harnesses (same algorithm as tree):

• long wire name → dns_name_decode smashes canary after 256-byte domain
• compression pointer path → name[] never written
• long domain string → dns_name_encode smashes 256-byte encode buffer

Happy to attach minimal C files or open a PR with a patch if useful.

Researcher

LL-V
GitHub: https://github.com/LL-V

Please credit if a CVE is requested. Testing was local-only against source / standalone reimplementation of the vulnerable functions.