In the Linux kernel, the following vulnerability has been resolved:
ipv6: avoid overflows in ip6_datagram_send_ctl()
Yiming Qian reported :
I believe I found a locally triggerable kernel bug in the IPv6 sendmsg
ancillary-data path that can panic the kernel via skb_under_panic()
(local DoS).
The core issue is a mismatch between:
- a 16-bit length accumulator (
struct ipv6_txoptions::opt_flen, type
__u16) and
- a pointer to the *last* provided destination-options header (
opt->dst1opt)
when multiple IPV6_DSTOPTS control messages (cmsgs) are provided.
include/net/ipv6.h:struct ipv6_txoptions::opt_flen is __u16 (wrap possible).
(lines 291-307, especially 298)
net/ipv6/datagram.c:ip6_datagram_send_ctl():- Accepts repeated
IPV6_DSTOPTS and accumulates into opt_flen
without rejecting duplicates. (lines 909-933)
net/ipv6/ip6_output.c:__ip6_append_data():- Uses
opt->opt_flen + opt->opt_nflen to compute header
sizes/headroom decisions. (lines 1448-1466, especially 1463-1465)
net/ipv6/ip6_output.c:__ip6_make_skb():- Calls
ipv6_push_frag_opts() if opt->opt_flen is non-zero.
(lines 1930-1934)
net/ipv6/exthdrs.c:ipv6_push_frag_opts() / ipv6_push_exthdr():- Push size comes from
ipv6_optlen(opt->dst1opt) (based on the
pointed-to header). (lines 1179-1185 and 1206-1211)
opt_flen is a 16-bit accumulator:include/net/ipv6.h:298 defines __u16 opt_flen; /* after fragment hdr */.ip6_datagram_send_ctl() accepts *repeated* IPV6_DSTOPTS cmsgs
and increments
opt_flen each time:
- In
net/ipv6/datagram.c:909-933, for IPV6_DSTOPTS:
- It computes
len = ((hdr->hdrlen + 1) << 3);
- It checks
CAP_NET_RAW using ns_capable(net->user_ns,
CAP_NET_RAW). (line 922)
- Then it does:
opt->opt_flen += len; (line 927)opt->dst1opt = hdr; (line 928)
There is no duplicate rejection here (unlike the legacy
IPV6_2292DSTOPTS path which rejects duplicates at
net/ipv6/datagram.c:901-904).
If enough large IPV6_DSTOPTS cmsgs are provided, opt_flen wraps
while dst1opt still points to a large (2048-byte)
destination-options header.
In the attached PoC (poc.c):
- 32 cmsgs with
hdrlen=255 => len = (255+1)*8 = 2048
- 1 cmsg with
hdrlen=0 => len = 8
- Total increment:
32*2048 + 8 = 65544, so (__u16)opt_flen == 8
- The last cmsg is 2048 bytes, so
dst1opt points to a 2048-byte header.
- The transmit path sizes headers using the wrapped
opt_flen:
- In
net/ipv6/ip6_output.c:1463-1465:
headersize = sizeof(struct ipv6hdr) + (opt ? opt->opt_flen +
opt->opt_nflen : 0) + ...;
With wrapped opt_flen, headersize/headroom decisions underestimate
what will be pushed later.
- When building the final skb, the actual push length comes from
dst1opt and is not limited by wrapped
opt_flen:
- In
net/ipv6/ip6_output.c:1930-1934:
if (opt->opt_flen) proto = ipv6_push_frag_opts(skb, opt, proto);- In
net/ipv6/exthdrs.c:1206-1211, ipv6_push_frag_opts() pushes
dst1opt via
ipv6_push_exthdr().
- In
net/ipv6/exthdrs.c:1179-1184, ipv6_push_exthdr() does:
skb_push(skb, ipv6_optlen(opt));memcpy(h, opt, ipv6_optlen(opt));
With insufficient headroom, skb_push() underflows and triggers
skb_under_panic() -> BUG():
net/core/skbuff.c:2669-2675 (skb_push() calls skb_under_panic())net/core/skbuff.c:207-214 (skb_panic() ends in BUG())- The
IPV6_DSTOPTS cmsg path requires CAP_NET_RAW in the target
netns user namespace (
ns_capable(net->user_ns, CAP_NET_RAW)).
- Root (or any task with
CAP_NET_RAW) can trigger this without user
namespaces.
- An unprivileged
uid=1000 user can trigger this if unprivileged
user namespaces are enabled and it can create a userns+netns to obtain
namespaced
CAP_NET_RAW (the attached PoC does this).
- Local denial of service: kernel BUG/panic (system crash).
- ---truncated---
What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)
The EG score (5.5): severity, escalated by confirmed exploitation. This CVE's 5.5 is its impact severity (EG Severity), not escalated.
- EG Severity
- 5.5 MEDIUMCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
- EG Likelihood
- 0.1% within 30 daysSource: FIRST EPSS. FIRST's exploitation probability, not an EchelonGraph model.
- Confirmed exploitation
- None on record (CISA KEV, EG-KEV Tier 1)Reported apart from the likelihood: an observed fact, not a probability.
- EG Priority
- SSVC Track at medium mission impactTrack at low or medium mission impact; Track* at high (mission-essential systems).CISA BOD 26-04: 14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.Each input and its source: the SSVC and BOD 26-04 blocks below.
SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Track* at high (mission-essential systems).
A fix is available. Apply it within your standard update timelines.
Exploitation none (no CISA value, KEV listing or exploit code on record) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). EchelonGraph holds no SSVC assessment of this CVE from CISA: none of these inputs is from Vulnrichment. EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table
BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.
In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). Where an input reads “CISA's BOD 26-04 default”, no one has published it for this CVE, and CISA's implementation guidance says: “If “Automatable” and “Technical Impact” information items about a CVE ID are not available and the CVE ID is not in the KEV Catalog, CISA will treat its values as “no” and “total,” respectively, until CISA or a third party provides metadata about the CVE ID. CISA will always provide the Vulnrichment data for a CVE ID listed in the KEV Catalog.” Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it